Published by Electrotek Concepts, Inc., PQSoft Case Study: Distribution Feeder Wind Turbine Generator Analysis, Document ID: PQS1206, Date: January 26, 2012.
Abstract: This case study presents a distribution feeder wind turbine generator analysis. The investigation included several potential causes of turbine and feeder overcurrents and overvoltages, including wind speed variations, cut-out and cut-in switching operations, ground faults, and islanding during feeder isolation. Mitigation alternatives for high temporary overvoltages (TOV) on distribution feeders include application restrictions, grounding transformers, direct transfer trip schemes, and crowbar switches.
INTRODUCTION
A distribution feeder wind turbine generator analysis case study was completed for the system shown in Figure 1. The case study investigated several potential causes of turbine and feeder overcurrents and overvoltages, including wind speed variations, cut-out and cut-in switching operations, ground faults, and islanding during feeder isolation. The simulations were completed using the PSCAD® transient program. A transient model was created to simulate a relatively weak distribution feeder with a single wind turbine generator connected near the end of the feeder. The resulting voltages and currents during various switching and fault events were determined.
Figure 1 – Illustration of Oneline Diagram for a Distribution Feeder Turbine Analysis
SIMULATION ANALYSIS
The simulation model included a 12.47kV substation and distribution feeder supplying a 1.5 MW wind turbine generator connected near the end of the feeder. The model included a 600 kVAr capacitor bank connected at the substation bus and several additional 600 kVAr capacitor banks on the feeder. There was one distribution feeder included in the model.
The model was designed so turbine and feeder overcurrents and overvoltages during wind speed variations, turbine cut-out and cut-in switching operations, ground faults, and islanding during feeder isolation could be determined. The accuracy of the simulation model at 60 Hz was determined using simulated fault current magnitudes and other steady-state quantities, such as cable line charging (MVAr) and feeder load flow values (MW & MVAr). The representation of the system short-circuit equivalent at the 12.47kV source substation included:
Three-phase (I3φ) fault current: 4,000 A @ -85.0° (86 MVA) Single-line-to-ground (IφG) fault current: 3,000 A @ -85.0° (65 MVA)
These values were converted to ohms for the PSCAD representation, which included a three-phase voltage source with positive and zero sequence impedances and a 150 Ω damping resistor.
The 12.47kV distribution feeder and underground cable sections were modeled using the following impedance data:
Conductor: 4/0 AWG AL Length: 5,550 feet Positive sequence impedance (Z1): 0.1087 +j0.0653 Ω/1000’ Zero sequence impedance (Z0): 0.2567 +j0.0688 Ω/1000’ Line charging (B/2): 8.7 μmhos/1000’
It was assumed that positive and zero sequence line charging values were the same. The coupled π-section model was used to model each cable section. That assured accurate representation of both the series impedances, as well as the line charging of the underground feeder cable sections.
The turbine transformer was modeled using the three-phase, two-winding transformer model. The turbine step-up transformer data included:
Three Phase Rating: 2,000 kVA Secondary Voltage: 480 V (grounded-wye) Primary Voltage: 12.47 kV (grounded-wye) Nameplate Impedance: 5.75% (X/R Ratio = 10)
The 1.5 MW (1.67 MVA @ assumed 0.90 PF) wind turbine generator was modeled using a wound rotor induction machine model. The wound rotor induction machine can be used to represent an induction generator and it can be operated in either ‘speed control’ or ‘torque control’ modes. One 150 kVAr capacitor bank was connected to the 480 V secondary for reactive power support.
The wind turbine generator was controlled using PSCAD’s built-in wind turbine component. The inputs to the component include wind speed (Vw – m/s) and the mechanical speed of the machine connected to the turbine (W – rad/s). The pitch angle (Beta) of the turbine blades is entered in degrees if pitch control is enabled. If pitch control is disabled, the turbine is switched to stall control mode, where the turbine’s aerodynamic characteristics will determine the output torque. The component outputs include torque (Tm) and power (P), which are in per-unit based on the machine’s MVA rating.
The wind speed component models the wind speed available to the wind turbine generator. The input of the component is an external signal representing wind speed (ES – m/s) and the output is the wind speed available to the turbine (Vw – m/s). This component can be used to study a wind gust (or ramp) to determine the response of the control system. The wind turbine characteristics determine the input torque variations (torque control mode) supplied to the induction generator.
Case 1 included simulating a wind speed variation. The simulation time was 20 seconds and the timestep was 50μsec. The average wind speed was 15 m/s. The simulated wind speed is shown in Error! Reference source not found.. The figure shows the simulated wind speed between 5 and 20 seconds. The maximum speed gust was 27 m/s.
Figure 3 shows the corresponding turbine current (Phase A) for Case 1. The maximum value was 17.0 kA during the initial abrupt wind speed change between 8 and 9 seconds.
Figure 4 shows the simulated electromagnetic torque for Case 1, while Figure 5 shows the corresponding turbine power.
Figure 2 – Simulated Wind Speed for Case 1
Figure 3 – Simulated Turbine Current for Case 1
Figure 4 – Simulated Electromagnetic Torque for Case 1
Figure 5 – Simulated Turbine Power for Case 1
Case 2 included simulating cut-out and cut-in switching operations. The simulation time was 20 seconds and the average wind speed was 15 m/s. The sequence of events for the switching operation included opening (cut-out) the low voltage circuit breaker supplying the wind turbine generator at 10 seconds and closing (cut-in) the circuit breaker back in at 15 seconds.
Figure 6 shows the turbine current (Phase A) for Case 2. The peak current value was 78.2 kA when the circuit breaker opens. The oscillatory transient voltage is due to the interaction with the power factor correction capacitor.
Figure 7 shows the corresponding turbine voltage (Phase A) for Case 2. The peak voltage was 471 V (1.20 per-unit) when the circuit breaker opens.
Case 3 included simulating a ground fault at the low voltage wind turbine generator secondary bus. The simulation time was 20 seconds and the average wind speed was 15 m/s. The sequence of events for the ground fault included applying a single-phase-to-ground fault (Phase A) on the turbine generator bus. The fault did not clear during the simulation.
Figure 8 shows the turbine current (Phase A) for Case 3. The peak current value was 30.9 kA when the fault occurs.
Figure 9 shows the turbine voltage for Case 3. The fundamental frequency temporary overvoltage was 464 V (1.19 per-unit) during the single-phase fault. Figure 10 shows the corresponding 12.47kV feeder voltage. The temporary overvoltage was 12.014kV (1.18 per unit).
Figure 6 – Simulated Turbine Current for Case 2
Figure 7 – Simulated Turbine Voltage for Case 2
Figure 8 – Simulated Turbine Current for Case 3
Figure 9 – Simulated Turbine Voltage for Case 3
Figure 10 – Simulated Distribution Feeder Voltage for Case 3
Case 4 involved simulating an islanding condition by opening the circuit breaker on the 12.47kV primary side of the turbine transformer. The simulation time was 20 seconds and the average wind speed was 15m/s. The sequence of events for the islanding case included opening the circuit breaker at 10 seconds. There were no faults applied during the simulation.
Figure 11 shows the turbine current (Phase A) for Case 4. The peak current value was 32.5 kA when the circuit breaker opens. Figure 12 shows the corresponding simulated electromagnetic torque for Case 4.
Figure 11 – Simulated Turbine Current for Case 4
Figure 12 – Simulated Electromagnetic Torque for Case 4
SUMMARY
This case study presents a distribution feeder wind turbine generator analysis. The investigation included several potential causes of turbine and feeder overcurrents and overvoltages, including wind speed variations, cut-out and cut-in switching operations, ground faults, and islanding during feeder isolation.
Voltage regulation, islanding, and reverse power flow, and the resulting potentially severe temporary overvoltages, are three of the most important operating considerations for distributed generation applications having a single large wind turbine generator on a relatively weak distribution feeder. These conditions are typically not dependent on the wind turbine generator type and therefore may occur for any of the turbines.
Mitigation alternatives for high temporary overvoltages on distribution feeders include application restrictions, grounding transformers, direct transfer trip schemes, and crowbar switches.
REFERENCES
IEEE Recommended Practice for Measurement and Limits of Voltage Fluctuations and Associated Light Flicker on AC Power Systems, IEEE Std. 1453-2004, IEEE, 2005, ISBN: 0- 7381-4482-7.
IEEE Recommended Practice for Grounding of Industrial and Commercial Power Systems, IEEE Std. 142 (IEEE Green Book), IEEE, November 2007, ISBN: 0738156392.
IEEE Recommended Practice for Monitoring Electric Power Quality,” IEEE Std. 1159-1995, IEEE, October 1995, ISBN: 1-55937-549-3.
IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems, IEEE Std. 519-1992, IEEE, ISBN: 1-5593-7239-
RELATED STANDARDS IEEE Std. 142 IEEE Std. 1453
GLOSSARY AND ACRONYMS DFT: Discreet Fourier Transform PCC: Point of Common Coupling TDD: Total Demand Distortion TOV: Temporary Overvoltage
Published by Stanisław CZAPP , Gdańsk University of Technology, Faculty of Electrical and Control Engineering
Abstract. In the paper spectral analysis of the distorted earth fault currents in circuits with variable speed drives is performed. Effectiveness of protection against electric shock using residual current devices in circuits with variable speed drives for various PWM frequencies is evaluated. Operational characteristic of the proposed residual current device is presented.
Streszczenie. W artykule przedstawiono analizę widmową odkształconego prądu ziemnozwarciowego z obwodu silnika o regulowanej prędkości obrotowej za pomocą przemiennika częstotliwości. Oceniono skuteczność ochrony przeciwporażeniowej wykorzystującej wyłączniki różnicowoprądowe przy różnych wartościach częstotliwości PWM przemiennika. Przedstawiono charakterystykę działania proponowanego wyłącznika różnicowoprądowego. (Wpływ częstotliwości PWM na skuteczność ochrony przeciwporażeniowej wykorzystującej wyłączniki różnicowoprądowe).
Keywords: variable speed drives, protection against electric shock, residual current devices. Słowa kluczowe: napędy o regulowanej prędkości obrotowej, ochrona przeciwporażeniowa, wyłączniki różnicowoprądowe.
Introduction
Electrical devices should not introduce risk of electric shock either in normal service or in the case of a single fault condition. In each part of an installation one or more protective measures shall be applied [1]:
– automatic disconnection of supply (with overcurrent and/or residual current devices), – double or reinforced insulation, – electrical separation for the supply of one item of the current-using equipment, – extra-low-voltage (SELV or PELV).
The most commonly used of the above mentioned protective measures is the automatic disconnection of supply. Specific and difficult for analysis of the effectiveness of protection against electric shock in the case of direct or indirect contact are circuits with variable speed drives. Earth fault in the output terminals of inverter or in motor terminals (Fig. 1) results in significant current in the PE conductor. Overcurrent or residual current devices should operate. Direct contact (Fig. 1) in the line between inverter and motor gives a low value of current but it is very dangerous for persons because the whole fault current flows through the human body. Only high sensitivity residual current devices may operate effectively. The earth fault current in circuits with variable speed drives is distorted and it has the fundamental significance for proper operation of protective devices, especially the residual current devices. For sinusoidal waveform, operational characteristics of residual current devices depend on earth fault current frequency, for a distorted waveform the most important is the order of harmonics [2–9]. Earth fault current in the output terminals of inverter comprises harmonics whose order depends on the PWM (Pulse Width Modulation) frequency. For the specified motor speed range, the PWM frequency component dominates in the earth fault current. The next paragraphs present analysis of the earth fault current distortion and impact of the PWM frequency on the residual current devices tripping current.
Earth fault current analysis
Spectral analysis of the earth fault current in circuits with variable speed drives was performed. Model of a circuit with variable speed drives was generated using TCad software [10, 11]. In the model voltage source inverter with PWM modulation and U/f = const is used. Computer simulation was verified under laboratory conditions. Both the computer simulation and the laboratory test gave similar results.
Fig. 2 presents computer simulation of the earth fault currents in a circuit with variable speed drives. This computer simulation was performed for 50 Hz motor supply voltage frequency (in this paper named “motor frequency”) and for the following PWM frequencies: 1 kHz, 3 kHz. The currents comprise harmonics, especially high-order harmonics. The spectrum of these currents is presented in Fig. 3.
Fig.1. Earth fault in motor terminals (indirect contact) and direct contact in circuit with variable speed drives. RCD – residual current device, FC – frequency converter, M – motor, IE– earth fault current, IT – touch current
Fig.2. Earth fault currents in the case of fault in the output terminals of frequency converter (TCad simulation). Motor frequency 50 Hz, PWM frequency: a) 1 kHz, b) 3 kHz
Fig.3. Harmonic spectrum of earth fault currents presented in: a) Fig. 2a, b) Fig. 2b. Horizontal axis – amperes, vertical axis – harmonic order
Fig.4. Earth fault current in the case of fault in the output terminals of the frequency converter. Motor frequency 1 Hz, PWM frequency 3 kHz: a) TCad simulation, b) harmonic spectrum, c) experimental verification – oscillogram from laboratory test
The order of harmonics corresponds with the applied PWM frequency. Apart from the PWM frequency component, current components appear which are a multiple of the PWM frequency. The lower the motor frequency, the higher the participation of PWM frequency component.
Fig. 4 presents, as an example, the earth fault current for very low motor frequency, equal to 1 Hz. This is the result of TCad simulation (Fig. 4a, Fig. 4b) and laboratory test (Fig. 4c). For a very low motor frequency, the PWM component significantly exceeds other components of the earth fault current – the dominating component is PWM component. In this waveform the PWM component becomes fundamental. Wide spectral analysis performed during laboratory test (Fig. 4c) indicates that the earth fault current also contains high level harmonics which are the multiple of PWM frequency.
PWM frequency versus tripping current
The effect of PWM frequency on the operational characteristics of residual current devices and effectiveness of protection against electric shock was tested under laboratory conditions. Numerous groups of residual current devices (over forty devices) were tested. The residual current devices under test were marked RCD1, RCD2, etc.
Taking into account spectral analysis of earth fault current, distorted currents were generated using programmable power supply [12]. It allows to avoid the impact of noises on the tripping current, which occur in real circuits with variable speed drives. The programmable power supply enables to generate waveforms comprising fundamental and one or several harmonics. The percentage of harmonic-to-fundamental value of individual harmonic and phase angle of individual harmonic may be specified precisely. It is possible to obtain current waveforms similar to earth fault currents in real circuit with variable speed drives. There were three types of test currents reflecting the earth fault current in circuit with variable speed drives. The first type of the current comprised harmonics which dominate in the earth fault current in the case of fault in the motor terminals for motor frequency equal to 50 Hz. Similarly, the second type of current for motor frequency equal to 25 Hz and the third type of current for motor frequency equal to 1 Hz.
As it was presented in [7], the tripping current for the same PWM frequency, reaches the highest value for the lowest motor speed. For this reason an extended test, for various PWM frequencies, was performed for motor frequency equal to 1 Hz (very low motor speed).
Fig. 5 and Fig. 6 present results of the extended test –tripping current of residual current devices: IΔn = 30 mA and IΔn = 300 mA, respectively. The tripping current was checked for motor frequency equal to 1 Hz and the following PWM frequencies: 500 Hz, 1000 Hz, 1500 Hz, 2000 Hz and 2500 Hz. Comparison of the tripping currents of various residual current devices allows to affirm that the higher the PWM frequency the higher the tripping threshold.
Apparently similar residual current devices in terms of the technical data may have significantly different operational characteristics. It is clearly visible comparing RCD6 to RCD10 (Fig. 5a) and RCD27 to RCD30 (Fig. 6a). RCD6 trips out for all the test currents but RCD10 does not trip out at all for the PWM frequency equal to or higher than 1000 Hz. Regardless of the PWM frequency, for very low motor speed the tripping current of the tested residual current devices either significantly exceeds the rated residual current IΔn or the residual current devices do not trip at all. Residual current device which has the tripping current many times higher than IΔn or does not trip at all can not ensure effectiveness of protection against electric shock.
Fig.5. Tripping current of residual current devices (IΔn = 30 mA) for various PWM frequencies and very low motor speed (motor frequency equal to 1 Hz). Residual current devices: a) RCD6, RCD10, RCD11 – type AC, b) RCD12, RCD18, RCD19 – type A
Negative effect of the earth fault current with high frequency components can be eliminated by using a solution proposed by the author. Structure of the new residual current device of IΔn = 300 mA was described in [13]. It allows to achieve steady tripping current for strong distorted earth fault current. Properties of this residual current device for strong distorted earth fault current are presented in Fig. 6c and compared with other residual current devices (Fig. 6a, Fig. 6b) of IΔn = 300 mA. The operational characteristics presented in Fig. 6a, Fig. 6b are typical of most residual current devices used in practice. Their tripping threshold rises with rising PWM frequency. The operational characteristic of the proposed residual current device presented in Fig. 6c indicates that tripping current does not exceed the rated operating residual current IΔn regardless of the PWM frequency. The tripping current is within 0,5IΔn ÷ 1,0IΔn as for the sinusoidal (50/60 Hz) earth fault current and is in accordance with the standard [14].
Such favourable operational characteristic for strong distorted earth fault current is a result of favourable operational characteristic for sinusoidal waveform within a wide frequency range. Fig. 7 presents comparison of the operational characteristics within the 50 Hz to 1000 Hz frequency range for the proposed RCD-P residual current device and the residual current device numbered RCD30 (as in Fig. 6a).
Fig.6. Tripping current of residual current devices (IΔn = 300 mA) for various PWM frequencies and very low motor speed (motor frequency equal to 1 Hz). Residual current devices: a) RCD27, RCD30 – type AC, b) RCD35, RCD38 – type A, c) RCD-P – the new solution
The proposed residual current device, contrary to the RCD30, has steady tripping current in the whole range. The new RCD-P residual current device is voltage independent, no auxiliary power is necessary for its operation. All power is derived from the earth (residual) current. It is an important advantage of this solution.
Fig.7. Tripping current of residual current devices (IΔn = 300 mA) within the 50 Hz to 1000 Hz frequency range: RCD30 – classical type AC RCD; RCD-P – the proposed RCD
Conclusions
The effectiveness of protection against electric shock in circuits with variable speed drives is difficult to evaluate in the case of direct or indirect contact. Disconnection of supply by residual current devices depends on the applied PWM frequency, present motor speed and real operational characteristics of the residual current device. For proper operation of residual current devices it is better to apply a relatively low PWM frequency. However, now there is a tendency to use high PWM frequency, so commonly used residual current devices are expected to have very high tripping current and in some cases may not trip at all. Special residual current devices, as proposed by the author, designed for strong distorted earth fault current with high-order harmonics should be used.
Author: dr hab. inż. Stanisław Czapp, Gdańsk University of Technology, Faculty of Electrical and Control Engineering, ul. Narutowicza 11/12, 80-233 Gdańsk, E-mail: s.czapp@ely.pg.gda.p
Source & Publisher Item Identifier: PRZEGLĄD ELEKTROTECHNICZNY (Electrical Review), ISSN 0033-2097, R. 87 NR 1/2011
Published by Yang HAN1, Lin XU2 University of Electronic Science and Technology of China (1), Shanghai Jiao Tong University (2)
Abstract. This paper presents the survey of the smart grid technologies, including the background, motivation and practical applications. The driving forces for the smart grid technologies are presented, including the blackout, global energy crisis and environmental protection requirement. The key technology issues for building the smart grid are discussed. The crucial elements of the smart grid and their applications are introduced, including the un-interruptible power supply (UPS), adaptive var compensator (AVC), static synchronous compensator (STATCOM), active power filter (APF), unified power quality conditioner (UPQC), micro-grid, solar and wind generation, and high voltage direct current (HVDC) transmission technologies.
