Published by Electrotek Concepts, Inc., PQSoft Case Study: Utility Capacitor Switching Causes Nuisance Tripping of ASD, Document ID: PQS0319, Date: July 18, 2003.
Abstract: The application of utility capacitor banks has long been accepted as a necessary step in the efficient design of utility power systems. Also, capacitor switching is generally considered a normal operation for a utility system and the transients associated with these operations are generally not a problem for utility equipment. These low frequency transients, however, can cause problems for low voltage power electronic-based loads.
PWM ASDs are susceptible to dc link overvoltage trips caused by utility capacitor switching. In general, an increase in input inductance (choke or isolation transformer) will reduce the possibility of nuisance tripping. However, if the customer has power factor correction capacitors on the same bus, it may be necessary to take additional remedial actions.
PROBLEM STATEMENT
A food processing manufacturer was experiencing a problem with a small PWM adjustable-speed drive tripping off-line each morning at the same time. Drive diagnostics indicated an overvoltage trip condition.
The short term solution was to have a technician available to reset the drive each morning.
SYSTEM MODEL
The oneline diagram shown in Figure 1 illustrates the system model used to investigate the nuisance tripping problem. The following parameters describe the system:
System Source Strength: 200 MVA Switched Capacitor Size: 1200 kVAr Step-down Transformer Size: 1500 kVA Substation Load: 5 MW 480 Volt Load: 300 kW
Figure 1 – Oneline Diagram for Nuisance Tripping Case
Capacitor energizing operations, that cause a transient oscillation on the utility system, can cause an overvoltage trip condition on overly sensitive drives.
PWM ASD Characteristics
Pulse-width modulation (PWM) adjustable-speed drives (ASDs) typically have a voltage source inverter (VSI) type of design and use PWM inverters to supply the motor. Drives that use a voltage source inverter design are sensitive to utility capacitor switching transients due to the capacitor used in the dc link between the rectifier and inverter. The protection on the dc link capacitor is very sensitive to transient voltages on the ac power side. It is not uncommon for the dc overvoltage control to cause tripping of the drive whenever the dc voltage exceeds 1.2 per-unit (typical value). Figure 2 illustrates the ASD configuration.
Figure 2 – Block Diagram for a PWM ASD
The potential for nuisance tripping is dependent primarily on the switched capacitor bank size (utility), the dc link capacitor size (ASD), and the inductance between the two capacitors. Figure 4.10 illustrates the transient overvoltages associated with energizing the 1200kVAr substation capacitor bank. Energizing transient overvoltages are generally not a concern for utilities because their magnitudes are usually below the protective level at which surge protective devices operate.
However, these transients can be magnified at the customer facility if the customer has power factor correction capacitors. Even if the customer does not have capacitors, the transient on the low voltage bus can cause a surge of current, at a relatively low frequency, into the dc link capacitor. The current surge charges the dc link capacitor and causes a overvoltage to occur. If this voltage exceeds the overvoltage trip setting, the drive will shut down. This is often refereed to as nuisance tripping because the situation can occur day after day, often at the same time. Figure 3 illustrates the dc current and voltage surge for this case.
Figure 3 – Overvoltages Associated with Utility Capacitor Switching
The problem can be solved by adding an isolating inductance in series with the individual drives. This inductance can be in the form of a simple ac choke or an isolation transformer. Typically, a choke size of 3-5% is chosen. If the choke has too much impedance harmonic voltage distortion levels will increase (increased voltage drop due to harmonic current flowing through higher impedance). Figure 4 summarizes the impact of choke size on the dc overvoltage for a case where the switched capacitor bank (utility) was increased to 3000kVAr.
Figure 4 – Effect of Choke Size on dc Link Overvoltage
SUMMARY
PWM ASDs are susceptible to dc link overvoltage trips caused by utility capacitor switching. In general, an increase in input inductance (choke or isolation transformer) will reduce the possibility of nuisance tripping. However, if the customer has power factor correction capacitors on the same bus, it may be necessary to take additional remedial actions.
Additional actions may include configuring the power factor correction capacitors as harmonic filters. Power factor correction capacitors can create the potential for voltage magnification. This increased voltage transient can often be controlled using harmonic filters, thereby reducing the transient voltage