Streszczenie. Artykuł prezentuje przegląd technologii smart grid”. Uwzględniono takie zjawiska jak blackout, globalny kryzys energetyczny i zalecenia ochrony środowiska. Omówiono podstawowe elementy sieci „smart grid” i jej zastosowania, uwzględniając systemy UPS, AVC, STATCOM, APF, UPQC, źródła słoneczne i wiatrowe oraz technologię transmisji napięcia stałego HVDC. (Przegląd technologii „smart grid” –tło, motywacje i praktyczne zastosowania)
The electric industry is poised to make transformation from a centralized, producer-controlled network to one that is less centralized and more consumer-interactive. The move to a smarter grid promises to change the industry’s entire business model and its relationship with all stakeholders, involving utilities, regulators, energy service providers, technology and automation vendors and all consumers of electric power [1-8].
As automated and distributed energy delivery network, the smart grid will be characterized by a two-way flow of electricity and information and will be capable of monitoring everything from power plants to customer preferences to the individual appliances. It incorporates into the grid the benefits of distributed computing and communications to deliver real-time information and enable the instantaneous balance of supply and demand at the device level.
A smart grid uses digital technology to improve reliability, security, and efficiency of the electric system. Due to the vast number of stakeholders and their various perspectives, there has been debate on a definition of a smart grid that addresses special emphasis desired by each participant. The following areas represent a reasonable partitioning of the electric system that covers the scope of smart grid concerns [9-13].
●Area, Regional and National Coordination Regimes A series of the interrelated, hierarchical coordination functions exists for the economic and reliable operation of the electric system, which includes independent system operators (ISOs), regional transmission operators (RTOs), electricity market operations, etc. Smart grid elements in this area includes measurement data to determine system state and health, and put forward coordinating actions to enhance efficiency, reliability, environmental compliance or response to network disturbances.
● Distributed Energy Resources (DERs) Technology This area includes the integration of distributed energy, storage, and demand-side resources for participation in the electric system operation. Consumer products such as the smart appliances and electric vehicles are expected to be important components of this area as are the renewable generation components such as those derived from solar and wind generation sources.
● Transmission and Distribution Infrastructure Smart-grid items at distribution-level include substation automation, dynamic limits, relay coordination and the associated sensing, communication and coordinated action. Distribution-level items include the feeder load balancing, capacitor switching, and the advanced metering, such as meter reading, remote service enabling and disabling and demand-response gateways.
●Information Networks and Finance It must be pointed out that the information technology and pervasive communications are cornerstones of smart grid. Though the information networks requirements, i.e., the capabilities and performance, will be different in different areas, their attributes tend to transcend application areas. The examples include interoperability and the ease of integration of the automation components as well as cyber security concerns. Moreover, the economic and investment environment for procuring smart grid is a vital part for the implementation progress.
The organization of this paper is as follows. Section II presents the driving forces for the smart grid technologies, such as the catastrophic blackout, energy crisis and global financial crisis, and the environment protection requirement. Section III presents the crucial elements of the smart grid and their applications, such as the un-interruptible power supply (UPS), adaptive Var compensators (AVC), static synchronous compensator (STATCOM), active power filter (APF), unified power quality conditioner (UPQC), microgrid, solar and wind generation, and high voltage direct current (HVDC) transmission technologies. Finally, Section IV concludes this paper.
II. The Driving Forces of the Smart Grid
The smart grid (SG) is the next generation intelligent electricity network which optimizes the energy efficiency to graft information technology onto the existing network and exchange real-time information between electric suppliers and customers.
Moreover, the smart grid is an integration of electrical and information infrastructures, and the incorporation of automation and information technologies with our existing electrical network. It provides comprehensive solutions that improve the utility’s power supply reliability, operational performance and overall productivity, deliver increases in energy efficiencies and decreases in carbon emissions, and empower consumers to manage their energy usage and save money without compromising their lifestyle. In addition, smart grid is also the solution that can optimize the renewable energy integration and enabling its broader penetration. To conclude, smart grid is the infrastructure that would deliver meaningful, measurable and sustainable benefits to the utility, the consumer, the economy and the environment.
To better understand the background and motivation of the smart grid technologies, the US blackout in 2003 is first briefly reviewed, followed by the introduction of low carbon emission target of various nations as well as the economic crisis and energy crisis.
Fig.1 The photo’s of the 2003 US blackout areas.(left: the photo taken on Aug 13, right: the photo taken on Aug 14)
Fig.1 shows the photo’s of 2003 US blackout stricken areas due to a cascaded power grid failure. The procedure of the catastrophic blackout is reviewed as follows:
● Time: August 14, 2003, at approximately 4:15 pm EDT. ● Affected 55 million people in eight US states, 1 province in Canada and 256 power plant went offline. ● 4:10:38 p.m. The Cleveland grid separates from the Pennsylvania grid; ● 4:10:46 p.m. New York grid separates from the New England Grid; ● 4:10:50 p.m. Ontario grid separates from the western New York Grid; ● 4:12:58 p.m. Northern New Jersey grid separates its power-grids from New York and the Philadelphia area; ● 4:13 p.m. End of cascading failure.
In total, nearly 85% of power plants which went offline after the grid separations occurred, due to the action of automatic protective control. The footprint of the blackout on both sides of the US-Canadian border includes large urban centers that are heavily industrialized and important financial centers (e.g., New York City and Toronto). Nearly half the Canadian economy is located in Ontario and was affected by the blackout. Service in the affected states and provinces was gradually restored with most areas fully restored within two days, but parts of Ontario experienced rolling blackouts for more than a week before full power was restored.
Fig.2 The transmission congestion of the US grid in 2002 (Source: US Department of Energy, National Transmission Grid study report)
Transmission congestion is one of the major problems of the modern electric networks throughout the world. Take the example of the US national grid for example, both the western and eastern interconnection networks suffer from significant transmission congestion problems. As indicated in Fig.2, thousands of miles of transmission networks in the western interconnection suffer from nearly 50% congestion, and the eastern interconnection networks also show heavy congestion in the middle and southeastern states in the year 2002.
Fig.3 The demonstration of the degradation of global environment
The ever-increasing global energy consumption causes severe damage to the environment. As the major producer of electricity, the fossil power plants produce almost one third of the carbon and sulfur pollution. Fig.3 shows a vivid illustration of the various pollution, which causes the global warming, acid rain and the melting of the polar ice caps. As a result, abnormal weather condition, unnatural ecology systems and desertification would be the unavoidable consequences. Hence, the new energy resources, such as the wind power generation, solar generation are excellent alternatives for the existing fossil power plants.
Fig.4 A comparison of the CO2 emission among China, United States, OECD and non-OECD countries (OECD: Organization for Economic Co-operation and Development).
Fig.4 shows a comparison of the CO2 emission among China, United States, the OECD and non-OECD countries. It shows that a few decades ago, China has the least CO2 emission, much less than the United States and the OECD countries. However, in recently years, China overtakes the United States and the rest of OECD countries in terms of CO2 emission, which is a threat for China’s environment and sustainable economic development. This is one of the major contributor for China’s strategic plan to develop the smart grid technologies to effectively reduce the emission and improve energy utilization efficiency.
Fig.5 shows the illustration of the CO2 reduction target of the United States for the year 2050.It shows that the US electric sector produces approximately one third of the total emission, reaching 2 billion tons. The total emission target in the coming decades shows a steady decline, and 83% reduction in the CO2 emission must be achieved compared to the emission in 2005. The ambitious plan of emission reduction is one of the major driving forces for developing the renewable energy sources, such as wind energy and solar energy, and the smart grid technologies become the most important issues for the electric industries.
Fig.5 The US CO2 emission reduction target for the year 2050
Fig.6 The driving force of the United States for the smart grid technologies. (US: Focus on businesses and infrastructure; EISA: Energy Independence and Security Act of 2007; ARRA: American recovery and reinvestment Act of 2009; NIST: National Institute for Standards and Technology).
Fig.6 shows the flowchart of the driving forces of the US for developing smart grid technologies. The wake-up calls were stimulated by the California energy crisis in 2000 and the blackout in the northeastern states in 2003. Hence, the EISA Act in 2007 decided to develop the smart grid in order to modernize the electricity network to improve the reliability and transmission efficiency. On the other hand, the global financial crisis originated from US stimulated the new economy, hence the US launched 4.3 billion dollars for smart grid technologies and 10 million dollars for the NIST to coordinate the smart grid standards. These factors are the major driving forces for the US smart grid boom.
Fig.7 The positive effect of the smart grid technologies.
Fig.7 shows the positive effect of developing smart grid technologies. The energy utilization efficiency is achieved by optimizing energy usage and extended asset utilization. The operational efficiency is improved by the increased operational productivity, reduced capital and operational costs and enhanced cyber security. Customer satisfaction is enhanced by the improved reliability metrics, additional services, and tighter communications with utility. The smart grid technology also has huge impact on the environment protection by the reduction of CO2 emissions.
III. The Vision and Technologies of the Smart Grid
Fig.8 shows the various focus areas that lead to various definition of smart grid, which can be summarized in the following aspects:
● Intelligent transmission and distribution automation; ● Distributed generation and storage; ● Advanced metering infrastructure; ● Demand response and load control
The intelligent transmission and distribution automation is the fundamental requirement of smart grid, which also includes reliability analysis, advanced monitoring facilities, energy management systems (EMSs) and demand side management systems (DMSs). The distributed generation and storage include the wind generation, solar generation, and micro-turbine and flywheel applications. The advanced metering system focuses on the communication networks, such as meter reading, remote sensing and control, home area network (HAN) and energy efficiency management. The demand response and load control focus on customer interaction with the smart grid.
Fig.8 Diverse focus areas lead to various definition of smart grid.
The present case of low energy efficiency for the US grid: 65.5% loss at the generation stage (coal-fired power plants efficiency = 60%), 5% auxiliary loss, 3.7%~4.8% loss at the transmission level, and 4.8%~5.1% loss at the distribution level. In total, approximately one third is delivered for the end user. Therefore, how to improve the energy efficiency, enhance the transmission capacity and reliability are the major concerns for the experts and engineers in the this area. The technological requirements for the smart grid can be summarized in the following aspects [2, 3, 7-10].
(1) Advanced control methods, such as: ● Real time and predictive control; ● Monitor and collect data from sensors; ● Determine and take action autonomously; ● Analyze data to diagnose and provide solutions; ● Provide information and solutions to the operators; ● Integrate with enterprise processes and technologies.
(2) Advanced components for the smart grid: ● Micro-grids; ● Fault current limiters (FCLs); ● Advanced switches and conductors; ● Next generation FACTS/PQ devices; ● Advanced distributed generation and energy storage; ● Superconducting cable & rotating machines, etc.;
(3) Improved interfaces and decision support: ● Visualization; ● Data reduction; ● Data to information; ● Speed of comprehension; ● System operator training, etc.
(4) Integrated communication for the smart grid: ● Micro-grids; ● Smart meters; ● Smart sensors; ● Markets feedback; ● Demand-side response; ● Distribution automation; ● Work-force management; ● Mobile premises (PHEV’s); ● Distributed generation (DG) dispatch, etc.
Fig.9 shows the vision and expectation of the smart grid. Notably, a large proportion of the electricity generated by conventional power plants will be displaced by distributed generation. Additional stand-by capacity might be required, which is called upon whenever the intermittent renewable resource ceases to generate power. Efficient integration of DG is unlikely to be made without changes to transmission and distribution network structure, planning and operating procedures. The key smart grid research and development areas can be summarized as follows [1-8, 16, 17]:
● Cyber security; ● Smart grid standards; ● FACTS/HVDC technologies; ● How the smart grid operates; ● Consumers respond to price signals; ● Communication architecture and technologies; ● Power electronics and advanced digital control, etc.
Fig.9 The vision and expectation of the smart grid.
Fig.10 shows the FACTS devices which can be utilized to adjust power flow of the transmission and distribution systems. The mechanical switched capacitors (MSCs) are used for the high voltage transmission systems to provide capacitive reactive power thus enhance the power factor. Moreover, the mechanical switched reactors (MSRs) are used to absorb the excessive capacitive power of the distribution network, normally installed at the 110kV and 35kV distribution systems to mitigate over phenomenon during light load conditions in the evening. The series capacitors (SCs) and the series reactors (SRs) are installed in series with the transmission line to adjust the effective electrical impedance of the network [9-13].
The phase shifting transformers (PSTs) are utilized at the distribution systems to modify the phase angles of the grid voltage, thus improve the harmonic properties. The static var compensators (SVCs) are a combination of thyristor controlled reactors and thyristor switched capacitors, which are widely used in distribution and transmission systems to dynamically modify the reactive power of the network, improve system stability and increase network efficiency. With the development of the power electronic devices, and with the advent of insulated gate bipolar transistors (ITBTs), the static synchronous compensators (STATCOMs) are developed, as the upgraded version of SVCs, which are widely utilized in the modern electric power networks and considered as one of the major building blocks of the smart grid. The D-STATCOM is abbreviated for the STATCOM used in the distribution system [16, 17, 20].
Fig.10 FACTS devices to improve system stability and reliability.
The thyristor controlled series capacitors (TCSCs) are the new generation FACTS controlled utilized in the high voltage transmission systems to enhance the transmission capacity, improve the transient stability of the generators and suppress the subsynchronous oscillations (SSR) of the HVDC transmission systems. The static synchronous series compensators (SSSCs) are installed in series with the transmission network, with the basic power electronic building blocks similar to the STATCOM systems. The dynamic voltage restorers (DVRs) are the customer power quality conditioner which are used to mitigate voltage sag and protect the sensitive load [9-13].
In the forthcoming subsections, the popular devices for smart grid applications are briefly reviewed, which includes the uninterruptible power supply (UPS), the adaptive/static var compensators (AVCs/SVCs), the static synchronous compensators (STATCOMs), active power filters (APFs) and the dynamic voltage restorers (DVRs). Next, the micro grid application is introduced, followed by the solar energy and wind power applications. Finally, the high voltage direct current (HVDC) technologies would be presented.
A. The Uninterruptable Power Supply (UPS)
Fig.11 shows the one line diagram of the uninterruptable power supply (UPS), which is fundamentally consisted of voltage source inverter (VSI) and the isolation transformer. During normal operation, the load is powered by the utility source through the closed power electronic switch (PES). The grid voltages are continuously monitored on all three phases. If a grid disturbance is observed by the disturbance monitor, which causes the voltage to sag or swell beyond 10% of its nominal value, it sends an “open” signal to the PES, thus coincident with a “run” signal to the inverter module(s). Hence the inverter module(s) provide regulated output to the critical load within 1ms. Load is transferred to the stored-energy source in 2~4ms, which is fast enough for computers and other sensitive electronic devices to ride through without malfunction [14, 15].
When the utility source voltage returns to its normal limits, the UPS synchronizes the output voltage with that of the utility source, and sends a “close” signal to the PES along with a “stop” signal to the inverter module(s). After a few seconds, the battery chargers turn on to restore the batteries to 100% capacity [15].
Fig.11 One line diagram of the uninterruptable power supply.
B. The Adaptive/Static Var Compensator (AVC/SVC)
Fig.12 Diagram of the adaptive/static Var Compensator (AVC/SVC)
Fig.13 Single-line diagram of the AVC/SVC application.
Fig.12 shows the diagram of the adaptive/static var compensator, which is an economical, distribution var compensator that provides dynamic reactive compensation and power-factor correction. By using the AVC systems, the problem of voltage fluctuations can be reduced, system stability and reliability can be enhanced, and the system capacity is increased, and light flicker is eliminated. The AVC can enable problem loads to coexist on the same feeder as more sensitive loads, eliminating the need for separate feeders [16, 17].
Fig.13 shows the single-line diagram of the AVC/SVC system. Normally, the AVC systems are installed in parallel with the dynamic load, which continuously monitors line-to-neutral voltage and current on each phase of the feeder serving the load. By measuring the inductive component of the current, the microprocessor-based controller of the AVC determines the needed capacitive compensation. The required reactive power is then injected into the system by closing the appropriate power electronic switches. By using an array of sophisticated algorithms, a response time close to 1⁄2 cycle can be achieved. Notably, the capacitors are always pre-charged and ready until the triggering signal is applied to the switches [16].
Besides, switching is synchronized to occur at peak of the grid voltage, i.e., the zero crossing of the capacitor current, when voltage across the power electronic switch is nearly zero. As a result, the transients associated with capacitor switching are eliminated. The micro-processor controller updates the capacitor switching patterns up to every half-cycle and sets the optimum firing sequences. Notably, the de-tuning reactors are inserted in series with the capacitors to eliminate undesirable system resonance.
C. The Static Synchronous Compensator (STATCOM) for the Distribution System (DSTATCOM)
Fig.14 Distribution static synchronous compensator (DSTATCOM).
Fig.15 Field tests of the DSTATCOM for flicker mitigation.
Fig.14 shows the circuit diagram of the DSTATCOM in a typical distribution network. The basic electronic block of a DSTATCOM is the voltage source converter (VSC), which in general converters an input dc voltage into three-phase output voltage at the fundamental frequency, with rapidly controllable amplitude and phase angle. In addition, the controller has a coupling transformer and a dc capacitor. The control system is designed to maintain the magnitude of the bus voltage constant by controlling the magnitude or phase shift of the VSC output voltage [18-20].
For the distribution system application, the STATCOM is controlled to inject or absorb reactive power to the grid, in order to support the dynamic load variations. From Fig.14, it can be observed that the currents supplied by the grid is pure sinusoidal waveform. Hence, the DSTATCOMs are widely utilized for dynamic compensation of the fluctuating loads, such as arc furnaces or other flicker producing loads. Fig.15 shows the typical experimental waveforms recorded in the field. It shows that the flicker effect in the dynamic load causes fluctuating grid voltages without compensation. However, when the DSTATCOM is used for compensation, the fluctuation in grid voltage is eliminated.
D. The Active Power Filter (APF)
The growing problems of power quality contamination also originated from the proliferation of nonlinear loads such as power converters in the distribution systems. For instance, voltage harmonics result from current harmonics produced by nonlinear loads, e.g., variable ac motor drives, arc furnaces and household appliances. These nonlinear devices result in a significant increase in the line losses, instability and voltage distortion, which corrupts the electric distribution systems. The active power filters (APFs) have been recognized as the most effective techniques for harmonic compensation. Their objective is to suppress the current currents and to correct power factor, especially in the fast-fluctuating nonlinear loads. In addition to their performances, APFs can favorably be widely used in the existing power systems and thus has a wide application. A lot of recent research work tries to improve the APFs by developing new topologies or control laws [21-26].
Fig.16 shows the circuit diagram of shunt active power filter, which has the similar power-stage configuration as the DSTATCOM. However, by exploiting a sophisticated control algorithm to the power electronic switches, the shunt APF is capable to generate the nonlinear current to cancel the load harmonics, hence making the grid currents free of harmonics. On the other hand, the series APFs are used for compensating voltage source type harmonics, as shown in Fig.17. In the series APF, each output phase leg is connected to the grid by series connection of coupling transformer, which serves the purpose of isolation and turn ratio adjustment between the inverter and the grid.
Fig.16 The circuit diagram of the shunt active power filter.
Fig.17 The circuit diagram of the series active power filter.
E. The Dynamic Voltage Restorer (DVR)
Fig.18 The circuit diagram of the dynamic voltage restorer (DVR).
The modern industrial plant is subjected to abnormal shutdown or malfunction due to the voltage sag problems. The dynamic voltage restorer (DVR) is considered as the best choice to protect the industrial facilities from voltage sag and other other voltage disturbances. Fig.18 shows the typical circuit diagram of the DVR in a distribution system. It demonstrates that voltage sag may be incurred by the fault from the adjacent feeder or the fault from the transmission network. Therefore, the DVRs can be applied to protect the sensitive loads of high-tech industries with adjustable speed drives and other power electronic based loads. For the industries with a high penetration of the induction motors, the energy storage might be used and a sophisticated controller must be adopted due to the inherit inertia of induction motors and their capability to withstand short duration, shallow sags and phase jumps [27,28].
F. The Unified Power Quality Conditioner (UPQC)
Fig.19 The circuit diagram of the unified power quality conditioner.
Fig.19 shows the circuit diagram of the unified power quality conditioner (UPQC), which is composed of shunt compensator and series compensator. The UPQC can be used for the simultaneous compensation of the currents and voltages, which provides a comprehensive solution for the harmonic and sag sensitive loads. The series APF is used for harmonic isolation between the load and the grid, which has the capability of voltage flicker and unbalance compensation as well as voltage regulation and harmonic compensation at the utility consumer side. The shunt APF is used to absorb current currents, compensate reactive power and negative-sequence current, and regulate the dc link voltage between two voltage source converters [29].
Fig.20 The application issues of the power quality conditioners.
Fig.20 shows the application areas of the power quality conditioners, such as STATCOM, APF, DVR and UPQC. These devices can be used for the dynamic ‘shock’ loads with rapid varying active and reactive power requirement. For instance, the heavy industries such as the automotive, mining systems, rolling mills, steel mills, arc furnaces, the pumping stations, the irrigation systems, large motor loads, cranes, hoists, ski lifts, roller coasters, resistive welding, and the metal fabrication industries, forestry pulps, paper processing industries, etc. These power quality conditions are the fundamental devices for the premium power quality and improving energy efficiency [30-39].
G. The Micro-Grid Applications
Fig.21 The configuration of the smart city with four houses.
Fig.22 The circuit diagram of the micro-grid.
Fig.21 shows the configuration of the smart city with four houses, where the energy storage systems are indicated. It shows that fuel cell and battery are designed to share the common dc bus, which is charged by using the 50kVA inverter with bi-directional power flow capabilities. During normal conditions, the four customers are supplied from the utility power system. In case of grid fault, the controller sends the islanding signal to the isolating device, and grid is isolated and the fuel cell or battery provide the energy to the four customers by the dc/ac inverter. At the same time, the thermal storage system is capable to provide heat to each house. In the smart city scenario, the major technique consideration includes the grid synchronization, grid fault detection, islanding detection and remote control, etc. [40].
Fig.22 shows the circuit diagram of the micro-grid, which is similar to the configuration of the smart city. Notably, the distributed generation (DG) systems are indicated and their circuit diagrams are denoted, which include the solar energy, wind power generation, etc. The micro-grid can work in the grid-connection manner or islanding mode. In case of grid fault or abnormal conditions, the micro-grid works in the islanding mode thus the DG supplies the electricity to the load from the same feeder. In case of grid connected mode, bidirectional energy flow is achieved hence the abundant energy generated by the DGs can flow into the grid.
H. The Solar Energy Applications
The photovoltaic (PV) system is appreciated for its ease of fabrication and declining cost in recent decades, which is most of the major driving forces for renewable energy. A lot of industrialized countries have put forward ambitious plan for developing solar energy as alternative for the fossil power generators, such as the US and Japan. Fig.23 shows the demonstration of Japan’s solar energy roadmap toward the year 2020. It shows that the total capacity of solar energy was only 1.4M kW in 2005, and a ten times increase of capacity is targeted in 2015 and the capacity would reach up to 28 M kW in 2020 based on the current plan. It is suggested that 5.3 million houses would install the solar panels in 2020 [41-45].
Fig.23 A demonstration of Japan’s solar energy roadmap.
However, the fast development of solar panels also face significant technique barriers owing to large unstable solar power connected to the grid. For instance, the solar panels face problems of deviation from voltage range by voltage rise of distribution grid, and the ac grid frequency regulation. Besides, the energy storage devices due to the excessive power generation is also crucial for the practical systems. The anti-islanding control and fault ride through abilities are also critical for high reliability applications.
Fig.24 The equivalent of PV array and its V-I characteristics.
Fig.24 shows the equivalent of the PV array and its V-I characteristics. Normally, the PV panel is modelled using a current source in parallel with a diode, the output current of the PV panel IPV is affected by the solar intensity. To maximize the efficiency of the PV panels, the maximum point point tracking (MPPT) algorithm must be adopted at the output stage of the PV panel, using a dc-dc converter.
Fig.25 shows the configuration of the PV panels with the electric distribution network. Generally, there are three types of circuit topologies, as illustrated in Fig.25. The first one is the conventional PV system based on the central converter ranging from 1~5 kW power rating. And the PV panels are connected in series to synthesize the output dc voltage, and the dc/ac inverter is adopted to connect the PV systems with the network. The string PV systems are based on the modular converter ranging from 0.5~1 kW to connect a string of PV panels to the network. Another type of PV system is named the integrated PV systems, which is based on the individual dc/ac converter integrated with each PV panel, which provides flexible solution and higher reliability with tradeoff of the increased cost.
Fig.25 The configuration of PV arrays with the electric network.
I. The Wind Power Generation
The increasing concerns over environmental issues and the depletion of fossil fuel demanded the search for more sustainable electrical sources. The wind power generation is the most effective solution that converts the energy contained by the wind into electricity. The wind is a vast and mainstream energy source and an important player in the world’s energy markets, with the 2008 market for wind turbine installation worth about 36.5 billion euros [46, 47].
The majority of current turbine models make best use of the constant variations in the wind by changing the angles of the blades through ‘pitch control’, by turning or ‘yawing’ the entire rotor as wind direction shifts and by operating at variable speed, which enables the turbines to adapt to varying wind speeds and increases its ability to harmonize with the operation of the electricity network [48-53].
Fig.26 shows the circuit diagrams of the wind generation systems, which can be classified as the induction generator type and the synchronous generator type. Fig.26(a) shows the induction generator with reactive compensation. The gear box is controlled to regulate the active power output. Fig.26(b) shows the scheme of induction generator with both reactive compensation and the resistance control. Fig.26(c) shows the scheme of the synchronous generator based wind generation system by using ac/dc and dc/ac inverter structures. Fig.26(d) shows the circuit diagram of the induction or permanent magnetic (PM) synchronous generator. Fig.26(e) shows the circuit diagram of the wind generation system using double-fed induction generator.
Modern wind technology is able to operate effectively at a wide range of sites, with low and high wind speeds, in the desert and in the freezing arctic climates. Clusters of turbines collected into wind farm operate with high speed, are generally well integrated with the environment. The main design drivers for the current wind technology are:
● Reliability; ● Grid compatibility; ● Offshore expansion; ● High productivity for low wind speeds; ● Acoustic performance (noise reduction); ● Maximum efficiency and aerodynamic performance.
Fig.26 The classification of wind generation systems.
Fig.27 Supercapacitor energy storage for wind energy applications
Fig.27 shows the circuit diagram of the supercapacitor energy storage for wind power applications. Notably, there is an increasing interest in both large scale storage system at transmission level, and smaller scale dedicated storage embedded in distribution networks. For large-scale storage, pumped hydro accumulation storage is best-known, which can also be implemented underground. On a decentralized scale storage operations include flywheels, batteries, and possibly in combination with the electric vehicles, fuel cells, the electrolysis and super capacitors [48, 49, 50].
Fig.28 The circuit diagram of the MV wind generator using tri-level converter and permanent magnetic (PM) generator.
Fig.28 shows the circuit diagram of the medium voltage (MV) wind generator suing tri-level inverter and permanent magnetic (PM) generator. The tri-level inverter topology is widely used in medium voltage high power applications. The power rating of the wind turbine is normally 3~5 MW and the tri-level inverter has the minimum components requirement for the power electronic switches hence result in high reliability. Moreover, the IGCTs are normally used to meet the requirement of high current and efficiency
Fig.29 The configurations of the wind generators with the network.
Fig.29 shows the circuit configuration of the wind power generators with the electric distribution network. In the first case, the wind farm is integrated with a shunt DSTATCOM for dynamic reactive power compensation. In the second case, the wind farm is made of multiple wind generators with a common ac grid, followed by the ac/dc and dc/ac converters to connect the wind farm to the grid. The third case denotes that the output of the individual wind power generator is connected to the grid through modular ac/dc converter, and a large capacity dc/ac converter is applied to convert the dc-link voltage into ac voltage [51, 52].
The technical requirements within grid codes vary from system to system, but the requirements for the generators normally concern tolerance, control of active and reactive power, the protective device and power quality. Specific requirements for the wind power generators are changing as penetration increases and as wind power is assuming more and more plant capabilities, i.e., assuming active power control and delivering grid support services [53].
In response to the increasing demands from the network operators, for example to stay connected to the grid during a fault event, the most recent wind turbine designs have been substantially improved. The majority of the MW-size turbines being installed today are capable of meeting the most severe grid code requirements, with advanced features including the fault ride-through capabilities. This enables them to assist in keeping the power system stable when disruption occur. In the past, the common practice was to disconnect the wind turbine generator unit during network disturbances. However, disconnection from the grid may deteriorate a critical grid situation and threaten the network stability and security with a high penetration of wind generators.
Furthermore, the fluctuating nature of wind arises issue of power quality such as flicker, voltage fluctuation, etc. At present, the variable voltage variable frequency converters are utilized for the wind generators. However, it introduces the problems of harmonics into the network and there is also high possibility of resonance effect due to reactance of wind turbine generator system electrical unit. Hence, most grid code will request wind power plant to maintain voltage fluctuations, flickers and harmonic currents in the desired range.
For secure grid operation, the frequency of the power system should be maintained to its rated value. However, in case of power imbalance between supply and demand, the undesirable frequency deviation occurs. The frequency control is a requirement for generating units to be able to increase or decrease output power with the falling or rising frequency. Besides, the wind power generator should be capable of automatically regulating its terminal voltage according to the given set point.
Fig.30 The schemes of HVDC transmission system.
J. The HVDC Technologies
The high voltage direct current (HVDC) transmission is widely recognized as being advantageous for long distance, bulk power delivery, asynchronous interconnections and long submarine cable crossings. HVDC lines and cables are less expensive and have lower losses than those for three-phase ac transmission. Owing to their controllability, the HVDC links offer firm capacity without limitation due to network congestion or loop flow on parallel paths. Hence, higher power transfer is possible without distance limitation.
Fig.30 shows the illustration of the HVDC transmission system. Fig.30(a) shows the scheme of the back to back connection, which is utilized for frequency changing, or synchronous connection. Fig.30(b) shows the scheme of the point-to-point overhead line for bulk transmission and overland construction. Fig.30(c) shows the scheme of the point-to-point submarine cable transmission for bulk power transmission [54, 55].
For the underground or submarine cable systems, there is considerable savings in the installed cable costs and cost of loss with HVDC transmission. Depending on the power level to be transmitted, these savings can offset the high converter station costs at distance over 40km. Besides, there is a rapid drop-off in the cable capacity with ac transmission over distance due to the reactive component of charging current. Although it can be compensated by the shunt compensators for the conventional schemes, for the underground cables, it is not practical for submarine cables. For a given cable conductor area, the line loss with HVDC cables can be less than half those of ac cables due to the skin effect and induced currents in the sheath and armor.
Fig.31 The circuit diagrams of monopolar/bipolar HVDC systems.
Fig.31 shows the circuit diagram of monopolar/bipolar HVDC systems. For very long distances and in particular for very long sea cable transmissions, a return path with ground/sea electrodes will be the most feasible solution, as shown in Fig.31(a). In many cases, existing infrastructure or environmental constraints prevent the use of electrodes. In such cases, a metallic return path is used in spite of the increased cost and power losses, as depicted in Fig.31(b).
A bipolar scheme is a combination of two poles in such a way that a common low voltage return path, if available, will only carry a small unbalance current during the normal operation conditions. During the maintenance or outages of one pole, it is possible to transmit part of the power. More than 50% of the transmission capacity can be utilized, limited by the actual overload capacity of the remaining pole. The advantages of a bipolar solution over a solution with two monopoles are reduced cost due to one common or no return path and lower losses [54, 55].
As shown in Fig.31(c), this bipolar configuration provides a high degree of flexibility with respect to operation with reduced capacity during contingencies or maintenance. Upon a single-pole fault, the current of the sound pole will be taken over by the ground return path and the faulty pole will be isolated. Following a pole outage caused by the converter, the current can be commutated from the ground return path into a metallic return path provided by the HVDC conductor of the faulty pole.
Fig.32 Perspectives of grid development in China-The AC and DC bulk power transmission from West to East via three corridors.
Fig.32 depicts the perspectives of the grid development in China using the AC and DC bulk power transmission from the western regions to the eastern economy centers. The focus is on the inter-connection of 7 large provincial grids of the northern, central and southern systems via three bulk corridors which built up a redundant ‘backbone’ for the whole grid. The north corridor is aimed to send bulk power from the fossil power plants in the inner-Mongolia and northern provinces to the capital. The central corridor is aimed to send bulk power from the three George’s hydro power plant to Shanghai. The south corridor is aimed to send the bulk power from the southwestern provinces to Guangdong and Hong Kong. Each corridor is planned for a sum of a bout 20 GW transmission capacity which shall be realized with both AC and DC transmission line with ratings of 4~10 GW each (at +/-800kV DC and 1000 kV AC). With these ideas, China investigated a total amount 900 GW installed generation capacity by 2020. The benefits of such a large hybrid power system interconnection are:
● Increase of transmission distance; ● Sharing of loads and reserve capacity; ● Flexible renewable energy integration; ● Reduction of power losses using ultra-high voltage; ● Utilization of cheap resources far from load center; ● Serves as stability booster and firewall against blackout
IV. Conclusions
This paper presents a critical survey of the smart grid technologies, including the background, motivation and the technique issues. Driven by the energy crisis and financial crisis, the smart grid provides the best solutions to improve the grid efficiency, reliability, flexibility and also provides interactive activities for consumers. The popular devices of smart grid and their application issues are reported, such as the UPS, AVC, DSTATCOM, APF, UPQC, micro-grid, the solar and the wind generation as well as the HVDC transmission systems. This paper can be used as useful reference for the engineers in the smart grid research and implementation field.
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Khan, Chen C., Pan J., Desynchronized processing technique for harmonic and interharmonic analysis based on cosine window interpolation, Int. Review of Electr. Eng., 4(2009), no.5, 943-956. [37]Biricik S., Ozerdem O., Investigation of switched capacitors effect on harmonic distortion levels and performance analysis with active power filter, Przeglad Elektrot., 85(2010), n.11a, 13-17. [38]Xu L., Han Y., Yao G., Zhou L., M. M.Khan, Chen C., Pan J.,Perfect harmonic cancellation strategy for three-phase four wire APF, Przeglad Elektrot., 85(2010), n.10, 65-70. [39]Xu L., Han Y., Chen C., Pan J., Yao G., Zhou L., M. M. Khan, Implementation of the PWM gating and IGBT protection scheme for the grid connected multilevel inverter applications, Przeglad Elektrot., 85(2010), n.7, 360-365. [40]Akagi H., Aredes M., Monteiro L., Afonso J., Pinto J., Watanabe E., Instantaneous p-q power theory for control of compensators in micro-grids, Przeglad Elektrot., 85(2010), n.6, 1-10. 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[47]Halinka A., Szablicki T., Possible ways of connecting of wind farms to power distribution grid 110kV and distance protection act for symmetrical faults, Przeglad Elektrot., 85(2010), n.8, 50-56. [48]Lubosny Z., Preparation wind farms influence on power stability, Przeglad Elektrot., 85(2010), n.8, 66-69. [49]Hradilek Z., Sumbera T., Simulator of power forecasting gained from wind power plants, Przeglad Elektrot., 85(2010), n.8, 196-199. [50]Cieslik S., Connection of 8th MW wind farm to MV switching station in HV/MV substation in distribution network, Przeglad Elektrot., 85(2010), n.6, 104-109. [51]Tomczewski A., The use of kinetic power storages with a view to improving the conditions of cooperation of a wind turbine and an electric power system, Przeglad Elektrot., 85(2010), n.6, 224-227. [52]Wlas M., Krzeminski Z., Szewczyk J., Pietryka J., The control system of the small wind turbine with induction generator, Przeglad Elektrot., 85(2010), n.2, 71-76. 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Authors: Dr. Yang Han is with the Department of Power Electronics, School of Mechatronics Engineering, University of Electronic Science and Technology of China (UESTC), No.2006 XiYuan Road, West Park of Chengdu Hi-Tech Zone, 611731, Chengdu, P.R.China, E-mail: hanyang_facts@hotmail.com; Dr. Lin Xu is with the department of electrical engineering, Shanghai JiaoTong University, #800 DongChuan Road, Shanghai, P.R.China.
Source & Publisher Item Identifier: PRZEGLĄD ELEKTROTECHNICZNY (Electrical Review), ISSN 0033-2097, R. 87 NR 6/2011
Published by Sohel UDDIN1, Hussain SHAREEF1, Azah MOHAMED1, M A HANNAN1 Dept. of Electrical, Electronic & Systems Engineering (1), Universiti Kebangsaan Malaysia (1)
Abstract. The purpose of this paper is to investigate harmonic generation from dimmable Light Emitting Diode lamps (LEDs) which are used in residential and commercial applications as an energy efficient lighting systems. It is done by conducting laboratory tests on various LED lamps and tapping the load current behavior under different conditions. Then the frequency domain analysis is performed to investigate the generated harmonics. Harmonic levels of different wattage, various branded dimmable and non dimmable LED bulbs along with dimmable compact fluorescent lamps are experimentally evaluated and compared. Experimental result shows that, all LED lamps generate very high level of harmonic during dimming operation which may affect the power quality of AC mains.
Streszczenie. W artykule opisano zagadnienie emisji harmonicznych przez ściemnianą diodę LED, wykorzystywaną do oświetlania pomieszczeń. Przeprowadzono próby laboratoryjne w różnych warunkach pracy, a na ich podstawie analizy generowanych harmonicznych. Pod tym kątem dokonano porównania działania badanych diod z innymi energooszczędnymi rozwiązaniami (diody LED nieściemniane, świetlówki). (Analiza generacji harmonicznych przez ściemniane lampy z diodami LED).
It is estimated that lighting accounts 20% of the electricity demand globally. Incandescent lamp has been the main source of lighting industry over 100 years. However, incandescent lamp produces insufficient lumens and generates high heat. Therefore to promote energy saving, many countries already banned energy inefficient incandescent light bulbs and replace it with other lighting technologies like light emitting diode (LED) lamps and compact fluorescent lamp (CFL) technology [1-2]. With technological advancement in semiconductors, LEDs are evolving in lighting industry because of their special features like power saving, environmental friendliness, dimmable and multi color features of solid state lighting system. However, a single LED is not sufficient to emit light like incandescent bulbs due to point source nature of LEDs and current concentration. Therefore a multi LED system is introduced where several LEDs are connected in series, parallel or series-parallel combinations [1] to produce dispersed light like in conventional bulbs.
In addition, dimming control is required to regulate lighting levels for human needs as well as reduction of electricity demand, visual comfort and better productivity at work place [2]. Besides, for architectural lighting systems, dimming is essential to fulfill the aesthetic requirements of a space. An analysis done by the lighting research centre shows that 6% of energy can be saved by individually controlling manual dimmers [2]. Rand et al. also reported that, daylight harvesting and light dimming can save around 30-40% of energy [3] by using traditional dimmable lighting source like incandescent lamps, fluorescent lamps. But rapid development of semiconductor technology, LED is showing promising dimmable characteristic. Mainly in all domestic LED light dimming systems, phase-cut (triac dimmer) control technique is used in which the current is switch on only for a certain period of the line cycle. In most schemes of phase control dimmer, amplitude modulation (AM) or pulse width modulation (PWM) are used [4-5]. In AM method, reduction of current can cause degradation of light illumination. On the other hand, PWM allow control in light output by changing duty cycle. However, a PWM controller connected in series with each LED string can increase circuit complexity and reduces life time of LED lamps [6]. Infect, due to fast response of LEDs and their drivers, most of the LED lamps cannot perform properly with the Triac dimmer [3]. To overcome this drawback many researchers design special driver which are compatible with Triac dimmer [7-10]. In the work of Lianghui, a primary side control single stage flyback converter with a dimmer is proposed [7]. The author realized the input voltage feedback with phase angle in primary side and hence there is no need of secondary side feedback current and the circuit become simple and increases the reliability. However, due to current chopping in dimmable ballasts, they may create harmonic distortion on the feeders. The deviation of waveform from perfect sinusoid is usually expressed in terms of harmonic distortion of the current and voltage waveforms. Normally, LED lamps creates harmonic. In addition with dimmer function, this harmonic may increase drastically because current drawn by these lamps has more deviation from sinusoidal wave shape. In the field of LED lamp research, a few contributions focus on harmonic emissions of conventional LEDs lamps [11-12]. But almost nothing is done about harmonic from dimmable LED lamps. In spite some contribution of harmonic is done with dimmable CFLs [13].
This paper presents some analysis on harmonic generation from dimmable LED lamps. This is characterized by measurement tests, using various available dimmable LED bulbs. In the investigations, laboratory tests are conducted for this purpose with 3 Watt and10 Watt LED lamps with dimming function from different manufactures. All tests are carried out to observe their current and voltage waveforms and analyze them in terms of power rating, and brands. The test results are also compared with IEC 61000-3-2 harmonic standard and harmonics from dimmable CFLs.
Basic operation of LED lamps and its harmonic standards
The principle operation behind LED bulbs and the harmonic emission limits for LEDs as defined by IEC 61000-3-2 are discussed in this section.
Operating Principal of LED Lamps
LEDs require a constant current source from a low DC voltage source, obtained from the AC mains. Therefore, it is necessary to use a converter to regulate the voltage and control the current applied to the LEDs. The buck, boost, flyback and resonant converters are well known in literature as a power source to the LEDs [14-15].
Fig. 1 depicts a block diagram of typical low-wattage LED ballast with dimming control. It includes the AC line input voltage, typically 220-240 VAC 50/60 Hz, an EMI filter to block circuit-generated switching noise, a dimmable control circuit, a rectifier with smoothing capacitor, a PWM controlled constant current source converter for DC to DC conversion and an array of LEDs. Moreover, the input current can be changed by the dimmer circuit to vary light output. Since the rated load powers are low in LED lamps, the directives governing the injection of harmonics are not particularly strict [16] and therefore power factor control circuits may or may not be found in low-wattage LED lamp ballasts. However, to reduce the generated harmonics and to improve the power factor it is possible to introduce either an input passive filter, valley filled circuits, IC controlled active filtering configurations.
Fig.1. Block diagram of LED ballast with dimmer
Harmonic Injection Limits for LED Lamps
Similar to any other appliance, LED lamps also must comply with several directives which are applicable to the product. The IEC 61000-3-2 standard assesses and sets the limit for equipment that draws input current ≤16A per phase [17-18]. Harmonic emission limits for lamps are subdivided based on their active power up to 25W and above in class C. Lamps having an active input power less than or equal to 25W must satisfy at least one out of the two following criterions. One of the criteria is that the third harmonic current should not exceed 86% of the fundamental and the fifth harmonic current should not exceed 61%. That gives the value of the current THD approximately 105%. The recommended voltage distortion limit for class C equipment is 3% and 5% for individual harmonics and total harmonic distortion (THDV) respectively.
The other criterion is given as a Table 1 for each harmonic order.
Table 1. IEC 61000-3-2 limits for class C equipment (P ≤ 25W)
.
Table 2. Technical data for tested LED lamps
.
Methodology
To analyze the characteristics of the LED lamps with dimming function, 5 samples of with different power ratings from various manufacturers as shown in Table 2 were tested. The lamps have build in ballast which is powered using E-27, E-14 or GU-10 type sockets, commonly available in retail stores. All the tested lamps are designed to operate at 220-240 V and have power consumptions rating of 3 W to 10 W.
Fig.2. Experimental setup
To obtain accurate data concerning the exact current harmonic content of LED bulbs, an experimental setup as shown in Fig. 2 is assembled. It consists of four components namely, Fluke 434 power quality analyzer, Fluke i30s current clamp, LED bulb(s) under test, and a personal computer to analyze the signals. Each lamp is kept switched on for 10 minutes before the measurements are taken for stabilization. Each lamp is tested for four times to eliminate any error during different period of the day. Furthermore, for comparisons purposes, a sample of dimmable CFLs indicated in Table 2 are also tested using the same procedure. The captured current waveforms were analyzed by using Fluke 434 power quality analyser and MATLAB software where the current waveforms of the lamps were transformed using the Fourier Theorem. It provide frequency spectrum of the lamp currents represented by the fundamental sinusoidal component and a series of higher order harmonic components at frequencies that are integer multiples of the fundamental frequency as in (1).
.
Where I(t) is the input current, In is the harmonic current component of order n. Io is the average current. Furthermore, the square roots of the sum of the amplitudes of the harmonic as in (2) are used to represent the total harmonic distortion (THD).
.
Where I1 is the rms (Route mean square) value of fundamental current and In is the harmonic current component of order n.
Experimental analysis
In this section, measurements of various dimmable LED lamp test were assessed to investigate the harmonic generation when the brightness of the lamps are varied using a Triac dimmer controller commonly used in indo lighting controls. For this findings from the tested lamps at dimming and non dimming mode are analyzed and discussed first. Then a performance comparison of different dimmable LED lamps from various manufactures is conducted. Furthermore, a comparison of dimmable LED and CFL lamps carried out.
General findings from dimmable LED lamps
In order to understand the harmonic patterns of dimmable LED lamps, we consider an Osram 10 W dimmable bulb. The current and voltage wave shape is shown in Fig. 3(a) when it is operated at 0° firing angle of the Triac dimmer representing full brightness of the lamp. From the figure it can be noted that the current waveform is not sinusoidal even at full brightness where the dimmer is not yet activated. It means that this bulb creates and inject harmonic into the power system. However, it is clear from Fig. 3(a), that the voltage wave shape is pure sinusoidal. Therefore only current is distorted. To further investigate, the corresponding harmonic spectrum at 0° firing angle or at non dimming mode Fig. 3(b) plotted. It is noticed that the magnitude of harmonic current decreases with increased harmonic order.
Fig.3. Test results of Osram 10 W dimmable LED lamps at full brightness: (a) Lamp current and voltage waveforms (b) Individual harmonic spectrum
In order to observe the effect of reducing brightness on current harmonics, the dimming angle is increased from 0° to 45°, 90° and 135° respectively. As shown in Fig. 4, it is found that increasing the firing angle of the dimmer, the current drawn by the lamp is more chopped and deviates further from sinusoidal pattern although the magnitude of the current decreases. As a result, harmonic level is increased as depicted in Fig. 5. As seen in Fig. 5, this lamp creates a THDI value of 65%-70% at 0° firing angle (full brightness) whereas it becomes 76%-80% and 230%-235% at 45°and 90° respectively. This increase in THDI may be due to dimming control switch which contribute some additional harmonics.
Findings from Same Wattage LED Lamps
These tests aim to identify the harmonic levels from same wattage lamps introduced by different manufacturers. For this purpose 3 Watt bulbs were investigated. Fig. 6 depicts the wave forms obtained from 3 Watt LED lamps from Philips and Aira brand with 0° delay angles.
From Fig. 6, it is clear that the current wave shape is totally different from Osram 10 Watt bulb because different manufacturers used different type of ballast circuit inside the bulb. The harmonic patterns at various dimming levels of these lamps are shown in Tables 3 and Table 4. For the case of Phillips 3 Watt lamp, it is observed that there is a very large variation of harmonic between 0° and 135°. These harmonic levels are not acceptable for IEC 61000-3-2 standard.
Fig.4. Tested current waveform of Osram 10 W at different dimming mode
Fig.5. Harmonic spectrum of Osram 10 W at different dimming mode
Fig.6. Current and voltage waveform of Philips 3 Watt and Aira 3 Watt lamps at 0° dimming angle
Table 3. Harmonic Content of Philips 3 W with Several Dimming Mode
.
However in the case of Aira brand 3 Watt dimmable lamp, it shows a different characteristic in which harmonic level decrease with decreasing brightness as shown in Table 4. This may be due to the rectangular shape characteristics of the current wave it maintain during the operation. From Fig. 6 it is also clear that the current peaks observed for the case of Aira lamp is much lower than that required by Philips lamp. These high peaks introduce more harmonics into the system. Fig. 7 ill starred current characteristic and harmonic spectrum of those same lamps as discussed in fig. 6 but 45° delay angles.
Table 4. Harmonic Content of Aira 3 W with Several Dimming Mode
.
Fig.7. Comparison of (a) Current waveform, (b) Individual harmonic spectrum of Philips 3 W and Aira 3 Watt lamp at 45° dimming angle
Findings from Same wattage dimmable and non dimmable LED Lamps
The third test investigates the effect of harmonic characteristics of dimmable LED lamps with conventional LED bulbs having same power ratings. For this purpose, 10 Watt normal LED lamp and 10 Watt dimmable LED lamp from Osram as mentioned in Table 2 is compared. Fig. 8 shows the current waveforms along with their harmonic levels for these two lamps at full brightness. From the figure it can be reviled that the distortion level of dimmable LEDs is lower than normal LED lamp at full brightness. However, in case of dimmable bulb, the distortion levels increases rapidly with reduction of brightness. As a result, dimmable lamp at lower brightness is more problematic than conventional LED bulbs.
Fig.8. Test results of Osram 10 W conventional LED lamp with Osram 10 W dimmable LED lamps at full brightness: (a) Lamp current and voltage waveforms (b) Individual harmonic spectrum
Comparison with Dimmable CFLs
Since CFLs are the most commonly used energy efficient lamps today, it is important to compare the performance of new dimmable LED lamps with dimmable CFLs in terms of harmonic generation. For this purpose, Osram brand 20 Watt LED dimmable lamps are compared with 20 Watt dimmable CFLs from same manufacturer. Currently, there is no 20 Watt dimmable LED bulb available in the market so two 10 Watt bulbs of same model is used in parallel for this purpose. This 20 Watt combination gives the same characteristics of 10 Watt LED bulbs. In fact, 20 Watt combination gives a little less harmonic than 10 Watt LED bulb alone.
Fig.9. Current waveform of 10 cycles at 0° dimming angle (a) Osram 20 W CFLs (b) Osram 20 W LED Lamps
Table 5. Harmonic Content of Osram 20 W (Leds and Cfls) Lamps with Several Dimming Mode
.
Fig. 9 depicts the experimental result of current characteristic for Osram 20 Watt dimmable LED and Osram 20 Watt dimmable CFL lamps for 10 cycles at 0° delay angle. From the figure it is clear that the current wave shapes are totally different due to different ballast circuit and the current peak of CFL is almost double to that of LED lamp. A side from current peaks, it is understood from Table 5 that CFL lamp performs better at low brightness but at full brightness LED creates less harmonic.
Conclusion
This paper has presented several experimental results on harmonic generation from dimmable LED lamps that are currently being used for domestic and commercial lighting. In the experiments various types of dimmable LED lamps from different manufactures were tested to evaluate their harmonic performance in terms of power rating, brand, type of ballast used. Furthermore a comparison of harmonic contents of LED lamps and CFLs were also made at dimming mode. Also a comparison of dimmable LED lamps with normal LED lamps in term of harmonic was discussed. Experimental results show that both types of LEDs produce harmonics and increase the value of current total harmonic distortion (THDI) due to the use of power electronic converter as a ballast to drive LED arrays in the bulbs. The value of THDI ranges between 47 % and 360 % for dimmable LEDs bulbs. Moreover, normal LED lamps generate lower harmonic than dimmable ones. Dimmable CFLs also shows similar characteristic like LED counterparts. It is also noted that harmonic characteristics of LED lamps and CFLs of equivalent wattage either at dimmable or non dimmable mode from same vendor depend on the type of ballast used. It is also noted that different manufactures of LED lamps use diverse ballast technologies. Currently there is no standard for dimmable lamps and it is recommended that an individual standard should employ for dimmable operation.
Acknowledgment
This work was carried out with the financial support from the Ministry of Higher Education of Malaysia (MOHE) under the research grant UKM-KK-02-FRGS0193-2010.
REFERENCES
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Authors: Sohel Uddin is a Masters student at the Department of Electrical, Electronic and Systems Engineering, Universiti Kebangsaan Malaysia (UKM), E-mail: sohel_091@yahoo.com. Dr. Hussain Shareef is a senior lecturer of Department of Electrical, Electronic and Systems Engineering, Universiti Kebangsaan Malaysia (UKM), E-mail: shareef@eng.ukm.my. Prof. Dr. Azah Mohamed is a professor of Department of Electrical, Electronic and Systems Engineering, Universiti Kebangsaan Malaysia (UKM), E-mail: azah@eng.ukm.my. Dr. M A Hannan is an Associate professor of Department of Electrical, Electronic and Systems Engineering, Universiti Kebangsaan Malaysia (UKM), E-mail: hannan@eng.ukm.m
Source & Publisher Item Identifier: PRZEGLĄD ELEKTROTECHNICZNY, ISSN 0033-2097, R. 89 NR 4/2013
Published by Electrotek Concepts, Inc., PQSoft Case Study: Evaluation of Capacitor Bank Switch Restrikes, Document ID: PQS0606, Date: April 1, 2006.
Abstract: The analysis of high voltage capacitor switching consists primarily of measurements and computer simulations. There are a number of important transient related concerns when transmission and distribution voltage level capacitor banks are applied, including insulation withstand level, switchgear capabilities, energy duties of protective devices, and system harmonic considerations. The considerations should also be extended to include distribution systems and sensitive customer equipment. This case study presents methods for determining transient overvoltage and arrester duties and during capacitor switch restrike events and sample simulation and field measurements of restrike waveforms.
CAPACITOR BANK RESTRIKE EVENTS
A capacitor switching device de-energizes a capacitor bank at a current zero (refer to Figure 1). Since the current is capacitive, the voltage at the time of current interruption is at a system peak. Successful interruption depends on whether the switch can develop sufficient dielectric strength to withstand the rate-of-rise and the peak recovery voltage. For a grounded-wye capacitor bank, two times (2 per-unit) the system voltage will appear across the switch contracts one-half cycle after interruption. If the switch cannot withstand this recovery voltage, the switch will restrike.
Determining Transient Overvoltages and Arrester Duties
The energy duty requirements for arresters at capacitor bank locations depend on the size of the capacitor and on existing arresters located at the substation. In general, the most severe duty for an arrester near a capacitor bank occurs during a switch restrike. This is due to the trapped charge on the capacitor at the instant the restrike occurs, and results in a greater magnitude of the voltage oscillation.
It is also important to consider the coordination of MOV arresters (at the capacitor location) with any conventional gapped type arresters in the substation. It is important that the protective level of the MOV arresters be low enough to prevent operation of the gapped arresters. This is often difficult to achieve. If coordination is not possible, there are three options for arrester protection at the substation involved:
Replace all of the gapped type arresters in the substation with MOV arresters. The arresters will share the energy duty in the event of a restrike and there should be no danger of arrester failure.
Add one set of MOV arresters. This will greatly decrease the probability that a conventional arrester will fail during a capacitor restrike event because the MOV arrester will reduce the chance of a conventional arrester sparkover. The minimum size MOV should be used for best coordination with existing arresters.
Use only conventional gapped type arresters at the substation. This option relies on the integrity of the capacitor switch to prevent a restrike event. If a restrike would occur, it is unlikely the conventional arresters would be able to withstand the associated energy duty.
The arrester energy during a restrike depends on the following parameters:
− Capacitor configuration (grounded vs. ungrounded) − Capacitor size − Existence of other parallel capacitors − Source strength − Number of lines leaving substation − Nearby capacitor banks − Arrester protective level
Arrester applications at large shunt capacitor banks need to be evaluated carefully due to the high-energy duties that can occur in the event of a restrike in the capacitor switch. The energy levels will depend on whether the capacitor bank is grounded or ungrounded.
Figure 1 – Illustration of Capacitor Bank Restrike Event
During normal grounded-wye capacitor bank de-energization, the capacitor current is interrupted at the peak system voltage thus leaving a 1.0 per-unit trapped charge on the capacitor. This trapped charge results in an offset in the transient recovery voltage (TRV) that reaches a magnitude of 2.0 per unit one-half cycle after opening. Significant transient voltages can occur if the switch restrikes during clearing. The worst restrike transient occurs when twice the normal system peak voltage appears across the switch contacts. Theoretically, in this case, the magnitude of the transient voltage approaches 3.0 per unit.
Ungrounded-wye capacitor banks may expose the capacitor switch to recovery voltages greater than 2.0 per unit. Recovery voltages may reach 2.5 per unit on the first phase to open when the other phases open at the next current zero. If two of the phases delay opening, the recovery voltage may reach 3.0 per unit on the first phase to open. Finally, if one of the other phases delays, the transient recovery voltage would be 4.1 per unit. If a restrike occurs on the first phase to open at 2.5 per unit, a recovery voltage of 6.4 per unit can occur on one of the other two phases because of the voltage that builds up across the neutral capacitance. The high recovery voltage on another phase can cause a second restrike, resulting in a two-phase restrike.
The transient voltages on a capacitor bank and the recovery voltages across the switch can be reduced by installing arresters on the capacitor side of the switching device. If the switch is rated for the recovery voltages involved, then the arresters can be located on either the capacitor side or source side of the switch.
To evaluate arrester energy duty, simple expressions can be derived for grounded and ungrounded capacitor banks in terms of capacitor size, source inductance, peak system voltage, and arrester protective levels. The equations for evaluating the energy duty are given in
Table 1 – Arrester Duty during a Capacitor Restrike
.
Assuming a given capacitor bank rating, the arrester energy duty (in joules) versus the arrester protective level can be determined. Figure 2 and Figure 3 illustrate the arrester duty for Metal-Oxide Varistors (MOV). Silicon-Carbide (SiC) arresters generally have more severe energy duties because of the partial capacitor discharge that occurs when the arrester sparks over.
Figure 2 – Theoretical Arrester Duty during a Capacitor Switch Restrike (per-unit of normal peak line-to-neutral voltage)
Figure 3 – Theoretical Arrester Duty, Arrester Capability, and Simulation Results (per-unit of normal peak line-to-neutral voltage)
While the placement of an MOV arrester on the capacitor side of the breaker is not required, it is generally recommended. This location provides overvoltage protection for the bank itself, as well as limiting the recovery voltage seen by the breaker. Another benefit of the arrester is that its presence should help to minimize the possibility of multiple restrike events. Previous experience has indicated that if a breaker experiences multiple restrikes during clearing, equipment failure will more than likely occur.
Figure 4 illustrates an example of a computer simulation showing arrester (MOV) voltage, arrester current, and arrester energy duty during a capacitor switch restrike.
Figure 4 – Simulated Arrester Voltage, Current, and Energy during Switch Restrike
Sample Simulations and Field Measurements of Restike Events
Figure 5 shows the bus voltage (in per-unit) during a multiple restrike event on a 50 MVAr, 230kV transmission capacitor bank. The capacitor bank is protected with an 180kV MOV arrester.
Figure 6 shows the bus voltage during de-energization and switch restrike of a 161kV transmission capacitor bank. The worst-case transient voltage was approximately 2.02 per-unit (202%).
Figure 6 – 161kV Capacitor Switch Restrike
Figure 7 shows the bus voltage during a multiple restrike event on a 34.5kV capacitor bank. The worst-case transient voltage was approximately 1.55 per-unit (155%).
Figure 7 – 34.5kV Multiple Capacitor Switch Restrike Voltage
Figure 8 shows the transformer secondary current during a multiple restrike event on a 34.5kV capacitor bank.
Figure 8 – 34.5kV Transformer Current during Multiple Capacitor Switch Restrike
SUMMARY
Arrester energy during a capacitor switch restrike event is dependent on the capacitor configuration, ratings, source strength (including nearby capacitors and number of transmission lines), and arrester protective level (e.g., maximum switching surge protective level – MSSPL).
A properly sized MOV arrester, placed between a capacitor switch and a capacitor bank, will provide overvoltage protection for a single restrike event. In addition, the arrester will protect the bank from excessive overvoltages, as well as reduce the likelihood of multiple restrike events that can result in equipment failure.
REFERENCES
G. Hensley, T. Singh, M. Samotyj, M. McGranaghan, and T. Grebe, Impact of Utility Switched Capacitors on Customer Systems Part II – Adjustable Speed Drive Concerns, IEEE Transactions PWRD, pp. 1623-1628, October, 1991.
G. Hensley, T. Singh, M. Samotyj, M. McGranaghan, and R. Zavadil, Impact of Utility Switched Capacitors on Customer Systems – Magnification at Low Voltage Capacitors, IEEE Transactions PWRD, pp. 862-868, April, 1992.
T.E. Grebe, Application of Distribution System Capacitor Banks and Their Impact on Power Quality, 1995 Rural Electric Power Conference, Nashville, Tennessee, April 30-May 2, 1995.
M. McGranaghan, W.E. Reid, S. Law, and D. Gresham, Overvoltage Protection of Shunt Capacitor Banks Using MOV Arresters, IEEE Transactions PAS, Vol. 104, No. 8, pp. 2326-2336, August, 1984.
S. Mikhail and M. McGranaghan, Evaluation of Switching Concerns Associated with 345 kV Shunt Capacitor Applications, IEEE Transactions PAS, Vol. 106, No. 4, pp. 221-230, April, 1986.
T.E. Grebe, Technologies for Transient Voltage Control During Switching of Transmission and Distribution Capacitor Banks, 1995 International Conference on Power Systems Transients, September 3-7, 1995, Lisbon, Portugal.
Electrotek Concepts, Inc., An Assessment of Distribution System Power Quality – Volume 2: Statistical Summary Report, Final Report, EPRI TR-106294-V2, EPRI RP 3098-01, May 1996.
Electrotek Concepts, Inc., Evaluation of Distribution Capacitor Switching Concerns, Final Report, EPRI TR-107332, October 1997.
RELATED STANDARDS IEEE Std. 1036
GLOSSARY AND ACRONYMS MOV: Metal Oxide Varistor Arrester MSSPL: Maximum Switching Surge Protective Level SiC: Silicon Carbide Arrester TRV: Transient Recovery Voltage
Published by Electrotek Concepts, Inc., PQSoft Case Study: Customer Adjustable-Speed Drive Motor Failure Evaluation, Document ID: PQS1010, Date: October 15, 2010.
Abstract: This case study presents a customer adjustable-speed drive motor winding failure analysis. The study investigated the potential for severe high frequency transient overvoltages at induction motor terminals for an adjustable-speed drive that utilized a pulse-width modulation inverter, along with a significant length of cable between the inverter and motor. Several power conditioning mitigation alternatives including series reactors and motor terminal filters were evaluated using computer simulations.
INTRODUCTION
A customer adjustable-speed drive (ASD) motor winding failure case study was completed for the system shown in Figure 1. The case study investigated the potential for severe high frequency transient overvoltages at induction motor terminals for an adjustable-speed drive that utilizes a pulse-width modulation (PWM) inverter, along with a significant length of cable between the inverter and induction motor. Several power conditioning mitigation alternatives, such as series reactors/chokes and motor terminal filters, were evaluated using computer simulations.
The simulations for the case study were completed using the PSCAD program. The accuracy of the simulation model was verified using three-phase and single-line-to-ground fault currents and other steady-state quantities. The circuit consisted of a 34.5 kV utility substation supplying a 1,500 kVA customer step-down transformer, along with a 10 hp PWM adjustable-speed drive with a 500 foot cable segment between the inverter and motor terminals. A high frequency, distributed parameter transmission line model was required to accurately represent the traveling wave (reflections) effects of the motor cable. There was also a standard 3% input choke on the drive, which resulted in a current distortion value of approximately 40.9%. A detailed drawing of the adjustable-speed drive and induction motor configuration is shown in Figure 2.
Figure 1 – Illustration of Oneline Diagram for Customer Motor Failure Evaluation
Figure 2 – Illustration of Adjustable-Speed Drive and Motor Circuit
Adjustable-speed drives for most small and medium induction motors utilize voltage source inverters to provide variable frequency ac output. The common drive structure adopted by the industry consists of an uncontrolled diode-bridge rectifier, dc link, and pulse-width modulation voltage source inverter as illustrated in Figure 3. The dc link for this type drive includes a ripple smoothing capacitor. The inverter output waveform is generated by a series of step-like functions. An ideal step-change in the output voltage is prevented by stray parameters of the circuit and commutation of switching devices from one phase to another. Steep-front waveform generation is one of the inherent characteristics of a high switching frequency voltage source inverter.
SIMULATION RESULTS
The output voltage of the pulse-width modulation inverter is a potential problem for the induction motor. Both the frequency and magnitude of the output voltage are adjusted by controlling the inverter’s operation. State-of-the-art voltage source inverters are based on insulated gate bipolar transistor technology. With these devices, the inverter operates with a switching frequency ranging from tens of Hz to tens-of-thousands of Hz. Figure 4 shows an example output voltage of a pulse-width modulation drive (measurement location #1 on Figure 3). The switching frequency of the most commonly used pulse-width modulation drives is in the range of 1,000 Hz to 5,000 Hz. The rise times of the pulses can be approximately 10μs to 0.1μs.
The problem occurs on the output of the inverter at the drive terminals. The high switching frequency of the inverter allows sophisticated control schemes to be implemented. One of the advantages of the high switching frequency inverter is the reduction of low order harmonics, which results in a reduction of output filter duty. However, this benefit can only be achieved under certain circuit conditions. Under some conditions, the fast changing voltage resulting from high frequency switching operation of inverter can create severe insulation problems for induction motors.
Machine insulation integrity is influenced by the rate-of-change of voltage as well as the transient overvoltage magnitude. A voltage with a high rate-of-change tends to be distributed along a motor’s windings unevenly. This uneven distribution causes a significant over-stress across ending turns resulting in turn-to-turn insulation failure. In practice, it is common for the drive and the motor to be separated by relatively long lengths of cable. In addition, the characteristic impedance of the induction motor can be ten to one hundred times that of the characteristic impedance of the cable connecting the drive to the induction motor.
Figure 3 – Oneline Diagram Showing Power System and Inverter Circuit
Figure 4 – Measured Example Line-to-Line Output Inverter Voltage
Figure 5 shows a transition in one of the pulses at the inverter (measurement location #1). Notice that when the voltage changes from zero to its full negative value, there is no significant over-shoot or overvoltage. At the motor terminals, however, the transition of one of the pulses at the motor terminals shows an overvoltage of approximately 1.7 per-unit, as shown in Figure 6 (measurement location #2). The overvoltage and the resulting ringing occur at both the front and rear of each pulse. Depending on the operation pattern of the adjustable-speed drive, similar transients may occur 20 to 100 times per 60 Hz cycle.
The most harmful effect of the inverter output occurs when the connection cable is relatively long with respect to the wave front of an incidental voltage wave and when the ratio of characteristic impedance of the machine and the cable is high. In the worst case, an inverter output voltage pulse magnitude can be doubled at the induction motor terminals. If a voltage wave travels at a velocity of 250 feet per microsecond, an incident voltage wave with a front time of 0.3μs is sufficient to create a voltage doubling at the open end of 75 feet of cable. Under this condition, motor windings experience a near 2.0 per-unit over voltage, if the maximum voltage seen at the inverter output terminal is 1.0 per-unit.
Figure 5 – Measured Example Phase-to-Phase Voltage at Inverter Terminals
Figure 6 – Measured Example Phase-to-Phase Voltage at Motor Terminals
The reflection of an incident traveling voltage wave at the motor connection termination is determined by surge impedance ratio at the junction point. The characteristic impedance of a small motor is usually higher than the low surge impedance of the cable. Therefore, when compared with the low surge impedance of cable, the motor connection may look like an open circuit.
The initial simulation (Case 4a) involved the basecase condition with no mitigation added to the adjustable-speed drive or induction motor.
Figure 7 shows the simulated current waveform (single phase shown) for the 10 hp adjustable-speed drive operating at an 80% power factor and with a 3% ac choke. The current has a fundamental frequency value of 8.5 A, an rms value of 9.1 A, and a THD value of 40.9%.
Figure 7 – Simulated AC Drive Current Waveform
Figure 8 shows the simulated line-to-line voltage at the inverter terminals for the basecase conditions. The dc voltage for the drive was approximately 650 V. The inverter switching frequency (Fs) for the case was 675 Hz and the motor frequency was 45 Hz.
Figure 8 – Simulated Line-to-Line Inverter Voltage Waveform
Figure 9 shows the simulated line-to-line voltage at the motor terminals for the case with no mitigation added. Figure 10 shows an expanded view of the waveform highlighting several of the ringing transients. The peak simulated transient voltage was 1,153V, which was approximately 1.77 per-unit (similar to the measured waveform previously shown in Figure 6).
Figure 9 – Simulated Line-to-Line Motor Voltage Waveform
Figure 10 – Expanded View of Line-to-Line Motor Voltage Waveform
The second simulation case (Case 4b) evaluated the power conditioning alternative of adding a series choke between the inverter and the induction motor. Inductive chokes (a.k.a., reactors) are similar to isolation transformers, except that they do not define a separately derived system. Inductive chokes provide additional impedance in the circuit in much the same manner that an isolation transformer does, but at a much-reduced cost.
Chokes are often applied to the front-end of adjustable-speed drives to protect the drives from nuisance tripping caused by utility capacitor bank switching and other normal power system switching operations. Some drive manufacturers now produce drives with chokes as part of their standard design. Chokes also help prevent voltage notching, caused by power electronic switching, from disturbing other sensitive customer equipment. They can limit notching to the drive side of the inductive choke.
Figure 11 shows the simulated line-to-line voltage at the induction motor terminals for the case with a 5% choke added between the inverter and motor terminals. Generally, a choke is specified in %X and hp. The inductance of the simulated choke rating was approximated using the following expression:
.
where: fdrive = inverter output fundamental frequency (Hz) X = inductive reactance of choke (%) kVϕϕ = system rms phase-to-phase voltage (kV) hp = horsepower rating of the motor (hp)
The resulting transient voltages at the motor terminals were significantly reduced with the 5% choke. It should be noted that the fundamental drive frequency voltage was somewhat lower due to the voltage drop across the choke.
The final simulation case (Case 4c) evaluated the power conditioning alternative of adding a motor terminal filter to the induction motor. A motor terminal filter is a type of low-pass filter that passes signals with low frequencies and reject signals with high frequencies. These filters can improve power quality by reducing the effect of the transient energy and by removing noise from the electrical system. Low-pass filters can be used to provide even better protection than inductive reactors for high frequency transients.
A first-order filter consisting of a capacitor in series with a resistor can be designed to have minimal losses and to match the surge impedance of the cable that supplies the motor.
Figure 11 – Simulated Line-to-Line Motor Voltage Waveform with 5% Choke
Figure 12 shows the simulated line-to-line voltage at the induction motor terminals for the case with a shunt motor terminal filter added at the motor terminals. The simulated filter component values were 1μF (capacitor) and 100Ω (resistor). The transient voltages were significantly reduced with the motor terminal filter, as compared with the basecase conditions. It should be noted that there was no fundamental drive frequency voltage drop for this case because the filter was connected in shunt, rather than in series like the previous case.
Figure 12 – Simulated Line-to-Line Motor Voltage Waveform with Motor Terminal Filter
Figure 13 shows an expanded view of the simulated line-to-line voltages at the motor terminals for the three simulated cases. The figure illustrates the reduced transient voltages with the mitigation alternatives and the voltage drop for the 5% series choke case (Case 4b).
Figure 13 – Simulated Line-to-Line Motor Voltage Waveforms
SUMMARY
This case study presented a customer adjustable-speed drive motor winding failure analysis. The study investigated the potential for severe high frequency transient overvoltages at induction motor terminals for an adjustable-speed drive that utilized a pulse-width modulation inverter, along with a significant length of cable between the inverter and motor.
In the past, the inverters for many drives were thyristor based with either forced-commutation or loadcommutation. For current source inverter (CSI) drives based on thyristor or gate turn-off (GTO) devices, the inverter switching frequency was limited to several hundred Hz. This low switching frequency means that these devices have relatively high commutation losses and need a relatively long commutation period. Consequently, induction motors supplied from current source inverter drives have a lower probability of experiencing fast-front transient voltages.
In an effort to improve the efficiency of many industrial processes, standard induction motors have been retrofitted with adjustable speed drives. The drives allow for better speed control, soft starting of motors, and increased efficiency of the overall process operation. Unfortunately, there can also be some power quality-related drawbacks when using these drives.
A number of drive manufacturers are working with motor manufacturers to match drive-duty induction motors to their adjustable-speed drives. The adjustable-speed drive and motor are provided as a complete package. The induction motors are designed to withstand the severe duties imposed on them by the high switching frequencies of the PWM inverters.
This case study investigated one of potential problems with applying new adjustable-speed drives with older induction motors, which is motor winding failure due to transient overvoltages. The power conditioning solutions that were evaluated included series chokes and shunt motor terminal filters. Other potential solutions include changing the cable length, which is generally not practical for the customer; and changing the inverter switching frequency, which may also not be practical and may not significantly reduce the transient overvoltages.
REFERENCES
IEEE Recommended Practice for Monitoring Electric Power Quality,” IEEE Std. 1159-1995, IEEE, October 1995, ISBN: 1-55937-549-3.
IEEE Recommended Practice for Emergency & Standby Power Systems for Industrial & Commercial Applications (IEEE Orange Book, Std. 446-1995), IEEE, ISBN: 1559375981.
IEEE Recommended Practice for Powering and Grounding Electronic Equipment (IEEE Emerald Book, Std. 1100-1999), IEEE, ISBN: 0738116602.
Melhorn, C.J., and Tang, L., “Transient Effects of PWM Drives on Induction Motors,” IEEE Transactions on Industry Applications, Volume 33, Issue 4, pp. 1065-1072, Jul/Aug 1997.
R.C. Dugan, M.F. McGranaghan, S. Santoso, H.W. Beaty, “Electrical Power Systems Quality,” McGraw-Hill Companies, Inc., November 2002, ISBN 0-07-138622-X.
Published by Electrotek Concepts, Inc., PQSoft Case Study: Study of 345 kV Transient Recovery Voltages, Document ID: PQS0602, Date: January 1, 2006.
Abstract: Transient recovery voltage (TRV) is the voltage across the terminals of a pole of circuit breaker following current zero when interrupting faults. TRV waveshapes can be oscillatory, exponential, cosine-exponential or combinations of these forms. TRVs due to short-line faults (SLFs) are characterized by triangular-shaped waveshapes and a very steep initial rate-of-rise. An engineering study found that for a number of cases, the TRV waveshapes exceeded their related TRV capability limits for the first 10-50 μsec. The results also indicated that clearing SLFs on lines leaving the 345kV substations would result in an initial rate-of-rise of the recovery voltage (RRRV) that exceeds the breaker’s SLF capability. The study evaluated the application of an additional capacitance on the line side of the circuit breakers. This capacitance reduces the initial RRRV to within the related SLF capability. This case study presents a summary of the model development and simulations completed during the 345kV TRV study.
INTRODUCTION
Due to the concern for excessive TRVs during breaker operations, an engineering study was preformed to evaluate the proposed 345kV substation design, as well as the impact on nearby utility equipment. The study evaluated the concerns and possible solutions, such as adding capacitive devices, to protect against the harmful transients that may damage the surrounding equipment and power system.
The analysis of high-frequency TRVs frequently requires the use of sophisticated digital simulation tools. Simulations provide a convenient means to characterize transient events, determine resulting problems, and evaluate possible mitigation alternatives. Occasionally, they are performed in conjunction with system monitoring for verification of models and identification of important power system problems. The complexity of the models required for the simulations generally depends on the system characteristics and the transient phenomena under investigation.
The transient analysis for the study was performed using the PSCAD program. This program can be used for the analysis of circuit switching operations, capacitor switching, lightning transients, and transients associated with the operation of power electronic equipment.
STUDY METHODOLOGY
The TRV evaluation for various fault conditions was based on the methods provided in IEEE Std. C37.06, IEEE Std. C37.04, and IEEE Std. C37.011. This involved analysis of the most severe conditions, including the clearing of a three-phase ungrounded symmetrical fault at the breaker terminal when the system voltage is at a maximum and SLFs.
The study considered normal cases where the system operates with all breakers and lines in service and various contingencies where only one breaker is available to clear a fault. For both of these conditions, three-phase ungrounded and single-line-to-ground faults were evaluated.
TRV is the voltage across the terminals of a pole of circuit breaker following current zero when interrupting faults. TRV waveshapes can be oscillatory, exponential, cosine-exponential or combinations of these forms. TRVs due to SLFs are characterized by triangular-shaped waveshapes and a very steep initial rate-of-rise. The triangular shape of the recovery voltage arises from positive and negative reflections of the traveling waves that oscillate between the open breaker and the fault. Due to the short distance involved between the fault location and the open breaker, the initial RRRV can be very steep.
According to IEEE Std. 37.011-1994, the most severe oscillatory or exponential recovery voltages tend to occur across the first pole to open of a circuit breaker interrupting a three-phase ungrounded symmetrical fault at its terminal when the system voltage is at a maximum. When the TRV performance meets the withstand criteria when subjected to the fault condition mentioned above, a SLF evaluation is not necessary. This is due to the fact that SLF TRV capability is higher than that of a three-phase ungrounded fault.
MODEL DEVELOPMENT
The model development process included steps for data collection, data approximation, data simplification and model verification.
The TRV system model was based on short-circuit data that consisted of positive and zero sequence impedance data in the ASPEN Oneliner format. The study area included the substation and the adjacent system (see Figure 1). The boundary of the study area was represented with equivalent sources and transfer impedances such that the electrical representation of the study area (at 60 Hz) was nearly identical to the original representation.
Figure 1 – System Model for the 345kV TRV Study
In the study, all transmission lines were represented with a frequency dependent line model to account for traveling wave phenomena. Generating units were represented with ideal sources behind sub-transient impedances. The accuracy of the transient model was verified by comparing three-phase and single-line-to-ground fault currents at all buses. A subset of the fault cases is summarized in Table 1.
Table 1 – Steady-State Fault Simulations Completed for Model Verification
.
The model represented a reduction of the entire system to determine the system equivalents and corresponding fault levels. It should be noted that the corresponding PSCAD model did not include mutual coupling between transmission lines. In addition, typical X/R ratio values were used where the short-circuit model did not include resistance (e.g., lines, transformers, etc.), and relatively large transfer impedances were ignored. Considering these factors, accuracy within 3% was considered acceptable for the 60 Hz short-circuit model verification.
Circuit Breaker Data
In evaluating the TRV withstand capability for the 345kV breakers, the following references were used:
ANSI C37.06-2000 Tables 3 and 6 (Note 6 for Table 3)
IEEE C37.04-1999, Section 5.9, Table 2 and Figure 5
The new 345kV breakers have the following ratings:
Rated Maximum Voltage: 362 kV Rated Continuous Current: 3000 A Rated Short-Circuit Current: 63 kA Rated Interrupting Time: 2 Cycles Rated Transient Inrush Current: 25 kA Rated Transient Inrush Current Frequency: 4250 Hz
TRV-related data is shown in Table 2 and Table 3.
Table 2 – Rated TRV Capability of 362kV, 3000 A, 63kA Breaker
.
Table 3 – Multipliers for Various Interrupting Levels for Terminal Faults
.
The waveshape of the exponential component E1 for terminal faults below 30% of the breaker rating is 1-cosine. Based on Table 2 and Table 3 and the discussion in Section 5.9 of IEEE Std. C37.04-1999, the TRV limit envelopes were derived and graphically represented using a MATLAB program. Figure 2 shows the TRV envelopes (or withstand capabilities) for several fault levels. Capability envelopes when interrupting fault currents below 30% of its rated short-circuit current have a waveshape of 1-cosine, while for fault currents above 30% of breaker rating, the waveshape has an exponential-cosine form.
Figure 2 – TRV Withstand Capability for a 362kV, 3000 A, 63kA Breaker
Capacitance Values for Substation Equipment
Equivalent values of capacitance for substation equipment were the lumped values at the breaker terminals. Since the capacitance values for the 345kV equipment at the studied substations were not supplied by the utility, it was agreed that typical capacitance ranges based on Annex B of IEEE Std. C37.011-1994 would be used. Three equivalent capacitance values (minimum, maximum, and average) were determined. Table 4 shows an example of the collection of typical capacitance values for each bus section in the model.
Table 4 – Typical Capacitance Values Based on Annex B of IEEE Std. C37.011-1994
.
This process was repeated for all of the 345kV substation equipment in the system model. The minimum values of equivalent capacitance were used throughout the simulation process for both normal and contingency cases.
SIMULATION RESULTS
The TRV evaluation was conducted for the most severe operating conditions, including both three-phase ungrounded faults at the breaker terminal and SLFs. The study considered both normal cases where the system operates with all breakers and lines in service and contingency cases where the only one breaker is available to clear the fault.
Three-Phase Ungrounded Terminal Faults
The simulation results for the three-phase ungrounded fault clearing cases were summarized in tables similar to Table 5. The table shows the respective case identifier, the breaker number, the peak current that the breaker interrupted, this peak current as a percentage of the rated value (63kA), the peak TRV in kV, and a note to report whether the TRV was within the breaker’s capability envelope. A “YES*” note signifies that the TRV waveshape slightly exceeded the TRV capability for the first 10-50 μsec, but it met the TRV SLF capability. A “NO” note signifies that the TRV waveshape did not meet the TRV capability limit.
Table 5 – TRV Evaluation of Three-Phase Terminal Faults
.
Figure 3 and Figure 4 show several examples of the simulation results for the three-phase ungrounded fault clearing cases summarized in Table 5. Figure 3 shows the recovery voltage for breaker 4560 for Case A1 and Figure 4 shows the recovery voltage for breaker 4592 for Case A3. Each graph of TRV includes an overlay of the withstand capability.
Figure 3 – TRV Withstand Capability for Breaker 4560 for a Three-Phase Ungrounded Fault
Figure 4 – TRV Withstand Capability for Breaker 4592 for a Three-Phase Ungrounded Fault
Short-Line Faults
The simulation results for the SLF cases were recorded and compared to their respective TRV withstand and SLF capabilities. Figure 5 shows an example of the simulation results for a SLF clearing case. When compared to their respective terminal fault case, the magnitude of the peak fault current interrupted was lower due to the additional line impedance between the fault location and the breaker terminals. However, the RRRV was higher due to the traveling waves that oscillate between the fault location and breaker terminals.
Figure 5 – TRV Withstand Capability for Breaker 4564 for a Three-Phase Ungrounded SLF
As can be seen in Figure 5, the initial TRV for the case with no added capacitance exceeds the related SLF capability. Additional cases were then completed for each faulted transmission line to evaluate the effectives of various capacitance values for reducing the RRRV for each 345kV substation breaker. The case with 15ηF added is shown in Figure 6.
Figure 6 – TRV Withstand Capability for Breaker 4564 with 15nF Added
SUMMARY
The engineering study included an evaluation of the TRV performance for various breaker operations for new 345 kV breakers. A number of observations and conclusions based on the simulation results included:
1.The TRV evaluation for the new 345kV circuit breakers in the substations was conducted for the most severe operating conditions, including clearing both three-phase ungrounded faults at the breaker terminal and SLF.
2.Three capacitance values, representing a range of equivalent capacitances for substation equipment, were determined based on information provided by the utility and from Annex B of IEEE Std. C37.011-1994.
3.The TRV evaluation considered both normal cases where the system operates with all breakers and lines in service and contingency cases where only one breaker is available to clear a fault. Both three-phase ungrounded and single-line-to-ground faults were evaluated for these conditions.
4.For a number of cases, the TRV waveshapes exceeded their related TRV capability limit for the first 10-50 μsec after the breaker had opened. These cases were then compared to their corresponding SLF capability.
5.For a number of normal and contingency cases, the TRV waveshapes exceeded their related capability limit. For these cases, the breaker’s withstand capability was exceeded due to the peak of the recovery voltage, rather than the initial rate-of-rise.
6.With respect to clearing SLF on lines leaving the 345kV substations (2 km from the substation), the simulations indicated that the initial RRRV will exceed the related SLF capability. One method for mitigating this condition is with the application of an additional capacitance on the line side of the breaker. This capacitance reduces the initial RRRV to within the related SLF capability.
7.Simulations were completed to evaluate the application of an additional capacitance on the line side of breakers. These cases used the same capacitance values at each of the line terminals. The additional capacitance of 15ηF/phase generally reduced the initial RRRVs to within the related SLF capability.
REFERENCES
Study of 345kV Transient Recovery Voltages on the Illinois Power System, Sixth International Conference on Power System Transients (IPST), Montreal, Canada, June 19-23, 2005.
IEEE AC High Voltage Circuit Breakers Rated on a Symmetrical Current Basis – Preferred Ratings and Related Required Capabilities, IEEE Standard C37.06, May. 2000.
IEEE Standard Rating Structure for AC High-Voltage Circuit Breakers, IEEE Standard C37.04, June. 1999.
IEEE Application Guide for Transient Recovery Voltage for AC High Voltage Circuit Breakers Rated on a Symmetrical Current Basis, IEEE Standard C37.011, September. 1994.
Published by Electrotek Concepts, Inc., PQSoft Case Study: Common Power Quality Waveform Signatures, Document ID: PQS0901, Date: October 15, 2009.
Abstract: Power quality problems encompass a wide range of disturbances and conditions on utility and customer power systems. They include everything from very fast transients (microseconds) to long duration (hours) outages. Problems also include steady state (e.g., harmonic distortion) and intermittent (e.g., voltage flicker) phenomena. This wide variety of conditions makes the development of standard measurement and analysis procedures very difficult. Therefore, it is beneficial to characterize power quality measurements and their related problems into common categories using standard data formats.
This case study presents a collection of representative waveforms for various power system fault and power quality events, including voltage sags, momentary interruptions, voltage swells, harmonics, capacitor switching transients, transformer energizing transients, and ferroresonance.
INTRODUCTION
The term power quality refers to a wide variety of different parameters that characterize the voltage and current at a given time and at a given point on the power system. It is important to have a clear understanding of these parameters and the variations in them that can cause customer problems. Definitions are required to develop a method of categorizing problems so that conditions at different sites can be compared and analyzed.
This case study refers to power quality variations and disturbances. Disturbances signify onetime, momentary events while power quality variations refer to the full range of conditions that can occur, including variations in steady-state voltage and current characteristics (e.g., harmonic distortion). There are currently no clearly accepted definitions for many categories of power quality variations because different manufacturers of measurement equipment often use non-standard definitions to categorize events. In addition, individual industry standards address only a small segment of the total range of power quality variations. Several important factors that should be considered when using power quality categories include:
− The characteristics of the power quality variation. Important characteristics include the magnitude, frequency content, and duration. Some combination of these characteristics can be used to describe virtually any power quality variation.
− The cause of the power quality variation. The condition could be caused by a switching event, lightning, a system fault, or operation of customer equipment. It is important to consider the possible causes of power quality variations in each category.
− Requirements for measurement. Some types of power quality variations can be characterized with simple voltmeters, ammeters, or strip chart recorders. Other conditions require special-purpose disturbance monitors or harmonic analyzers. The characteristics of the power quality variation in each category determine the requirements for monitoring.
− Methods to improve the power quality. Solutions to power quality problems depend on the type of power quality variation involved. Transient disturbances can often be controlled with surge arresters while momentary interruptions could require an uninterruptible power supply (UPS) system for equipment protection. Harmonic distortion may require special-purpose harmonic filters.
− Existing standards and power quality terminology. Existing terminology has become almost standard in describing many types of power quality variations. This terminology has resulted from the definitions used to describe power quality by popular monitoring equipment manufacturers and from the development of standards for some aspects of power quality. When developing a new set of definitions for power quality variations, the existing terminology should be carefully considered.
POWER QUALITY CATEGORIES
The relative importance of a particular category of power quality phenomena for a specific customer will depend on the type of installed electrical equipment. The type of interaction between customer equipment and the power quality phenomena – equipment damage, equipment/process trip, compromised product quality, etc. – and the frequency at which it occurs or could be expected to occur are also critical factors in the evaluation process once the cause has been identified. The range of power quality phenomena is defined by IEEE Std. 1159-1995: Recommended Practice for Monitoring Electric Power Quality (refer to Table 1) [1].
Approaches for resolving equipment or process problems related to each category of phenomena vary widely. Causes, impacts, and appropriate solutions for this range of electrical phenomena have been analyzed in numerous research and study efforts, resulting in the development of proven solution techniques for many common power quality problems.
These efforts have also contributed to a prioritization of the power quality phenomena categories. From the customer’s point of view, the most important problem categories:
− Have the highest negative impact on productivity − Are difficult to diagnose and characterize − Are more difficult and/or expensive to resolve
Using these criteria, research and case study investigations have identified the following categories of power quality phenomena to be of highest importance to customers:
− Transients, especially utility capacitor bank switching transients − Harmonic distortion, especially resonance conditions − Voltage variations, especially rms voltage sags and interruptions
This does not mean that there are never problems associated with other categories of power quality phenomena. Experience does indicate, however, that the majority of problems (especially from the customer’s perspective) are those listed above.
Table 1 defines power quality variation categories. Some of the categories also include subcategories for more accurate description of particular power quality variations. Three primary attributes are used to differentiate among the different categories and subcategories:
Frequency components
Magnitude
Duration
These attributes are not equally applicable to all the categories of power quality variations. For instance, it is difficult to assign a duration to an oscillatory transient, and it is not useful to assign a spectral content to variations in the fundamental frequency magnitude (e.g., sags, swells, overvoltages, undervoltages, and interruptions). Each category is defined by the most important attributes for that particular power quality condition.
These characteristics and attributes are useful for evaluating measurement equipment requirements, system characteristics affecting the power quality variations, and possible measures to correct power quality problems.
This case study presents a number of representative waveforms for an assortment of power system fault and power quality events, which are grouped by the categories provided in IEEE Std. 1159-1995 and shown in Table 1.
Table 1 – Categories of Power System Electromagnetic Phenomena (source IEEE Std. 1159-1995)
.
POWER QUALITY DATA FORMATS
This case study illustrates a number of representative power quality event waveforms that are stored using the common data interchange formats PQDIF and COMTRADE. PQDIF [2] provides a recommended practice for a file format suitable for exchanging power quality related measurement and simulation data. COMTRADE [3] provides a common format for digital data records of power system fault, test, or simulation events.
PQDIF (IEEE Recommended Practice for the Transfer of Power Quality Data) is an IEEE standard (1159.3-2003) that was developed by the Working Group on Monitoring Electric Power Quality, which is part of the Power Quality Subcommittee of the T&D Committee. It defines a file format suitable for exchanging power quality related measurement and simulation data in a vendor-independent manner. A variety of simulation, measurement and analysis tools for power quality engineers are now available from many vendors. Generally, the data created, measured, and analyzed by these tools are incompatible between vendors. PQDIF provides a set of requirements and attributes for a power quality data interchange format. Key among these is the ability to represent data from a variety of sources (e.g., measured, simulated, or manually created), in the time, frequency, and probability domains.
COMTRADE (IEEE Standard Common Format for Transient Data Exchange for Power Systems) is an IEEE standard (C37.111-1999) first published by the Power System Relaying Committee in 1991. It was updated in 1999 and reaffirmed in 2005. It defines a common format for data files and an exchange medium used for the interchange of various types of fault, test, or simulation data for electrical power systems. The standard also describes the sources of transient data such as digital protective relays, digital fault recorders, and transient simulation programs (e.g., PSCAD/EMTP/ATP) and discusses the sampling rates, filters, and sample rate conversions for the transient data being exchanged.
A viewing program that is capable of reading, displaying, and manipulating PQDIF and COMTRADE files is required for processing the power quality waveforms that are presented in this case study. A free program TOP, The Output Processor® [4] has this capability. The program is widely used in the utility industry for visualizing data from a variety of simulation and measurement sources. Figure 1 shows an example power quality event waveform signature that was measured using a Dranetz-BMI 8010 PQNode. The waveform shows the three-phase voltage on a 25 kV distribution feeder during a SiC arrester failure.
Figure 1 – Example of a Power Quality Event Waveform
REPRESENTATIVE POWER QUALITY WAVEFORM SIGNATURES
Power quality monitoring is used to characterize variations at various locations on utility and customer power systems. The length of the monitoring period is generally dependent on the nature of the power quality problem. For example, utility capacitor bank switching transients may be collected in several days, while harmonic distortion levels may need to be monitored for weeks, months, or even years to show the influence of load and seasonal variations. The current industry trend for power quality monitoring is fixed instruments that continuously monitor the power system.
Generally, it is advisable to begin monitoring as close as physically possible to the sensitive equipment being affected by the power quality variations. It is important that the monitor sees the same variations as the sensitive equipment. High-frequency transients, in particular, can be significantly different if there is significant separation between the monitor and the affected equipment. Another important monitoring location is the main service entrance. Transients and other voltage variations measured at this location can be experienced by all of the equipment in the facility. This is also the best indication of disturbances caused by the utility system (it is possible that disturbances at the service entrance are caused by events occurring within the facility). Monitoring site selection for diagnostic or evaluative monitoring is usually straightforward, being indicated by customer complaints, equipment failure reports, and other external factors.
This section includes a number of representative waveforms for various power system fault and power quality events, including voltage sags, momentary interruptions, voltage swells, harmonics, capacitor switching transients, transformer energizing transients, and ferroresonance. These waveforms all fall into one of the categories provided in IEEE Std. 1159-1995 (refer to Table 1) and are stored using either the PQDIF or COMTRADE formats. Each waveform includes background information regarding the source (e.g., measurement or simulation), cause, related utility or customer problem, and common solution.
Figure 2 shows a three-phase voltage sag waveform measurement for a remote three-phase fault on a distribution feeder. The magnitude of the resulting sag was approximately 60% for 9 cycles. The instantaneous voltage measurement was captured using a Dranetz-BMI 5530 DataNode and stored using the IEEE PQDIF file format. The customer power conditioning options for this event include UPSs and CVTs. The keywords for the waveform include sag and fault, while the slang terms that should be avoided include glitch, blink, wink, and outage.
Figure 2 – Remote Three-Phase Fault Voltage Waveform
Figure 3 shows a voltage rms trend during a distribution feeder momentary interruption sequence. The multiple reclosing interruptions, which are shown in per-unit, lasted approximately 1.2, 9.0, and 22.5 seconds respectively. The measurement was captured using a Dranetz-BMI 8010 PQNode and stored using the IEEE PQDIF file format. The customer power conditioning options for this event include UPSs and CVTs. The keywords for the waveform include interruption and fault, while the slang terms that should be avoided include glitch, wink, and outage.
Figure 3 – Reclosing Sequence during a Distribution Feeder Fault
Figure 4 shows a measured feeder voltage swell that occurred on the unfaulted phases close to a single line-to-ground fault on an overhead 34.5 kV distribution feeder. The swell was approximately 150%. The instantaneous voltage measurement was captured using a Dranetz-BMI 8010 PQNode and stored using the IEEE COMTRADE file format. The customer power conditioning options for this event include UPSs and CVTs. The keywords for the waveform include swell and fault, while the slang terms that should be avoided include glitch and surge.
Figure 4 – Voltage Swell on a Distribution Feeder
Figure 5 shows a measured 13.8 kV, 740 amp fundamental, 0.75 displacement power factor arc furnace load current. The waveform is an 18-cycle snapshot of one operating point for the furnace. The instantaneous current measurement was captured using a Dranetz-BMI 5530 DataNode and stored using the IEEE PQDIF file format. The power conditioning options for this event include harmonic filters and SVCs. The keywords for the waveform include current distortion, while the slang term that should be avoided is dirty power.
Figure 5 – Arc Furnace Current
Figure 6 shows the voltage on a customer secondary bus with moderate notching and distortion (VTHD ≈ 9%). It also shows a transient that was due to utility capacitor bank switching. The instantaneous voltage measurement was captured using a Dranetz-BMI 5530 DataNode and stored using the IEEE COMTRADE file format. The customer power conditioning options for this event include inductive chokes, and harmonic filters. The keywords for the waveform include notching and resonance, while the slang term that should be avoided is dirty power.
Figure 6 – Customer Voltage Notching
Figure 7 shows a 13.8 kV feeder current before-and-after energization of a 900-kVAr pole-mounted capacitor bank that creates a harmonic resonance that increases the current distortion (ITHD ≈ 13%). The instantaneous current measurement was captured using a Dranetz-BMI 8010 PQNode and stored using the IEEE PQDIF file format. The power conditioning options for this event include arresters and harmonic filters. The keywords for the waveform include capacitor and resonance, while the slang terms that should be avoided include surge, glitch, and spike.
Figure 7 – Feeder Capacitor Bank Switching and Harmonic Resonance
Figure 8 shows a 4.16 kV bus voltage waveform during utility capacitor bank switching. The resulting transient voltage was 1.35 per-unit, while the steady-state voltage rise was 1.2%. The instantaneous voltage measurement was captured using a Dranetz-BMI 8010 PQNode and stored using the IEEE PQDIF file format. The power conditioning options for this event include overvoltage control and arresters. The keywords for the waveform include oscillatory transient and overvoltage, while the slang terms that should be avoided include surge and spike.
Figure 8 – Substation Capacitor Bank Switching
Figure 9 shows a measured bus voltage waveform during a multiple restrike event on a 34.5 kV capacitor bank. The worst-case transient voltage was approximately 1.55 per-unit. The instantaneous voltage measurement was captured using a Dranetz-BMI 8010 PQNode and stored using the IEEE COMTRADE file format. The power conditioning options for this event include arresters. The keywords for the waveform include restrike and overvoltage, while the slang terms that should be avoided include surge and spike.
Figure 9 – Capacitor Bank Switch Multiple Restrike
Figure 10 shows the inrush current waveform for a distribution transformer energizing. Transformer inrush current typically decays over a period of about one second. The instantaneous current measurement was captured using a Dranetz-BMI 5530 DataNode and stored using the IEEE COMTRADE file format. The power conditioning options for this event include overcurrent protection, fuses, and reclosers. The keywords for the waveform include transient and overcurrent, while the slang terms that should be avoided include surge, and spike.
Figure 10 – Feeder Transformer Energizing
Figure 11 shows a phase-to-phase feeder voltage during a ferroresonance event that was caused by an unbalanced switching operation. The peak voltage was approximately 1.42 per-unit. The instantaneous voltage measurement was captured using a Dranetz-BMI 8010 PQNode and stored using the IEEE COMTRADE file format. The power conditioning options for this event include three-phase switches and secondary loads. The keywords for the waveform include ferroresonance and overvoltage, while the slang term that should be avoided is surge.
Figure 11 – Ferroresonance on Distribution Feeder
Figure 12 shows the simulated voltage waveform for a distribution system ferroresonance event on a 13.8 kV feeder. The peak voltage is approximately 2.89 per-unit. There were no arresters included in the model. The instantaneous voltage was created using an EMTP program and stored using the IEEE COMTRADE file format. The power conditioning options for this event include three-phase switches and secondary loads. The keywords for the waveform include ferroresonance and overvoltage, while the slang term that should be avoided is surge.
Figure 12 – Distribution Cable/Transformer Ferroresonance
SUMMARY AND CONCLUSIONS
Power quality problems encompass a wide range of disturbances and conditions on utility and customer systems, ranging from very fast transients to long duration outages. Problems also include steady state and intermittent phenomena, such as harmonic distortion and voltage flicker. This wide range of conditions makes the development of standard measurement and analysis procedures difficult. It is therefore beneficial to characterize power quality measurements and their related problems into common categories using standard data formats.
This case study includes a summary of power quality categories and characteristics. These characteristics and attributes are useful for evaluating measurement equipment requirements, system characteristics affecting the power quality variations, and possible measures to correct power quality problems.
This case study also includes an introduction to the commonly used data interchange formats PQDIF and COMTRADE and a collection of representative waveforms for various power system fault and power quality events. Each waveform includes a brief summary regarding the source (e.g., measurement or simulation), cause, related utility or customer problem, and common solution. Keywords and slang terms that should be avoided are also included with each waveform.
REFERENCES
IEEE Std. 1159-1995, IEEE Recommended Practice on Monitoring Electrical Power Quality, ISBN 1-5593-7549-3.
IEEE Std. 1159.3-2003, IEEE Recommended Practice for the Transfer of Power Quality Data, ISBN 0-7381-3578-X.
IEEE Std. C37.111-R2005, IEEE Standard for Common Format for Transient Data Exchange (COMTRADE) for Power Systems, ISBN 0-7381-1666-1.
Published by M.Sc Marko Pikkarainen, M.Sc Pekka Nevalainen, Dr. Pertti Pakonen, Prof. Pekka Verho, Tampere University of Technology, marko.pikkarainen@tut.fi
ABSTRACT
Two matters have strongly affected use of energy in Finland during recent years. The one is the greenhouse effect and the other is the energy price which has risen. These points have brought to the market many new devices that are more energy efficient, for example, heat pumps and energy saving lamps. Also the reduced manufacturing costs of devices have brought new kinds of loads available in the market. These new devices have changed the usage of electricity and usage of the distribution network. The distribution network has been designed to fulfil different kind of electricity usage need. That is why more power quality problems may occur in the distribution network in the future.
This paper will describe some power quality problems caused by the usage of ground source heat pumps and a wood splitter. This examination is based on the measurements that have been carried out in the real distribution network at low voltage level in Finland.
The examination showed that power quality problems may appear when using these typical new loads. Especially one big problem is a starting current of a wood splitter. Because of high starting current the voltage will drop and it may cause flicker.
I. INTRODUCTION
During recent years, the use of energy has changed because new kinds of loads have been connected to the distribution network. Three main issues have driven this change of loads. One is the green house effect and second is the energy price, which has risen, and third is the reduced manufacturing costs of devices.
The main act in preventing the green house effect and global warming is to decrease carbon dioxide emissions. Because of this more and more loads, which are reducing carbon dioxide emissions, have entered the market. For example compact fluorescent lamps are one group of such loads. Replacing incandescent bulbs with compact fluorescent lamps is one such act that should reduce carbon dioxide emissions because of the efficient light produce of compact fluorescent lamps compared with incandescent bulbs. The result of European Commission Regulation number 244/2009 is that incandescent bulbs will be gradually phased out from the market. [1, 6]
The increasing energy price has affected the use of energy so that customers have invested in devices which reduce costs and energy consumption. One good example of this kind of behavior is to replace an oil heating system with a heat pump or to add an air-to-air heat pump in complement electric heating. In Finland support from government has speeded up this change [4]. Figure 1 shows the number of installations of different types of heat pumps in Finland during the years 1996-2008. As shown in Figure 1 the number of heat pumps has grown very rapidly during past few years [2]. The growing trend has been the same all over Europe. The overall percentage of heat pumps of all heating types is not very massive in Europe but for example in Sweden heat pumps are the most common heating system in single-family houses with an approximately 34 % share of all. [3]
Figure 1. Number of installations of different heat pump types in Finland during years 1996-2008.[2]
Different heat pumps may have different effects on electrical energy consumption and to the way electricity is used. For example, if an oil heating system is replaced with a ground source heat pump, the overall consumption of electrical energy of a house will increase, but if an air-to-air heat pump is added to complement electric heating the consumption will decrease. However in both cases if the earlier consumption of the primary energy source is compared with the new consumption of electrical energy the consumption is decreased because most of the heating energy of heat pump is coming from ground or from air. Because of decreased overall primary energy consumption also carbon dioxide emissions are decreased depending on how electricity is produced. The greater usage of fossil fuels in electricity production the greater cutting can be achieved using heat pumps.
Heat pumps are a good example of loads which have become more common because of technical development and reduced manufacturing costs. Nowadays in Finland an air-to-air heat pump costs about 1200-3500 € including installation which is quite feasible price in Finland [5].
Ground source heat pumps cost more because it will need a ground circuit and the device is bigger in the power scale. Another example of a load which is becoming more and more common because of a cheap price is a wood splitter. Wood splitters are used for splitting thick woods to smaller ones so that woods can be used in fireplaces or in sauna stoves. Especially the cheaper versions of wood splitters that are designed for regular customers are very tempting devices because of the easiness of wood splitting.
These new loads are changing the use of electrical energy. Even though these loads may have a favorable effect on overall energy consumption some power quality problems may occur when using these loads. This is mainly caused by the new electrical characteristics of these loads. The planning principles of distribution networks are becoming old-fashioned and do not always fulfill the requirements of these new loads. From the power quality point-of-view the trickiest part will be the commonness of these loads because it means that power quality problems are also becoming more common.
This paper will study some power quality problems caused by the use of ground heat pumps and wood splitters. Devices have been selected to this study based on power quality complaints received by one distribution utility. The study is based on practical case study measurements which were carried out in real distribution networks in Finland. In the paper there is first a theoretical examination about power quality problems and previous mentioned loads. This is followed by a description of case study measurements carried out. Finally there are results and conclusions of those measurements.
II. THEORETICAL BACKROUND OF POWER QUALITY
Power quality is defined as “Set of parameters defining the properties of power quality as delivered to the user in normal operating conditions in terms of continuity of supply and characteristics of voltage (symmetry, frequency, magnitude, waveform)” [7]. In this paper, we are observing power quality in terms of quality of voltage. The limits for voltage quality are defined in standard EN 50160 Voltage characteristics of electricity supplied by public distribution networks. The standards object is to define and describe characteristics of the supply voltage concerning: frequency, magnitude, wave form and symmetry of the line voltages. “These characteristics are subject to variations during normal operation of the system because of changes of load, disturbances generated by certain equipment and the occurrence of faults which are mainly caused by external events”. Variation of the characteristics is random in time and location. Therefore on small number of occasions the limits can be expected to be exceeded. [8]
The standard EN 50160 defines the characteristics of voltage in low voltage and medium voltage networks [8]. Because we are interested power quality problems caused by devices which are connected to low voltage networks only the characteristics of voltage in low voltage networks are considered in this paper. The examination is focusing on a flicker caused by rapid voltage changes, voltage levels and voltage dips because those are a very common cause for power quality complaints as shown in Figure 2. Figure 2 presents the distribution of power quality complaints in one distribution utility in Finland during the years 2003-2005. About 70 % of all power quality complaints were caused by voltage changes.
Figure 2. Distribution of power quality complaints in one distribution utility in Finland during years 2003-2005 [9]
The other reason to focus only on flicker and voltage levels, when studying power quality, is that devices very commonly cause these disturbances. This mainly happens because a device requires power to operate. In addition voltage, a device will need current from the grid. This current will cause a voltage change over the impedance of a distribution network. This is seen in Equation 1. Depending on the connection of a device it may have different effects to phase voltages. If the connection is a three-phase connection and connection is symmetrical, all phase voltages will experience the same kind of effect voltage drop or voltage rise. If the connection is a single-phase connection, every phase will experience different kind of voltage change because of the star point displacement for example one could rise and the others drop.
where
ΔU = vector of voltage change ZN = vector impedance of distribution network IDEV = vector device current
The standard EN 50160 defines permitted levels for a flicker so that 95 % of long term flicker severity in any week should be lower or equal than 1. For voltage levels, standard defines that 95 % of the 10 min mean r.m.s. values of the supply voltage shall be within the range of Un ± 10 % during each one period and all of 10 min mean r.m.s. values shall be within range -15 % < Un < +10%. Exceptions for voltage levels can appear for example in remote areas with long feeder lines or not connected to a large interconnected network. In these cases voltage levels could be outside previous mentioned range but a customer or user should be informed of the conditions. [8]
III. THEORETICAL BACKROUND OF HEAT PUMP AND WOOD SPLITTER
In both devices, heat pumps and wood splitters, the source of power comes from induction motor. This component is the most significant component in these devices from the current usage and power quality point-of-view. In wood splitters the motor is usually a single phase induction motor. In heat pumps the motor can be either a single phase or a three phase induction motor. The connection type varies with different type of heat pumps. In bigger heat pumps, ground source heat pumps and air-to-water heat pumps, the induction motor is three-phase connected. In others heat pumps the induction motor is typically single-phase connected, because those heat pumps are smaller.
The biggest impact of an induction motor on the distribution network appears when the motor is started. The induction motor takes a high starting current. High starting current causes voltage change over the impedance of the distribution network. This phenomenon is seen in Figure 3 and equation 2. Figure 3 a presents an equivalent circuit of a polyphase induction motor and figure 3 b presents an approximate equivalent circuit of a polyphase induction motor. The approximate equivalent circuit is based on assumptions that the reactive component of impedances z1 and zm is much greater than the resistive component and the voltage E2 is only little smaller and nearly in the same phase with voltage V. These assumptions are valid in conventional induction motors in the normal running range. Equation 2 can be defined from figure 3 b. From Equation 2 can be seen that when the slip of the induction motor is small the motor takes a high current from a network. The slip is 1 at the moment of starting the induction motor and will decrease close to 0 after a starting. Single phase induction motors have a different equivalent circuit and equation for current taken from network compared to polyphase induction motors. Nevertheless the high starting current effect is similar to that in polyphase motors. [10]
Figure 3. a) Equivalent circuit of three-phase induction motor b) Approximate equivalent circuit of a three-phase induction motor [10]
r1 = resistance of stator x1 = leakage reactance of stator xm = magnetizing reactance r2 = resistance of rotor x2 = leakage reactance of rotor s = slip of motor
The starting current causes remarkable power quality problems when using induction motors because current reaches highest value at the beginning of start up and it won’t fluctuate much during normal operation. From power quality point-of-view, the critical factor is how often the motor is needed to start up. If the start up frequency is very high more voltage changes will appear. For the heat pumps length of the running cycle depends on the need of the heating energy, the dimensioning and parameter settings of a heat pump. If the heating power demand is close to the nominal heating power of the heat pump, the pump may run long times continuously. If the heating power need is clearly lower than the nominal heating power of the heat pump, stopping and reclosing of the pump will appear. The time between the stopping and reclosing the pump depends on the restrictions of the process and, for example, for one ground source heat pump the shortest time between stopping and restarting the process is 10 minutes according to a heat pump supplier.
For wood splitters starting up frequency varies depending on the operation logics of the device. Basically there are two operation logics to move hydraulic piston of the wood splitter. One is to perform all piston movements with hydraulic control when the induction motor is running. The other is to do the pressing with a hydraulic control when the motor is running and the backward movement with a spring when the motor is stopped. Second logic means that the repetition frequency of starting ups will be very high. In our measurements we had a wood splitter that needed to start up again every time a new wood was split.
IV. DESCRIPTION OF PRACTICAL CASE MEASUREMENT STUDY
In our study, two practical case measurements were made in Finland one in Lempäälä and the other in Tampere. In both cases the scope and the environment were bit different. Dranetz PX-5 power quality analyzer and Dranetz 4400 power quality analyzer were used as measuring devices in our study. In this chapter both practical case measurements will be described in depth.
Lempäälä rural area network
In Lempäälä the scope of the study was to explore power quality problems caused by a wood splitter in a rural area network. The wood splitter was single phase device and the nominal power of induction motor of the wood splitter was 2.2 kW. Operation logic of this wood splitter was that it needed to start up again every time a new wood was split. Feeders in this low voltage network were mainly aerial bundled cables called AMKA with cross-sections from 70 mm2 to 35 mm2 and the rated power of the 20/0.4 kV transformer feding the low voltage network was 200 kVA. In this low voltage network computational single phase short circuit currents at customers varied from 1400 A to 148 A. We decided to study power quality problems caused by a wood splitter in three locations in which measured single phase short circuit currents were 146 A, 275 A and 350 A at customer supply terminal. There were 10 m extension cord between a customer supply terminal and a plug point of a wood splitter so single phase short circuit currents were 136 A, 200 A and 233 A at a plug point of device. Figure 4 A shows an overall picture of low voltage network in Lempäälä. Measuring locations were situated along feeders 1 and 2. When a wood splitter was operated measurements were made in three places: one in a plug point of device, one in a customer supply terminal and one near the transformer. In addition when the wood splitter was operated in the location where the short circuit current was 146 A measurements were performed also in the location with a short circuit current of 275 A because those were located along the same feeder. Measured quantities were voltage and current waveforms and quantities defined in standard EN 50160 with an exception that a measuring period of short term flicker severity was 5 min.
Figure 4. A) Overall picture of a low voltage network in Lempäälä. B) Overall picture of the low voltage network in Tampere with measuring point locations
Tampere urban area network
In Tampere the scope of the study was to explore power quality problems caused by ground source heat pumps in an urban area network. Feeders in this low voltage network were mainly underground cables with cross-sections from 300 mm2 to 120 mm2 and the rated power of 20/0.4 kV transformer was 315 kVA. In this low voltage network computational single phase short circuit currents varied from 9,7 kA to 445 A and three phase short circuit currents varied from 10.8 kA to 1110 A so the network was quite strong. At the one end of this low voltage network two terrace houses changed their shared oil heating system into a separate ground source heat pump systems. The installation was made so that two ground source heat pumps, nominal heating powers 25 kW and 36 kW and maximum electrical powers 9,9 kW and 13,2 kW, were installed to both terrace houses. Starting of all heat pumps was direct on line starting so every time heat pumps were started a high starting current appeared. This place was selected to this study because some customers in both terrace houses complained about flicker. Overall picture of low voltage network, customer supply terminal short circuit currents and measuring points are illustrated in figure 4 B. Measured quantities were voltage and current waveforms and quantities defined in standard EN 50160.
V. PRACTICAL CASE STUDY RESULTS
This chapter presents the results of practical case studies. This chapter is headlined similarly as the previous chapter so that it is easy to follow results.
Lempäälä rural area network
Power quality problems caused by a wood splitter were remarkable at the customer end. Every wood splitting produced high starting current compared with the short circuit current and a remarkable voltage dip in the phase in which the wood splitter was connected at the customer supply terminals. This is why the biggest problems appeared in flicker and in number of voltage dips. The waveform and the RMS value of a starting current of one start up of a wood splitter are illustrated in Figure 5.
Figure 5. Waveform and RMS value of one start up of a wood splitter in place where a short circuit current at customer supply terminal was 275 A
It was detected that phase voltages of other phases than the phase in which the wood splitter was connected were raised at a customer supply terminal. This is due to a star point displacement of low voltage network when using single phase devices as predicted in Chapter 3. This effect is illustrated in Figure 6. Figure 6 shows phase voltages at customer supply terminal during start up of a wood splitter. Because of this effect power quality problems also appeared in other phases than the phase where a wood splitter was connected. Also Figure 6 shows that the phase voltage in connection phase drops so dramatic that every start up produced a voltage dip according to standard EN 50160 [8].
Figure 6. Phase voltages in place where short circuit current was 275 A at a customer supply terminal during start up of a wood splitter
Overall network impact results of practical case study in Lempäälä at customer supply terminal are summarized in Table 2. In measurements 1 and 3 the wood splitter was connected in phase L2 and in measurement 2 the wood splitter was connected in phase 1. Table 2 shows very dramatic short term flicker severity index increase at customer supply terminal in every phase when using the wood splitter at a customer installation. This means very annoying flicker and it also means that even short use of a wood splitter will exceed the limit Plt=1 in long term flicker severity calculated with definition in standard EN 50160 [8]. It should be noticed that an electrical chainsaw was used at same time when the wood splitter was operated. This increased little a short term flicker severity index. The effect of an electrical chainsaw to phase voltage was clearly smaller than the effect of a wood splitter.
Table 2. Overall results of a practical case study in Lempäälä
In addition of Table 2 results it was noticed that when the wood splitter was operated in measurement place 1 also power quality problems were recorded in measurement place 2. Geographical distance between these two places was 250 m. Operation of the wood splitter raised the short term flicker index of the other measurement place up to 7,2 in phase where a wood splitter was connected and up to 2,8 and 1,1 in other phases even though a wood splitter was operated in measurement place 1. Even though remarkable power quality problems appeared at customer end no power quality problems appeared at transformer. One way to prevent power quality problems mentioned in this chapter is to use only wood splitters of which piston movements are controlled with hydraulic control while the induction motor is running continuously.
Tampere urban area network
In Tampere four heat pumps were operated in same low voltage network. The measurement period was one week. During this period the mean temperature of a day was 3…9 °C and at night the temperature fell under 0 °C. This meant that heating power need was not near heating capacity of pumps so start ups and stops of pumps should appear. Installations of pumps were made so that a bigger pump heated only a water circulation of radiators. A smaller pump heated mainly use water but could support a bigger pump to heat a water circulation of radiators. Because of a direct on line start up of pumps high starting currents were detected. Starting currents of bigger pumps were 220…230 Arms and duration approximately 4 cycles. Starting currents of smaller pumps were 185…195 Arms and duration approximately 3 cycles.
Running cycle of pumps varied between terrace houses. Bigger pump of customer 1 ran typically from 1 h to 3 h 20 min. Time between two start ups varied from 1 h 40 min to 4 h 10 min and the average time between two start ups was 2 h 20 min. The total number of start ups was 73. Bigger pump of customer 2 ran typically from 50 min to 11 h. Start up times of bigger pump of customer 2 varied from 2 h 20 min to 12 h 10 min and the average time between two start ups was 3 h 50 min. The total number of start ups was 43. In both cases the biggest running times appeared at night time and shorter ones at day time. This is due to bigger heating need at night time. The differences between running times of bigger pumps could result from different size of houses and different heating system specifications.
Because the current measurements were placed in the common feeder of two different sized heat pumps, running times of smaller pump were difficult to determine. Start up times could still be determined. For the smaller pump of customer 1 time between two start ups varied from 19 min to 2h 10 min and the average time between two start ups was 39 min. The total number of start ups in one week was 255. For the smaller pump of customer 2, time between two start ups varied from 24 min to 1 h 40 min and the average time between two start ups was 30 min. The total number of start ups in one week was 371.
The total number of different heat pump start ups at customer 1 in each hour during one week is shown in figure 7. The figure 7 presents the number of start ups so, that if a smaller pump of customer 1 has started up once every day between 8.00 am and 9.00 am the number for smaller pump of hour 8.00 will be 7. As shown in figure 7, start ups of the smaller heat pump most commonly occurred at evening and day time. At night time, the number of start ups of smaller pump decreased significantly. This happened because more water was used at day time and evenings than at night time. For the bigger pump there was no specific time for start ups, only there was less start ups at morning. This occurred because heat pump ran longer at morning. This was because of bigger heating power need due to decreased outside temperature. Because of this kind of distribution in start up times, there was lower long term flicker severity index at night than day time. The key issue of high short term flicker index was bigger pump start up time and common start ups in the same 10 minute period for both heat pumps of customer 1. Also if there was common start up in the same 10 minute period for heat pumps of customer 2, it raised short term flicker severity index above 1, which is the irritation threshold.
Figure 7. The total number of different heat pump start ups in each hour of customer 1 during one week
The total number of heat pump start ups at customer 2 in each hour during one week is shown in figure 8. There were more start ups of smaller pump for customer 2 than customer 1. The number of start ups remained quite high all day. There were only little less start ups at night time than at day time. For the bigger heat pump there were only a few start ups during whole week. This was because of long running times of the bigger heat pump. The long term flicker severity index got higher values at evening than at night time because of this kind of distribution of start up times of different heat pumps. For high short term flicker severity index the key issues were the same for customer 2 as for customer 1. The start up of bigger heat pump at customer 2 meant higher short term flicker severity index at customer 2 and if there was start up of both bigger and smaller heat pumps at customer 2 in the same 10 minute period short term flicker severity index got even higher. Also if there was start up of both heat pumps at customer 1 short term flicker severity index increased above 1. In Table 3 short term flicker severity indices are summarized at different customer ends when different heat pumps started up. Short term flicker severity indices are average 10 minute values of events mentioned in table 3. Table 3 shows previously mentioned cross disturbance from start up of heat pump of one customer to the short term flicker severity index of the other customer. Table 3 also shows the greater effect of the bigger heat pump start up to the short term flicker severity index.
Figure 8. The total number of different heat pump start ups in each hour of customer 2 during one week
Table 3. Average values of short term flicker severity index at different customer end when different heat pump is or pumps are started up
Even though there were sometimes relatively high short term flicker indices the long term flicker index never exceeded the level Plt = 1 during one week. The highest long term flicker severity index was 0.99 at customer 1. The standard EN 50160 defines the threshold level to the flicker so that “under normal operating conditions, in any period of one week the long term flicker severity caused by voltage fluctuation should be Plt ≤ 1 for 95 % of the time”. This also means that threshold defined in the standard was not exceeded.
Despite the fact that flicker did not exceed the threshold level of standard EN 50160, flicker from heat pump start ups was clearly visible. The start ups of heat pumps were easily seen from lighting. Some customers can be irritated from this kind of rapid voltage changes. Now in the distribution utility point of view it is easy to say, that no problems occurred and case is closed. But in the customers point of view there might be flicker problems so the result “there cannot be flicker” is not a good answer from customer service point of view. In this case the result was that the supplier of all these heat pumps will install softstarters to heat pumps.
VI. CONCLUSIONS
In this paper there were examinations about power quality problems caused by loads that are coming more and more common. The examination is based on practical case measurements made in real distribution network in Finland. Two groups of loads were selected to this examination: wood splitters and heat pumps.
Wood splitter, which nominal power of induction motor was 2.2 kW and which was single phase device, was selected to this examination. This wood splitter was operated in different locations in one rural area low voltage network in Finland. In this network calculated short circuit currents varied from 1.4 kA to 146 A. Measurement places were selected so that measured short circuit currents were 148 A, 275 A and 350 A at customer supply terminals. In these places the wood splitter caused lots of flicker problems because of a high starting current and because of operation logic which caused lots of start ups of induction motor of the wood splitter. Flicker problems occurred in all three phases because of the star point displacement due to operation of a single phase device. Flicker problems caused by the wood splitter extended also to nearby customers along the same feeder. One way to prevent these flicker problems is to accept in the market only wood splitters of which piston movements are controlled with hydraulic control while the induction motor is running continuously. In such a case there will be less start ups of the induction motor.
Power quality problems caused by heat pumps were studied in urban area network in Tampere. In urban area network there was four big heat pumps installed into two terrace houses. Here start ups of heat pumps caused short term flicker severity index increasing over irritation threshold 1. Also start ups of heat pumps in one terrace house increased the short term flicker index over 1 at the other terrace house so there was cross disturbance from one terrace house to the other. Even though the short term flicker index was sometimes over 1 the long term flicker index was always under 1 so from the standard point of view there was no flicker problem. Despite this every start up of heat pumps could easily be seen from lighting so someone could feel this to be irritating. In this case the result was that the supplier of the pumps will install softstarters to all heat pumps.
Published by Electrotek Concepts, Inc., PQSoft Case Study: General Reference – Using Simulations and Measurements in PQ Analysis, Document ID: PQS0309, Date: April 16, 2003.
Abstract: Simulations provide a convenient means to characterize power quality problems, predict disturbance characteristics, and evaluate possible solutions to problems. They should be performed in conjunction with monitoring efforts and measurements for verification of models and identification of important power quality concerns. The models required for the simulations depend on the system characteristics and the power quality variations being evaluated.
The specific power quality concerns that need to be evaluated will be different for each customer. A review of the electrical configuration, protection practices, types of equipment used by the customer, process requirements, and economic impacts of problems will lead to a list of concerns that need to be studied.
USING SIMULATIONS AND MEASUREMENTS IN POWER QUALITY ANALYSIS
Power quality concerns that need to be evaluated will be different for each utility and customer configuration. A review of the electrical configuration, protection practices, types of equipment used by the customer, process requirements, and economic impacts of problems will lead to a list of concerns that need to be studied. These concerns can include possible problems with both the utility system and customer facilities. Possible power quality problem categories include:
Voltage transients caused by circuit switching and load switching within the customer facility.
Harmonic distortion from the application of adjustable-speed drives or other nonlinear loads.
Transformer heating caused by harmonic current levels.
Transient voltage magnification at low voltage capacitor banks.
Sensitivity of adjustable-speed drives (ASDs) and control systems to utility capacitor switching transients.
Transients and notching associated with power electronic equipment operation.
Neutral conductor overloading due to harmonic producing loads in commercial installations.
Voltage flicker from arc furnace loads and arc welding loads.
Voltage sags due to faults on parallel circuits on the same distribution system or faults on the transmission system.
Momentary interruptions at industrial and commercial installations due to recloser operations on feeder circuit breakers.
Coupled voltages in customer facilities due to lightning transients on the primary distribution systems.
Identification of the particular concerns involved for an installation provides a focus for a simulation-based study. Development of a model for analysis of the problem is dependent on the frequency range of the power quality variations that need to be studied. The model can be for computer simulations, hand calculations, or application of simple rules. For example, analysis of voltage sags often requires modeling that includes the utility transmission system, while analysis of high frequency transients might only require a model for a very local part of the customer facility.
Monitoring requirements are also based on the particular concern involved. If harmonic distortion is a concern, monitoring of steady-state conditions with a harmonic analyzer is required. Analysis of disturbances requires a disturbance monitor. The duration of monitoring depends on how often the problems occur. Some problems with voltage sags or momentary interruptions might only occur a few times per year due to faults on the transmission system, while problems caused by capacitor switching might occur every day. Other voltage variations of interest will typically fall somewhere between these extremes.
Data Collection Process
A representation of the customer system and important parts of the utility system should be developed for preliminary analysis. This model can be used for preliminary simulations or analysis to predict power quality problems and evaluate possible solutions to problems. In cooperation with the customer, the data for the model is collected and compiled into a database for convenient reference during the study.
Important information includes:
1. Utility system characteristics: − Primary voltage − Short circuit / load levels − Feeder configuration and characteristics – underground/overhead − Transformer ratings / connections / impedances − Protection practices, switching procedures − Capacitor applications (locations, sizes, switching method and controls) − Arrester sizing and placement
3.Power conditioning equipment: − Surge suppressors (arresters, varistors, etc.) − Isolation transformers − Constant voltage transformers − Voltage regulators / power conditioners − UPS systems − Harmonic filters − Custom power devices (utility distribution system)
Computer Simulation Process
Simulations provide a convenient means to characterize power quality problems, predict disturbance characteristics, and evaluate possible solutions to problems. They should be performed in conjunction with monitoring efforts and measurements for verification of models and identification of important power quality concerns. The models required for the simulations depend on the system characteristics and the power quality variations being evaluated. The simulations fall into three major categories:
− Transient simulations are often performed using the Electromagnetic Transients Program (EMTP). There are a number of different versions and platforms available. This is a valuable tool for analysis of circuit switching operations, capacitor switching, lightning transients, and transients associated with power electronic equipment operation.
The most widely used program for transient analysis is the EMTP. This program is used to simulate electromagnetic, electromechanical, and control system transients in multiphase power systems. It was originally developed by Hermann Dommel at Bonneville Power Administration (BPA) during the late 1960’s. Since then, there have been significant developments by groups all over the world.
The EMTP is a general-purpose computer program for simulating high-speed transient effects on electric power systems. The program features an extremely wide variety of modeling capabilities encompassing electromagnetic and electromechanical oscillations ranging in duration from microseconds to seconds. Examples of its use include switching and lightning surge analysis, insulation coordination, shaft torsional oscillations, ferroresonance, and HVDC converter control and operations.
The EMTP is used to solve the ordinary differential and/or algebraic equations associated with an “arbitrary” interconnection of different electrical and control system components. The implicit trapezoidal-rule (second order) integration is used on the describing equations of most elements that are modeled by ordinary differential equations. The result is a set of real, simultaneous, algebraic equations, which is solved at each time-step. These equations are written in nodal-admittance form, and are solved by ordered triangular factorization.
Studies involving use of the EMTP have objectives that fall in two general categories. One is design, which includes insulation coordination, equipment ratings, protective device specification, control systems design, etc. The other is solving operating problems such as unexplained outages or equipment failures.
Currently, primary development and support for member utilities is coordinated by EPRI and the Development Coordination Group (DCG). Additional DCG/EPRI EMTP information may be obtained from: http://www.emtp96.com/.
Another version of the EMTP is the Alternative Transients Program (ATP). The ATP is the royalty-free version of the EMTP. ATP is distributed by the ATP User’s Group for your country. ATP licensing is free, and requires only that you agree to the licensing terms. Additional ATP information may be obtained from: http://www.ee.mtu.edu/atp/ and
− Harmonic investigations are typically performed using steady-state analysis techniques at the individual harmonic frequencies. In general, harmonic-producing loads can be modeled as harmonic current sources, and the simulations used to predict harmonic voltages and currents throughout the customer and utility systems. Overloading of neutral conductors, transformer heating considerations, resonances caused by capacitor applications, and harmonic currents injected onto the utility system can be evaluated in the simulations.
SuperHarm, a harmonic simulation program developed by Electrotek Concepts, Inc. as part of the EPRI HarmFlo+ Workstation, uses a steady-state frequency domain analysis. The solution technique is a direct admittance matrix solution. This method requires that the system admittance matrix be solved for each harmonic of interest. A nonlinear device is modeled as a shunt-connected constant current source. The magnitude and angle of current injected at each harmonic frequency is determined with a Fourier transformer of the device’s line current waveform. SuperHarm allows the user to input harmonic current sources without concern for the phase angle between the bus voltage and the fundamental current.
Most harmonic simulation programs in use throughout the world use the admittance matrix approach similar to the one utilized in SuperHarm. Examples include V-HARM (Cooper Power Systems), CYMHARMO (Cyme) and HARMZW (CEPEL).
− Variations in the fundamental frequency voltage can be evaluated with conventional steady-state analysis tools. Power flow programs give system voltages as a function of load levels on the system. Fault programs (short circuit analysis) can calculate system voltage profiles during fault conditions for analysis of voltage sag concerns.
Computer programs used to solve power flows are divided into two types – static and dynamic (real time). Most power flow studies for system analysis are based on static network conditions. Real time power flows are primarily used for optimization of generation, VAr control, dispatch, losses, and tie line control.
A power flow solution gives the voltages at all buses and the power flow in all branches for a given set of operating conditions. It represents a steady state in which the influential parameters are in balance and a solution has been found. A power flow study is a collection of such solutions made when certain equipment parameters are set at different values.
Power flow/stability program vendors include EPRI, PTI (PSS/U), BPA/WSCC, Power Computing Associates, General Electric, Philadelphia Electric, ABB, and CYME.
There are numerous short circuit programs that can be used for power quality investigations. The full list given above for power flow/stability programs also provides short circuit programs. An addition to the list is ASPEN OneLiner. This program includes a special feature that allows convenient evaluation of voltage sag area of vulnerability.
In general, a process of developing a simple system model and working towards a more complete (and often more complex) model for an overall analysis yields the best results. A simplified system model allows the user to develop an understanding of the phenomena of interest. In addition, data verification is more efficient using this method. Once an understanding has been developed and the initial model has been verified, the user can then expand the system model to include more components. The overall study is then performed and solutions to the particular problem may be developed and analyzed.
Monitoring Process
The utility and customer systems being evaluated should be monitored to characterize the power quality variations and to verify the analytical models developed for simulations. The measurement program should be designed based on initial simulation results and on the particular sensitive loads existing at the customer facility. Monitoring will typically be performed on the feeder, at the customer service entrance, and close to the sensitive load. This will permit characterization of disturbances originating on the utility system and disturbances that are localized at the load. A measurement program plan should be developed which specifies:
− quantities to monitor − monitoring durations − threshold levels which will trigger recording of disturbances − waveform sampling and data storage requirements − analysis procedures and data presentation formats
Available monitoring instruments should be evaluated for the measurements required. The problem of obtaining adequate representation of both harmonic and transient conditions must be addressed in particular, if both of these concerns exist at a facility.
A customer site survey should be part of the measurement program design. The site survey should characterize the wiring and distribution system integrity and provide basic information about circuit and equipment loading. The site survey should also include discussions with facility personnel regarding characteristics of equipment problems and known customer system conditions at the time power quality variations have occurred.
The actual monitoring effort requires close cooperation between the customer and utility personnel. Monitoring sites and instrumentation should be selected based on the particular concerns being characterized. The duration of monitoring effort will depend on the parameters that can affect the power quality concerns. It is likely that the customer will need to be responsible for verifying that the monitor is operating properly on a day-to-day basis. The monitoring results should be compiled and analyzed for verification of analytical models and to provide a concise description of the possible concerns.
Selecting the Appropriate Monitoring Equipment
Power quality problems encompass a wide range of disturbances and conditions on the system. They include everything from very fast transient overvoltages (microsecond time frame) to long duration outages (hours or days time frame). Power quality problems also include steady-state phenomena such as harmonic distortion, and intermittent phenomena, such as voltage flicker. This wide variety of conditions that make up “power quality” makes the development of standard measurement procedures and equipment very difficult. Table 1 indicates the equipment requirements for identifying and monitoring specific power quality problems.
Table 1 – Equipment Requirements
Instrument Types
Although instruments have been developed that measure a wide variety of disturbances, a number of different instruments are generally necessary, depending on the phenomena being investigated. Basic categories of instruments that may be applicable include:
− Wiring and Grounding Test Devices − Multimeters − Oscilloscopes − Disturbance Analyzers − Harmonic Analyzers/Spectrum Analyzers − Flicker Meters
Transducer Requirements
Monitoring of power quality on power systems often requires transducers to obtain acceptable voltage and current signal levels. Voltage monitoring on secondary systems can usually be performed with direct connections but even these locations require current transformers (CTs) for the current signal.
Many power quality monitoring instruments are designed for input voltages up to 600Vrms and current inputs up to 5Arms. Voltage and current transducers must be selected to provide these signal levels. There are two important concerns that must be addressed in selecting transducers:
− Signal levels. Signal levels should use the full scale of the instrument without distorting or clipping the desired signal.
− Frequency response. This is particularly important for transient and harmonic distortion monitoring, where high frequency signals are particularly important.
EVALUATING RESULTS
The measurement results are analyzed in conjunction with the results of simulations to correlate customer problems with the utility system power quality levels. The initial measurements and the site survey are used to identify the phenomena involved and the important parameters. The subsequent measurement results are used to verify the model and characterize the actual power quality variations. Using this information, the model can then be used for more detailed simulations of possible solutions to the power quality problem. The simulations provide the means to evaluate a range of possible solutions from a technical point of view.
Once the range of technical solutions is identified, economic analyses need to be performed to evaluate the possible alternatives for solving customer power quality problems. These alternatives will generally include the following options:
− Power conditioning and/or filtering at the sensitive loads − Central power conditioning and/or filtering at the customer service entrance − Changing operating procedures or system design on the utility distribution system − Modification to the design of sensitive loads to make them less sensitive to power quality variations
The requirements for each of these options will be developed from the simulation effort and the analysis of measurement results.
Power conditioning in this case includes surge suppression, voltage regulation, and possibly backup for momentary interruptions. Harmonic filtering to solve harmonic problems can be applied either at individual loads or at the main service for a facility. Customer system design modifications, such as changing power factor correction procedures and equipment, can have an important impact on power quality variations. If particular loads are much more sensitive that other loads in the facility, either power conditioning at the particular load or design changes to the load equipment should be considered.
Momentary interruptions and voltage sags require careful consideration. Utility system modifications could include implementation of switching procedures to minimize transients associated with capacitor switching events or addition of current limiting devices to minimize the voltage sags that occur during faults on parallel feeders. The impact of protection practices on power quality levels experienced by customers should be evaluated carefully using both the analytical and measurement results.
SUMMARY
Simulations provide a convenient means to characterize power quality problems, predict disturbance characteristics, and evaluate possible solutions to problems. They should be performed in conjunction with monitoring efforts and measurements for verification of models and identification of important power quality concerns. The models required for the simulations depend on the system characteristics and the power quality variations being evaluated.
The specific power quality concerns that need to be evaluated will be different for each customer. A review of the electrical configuration, protection practices, types of equipment used by the customer, process requirements, and economic impacts of problems will lead to a list of concerns that need to be studied.
REFERENCES
Potential Transformer Accuracy at 60 Hz Voltages Above and Below Rating and at Frequencies Above Hz, D. A. Douglass, presented at the IEEE Power Engineering Society Summer Meeting, Minneapolis, MN, July 13-18, 1980.
Current Transformer Accuracy with Asymmetric and High Frequency Fault Currents, D. A. Douglass, IEEE Transactions on Power Apparatus on Systems, Vol. PAS-100 No. 3, March, 1981.
Transducer Performances for Power System Harmonic Measurements, C. J. Cokkinides, L. E. Banta, A. P. Meliopoulos, Proceedings of the International Conference on Harmonics, Worcester, MA, October 1984.
Computation of Current Transformer Transient Performance, IEEE Transactions on Power Delivery, Vol. PWRD-3 No. 4, October 1988.
Electrical Transients in Power Systems, Second Edition, Chapter. 18, A. N. Greenwood, John Wiley and Sons, New York, 1991.
RELATED STANDARDS IEEE Standard 1159 IEEE Standard 1346 IEEE Standard 1250 IEEE Standard 1036 IEEE Standard 519
GLOSSARY AND ACRONYMS ASD: Adjustable-Speed Drive CT: Current Transformer EMTP: Electromagnetic Transients Program HVAC: High-Voltage Air Conditioning MOV: Metal Oxide Varistor PF: Power Factor PWM: Pulse Width Modulation TVSS: Transient Voltage Surge Suppressors
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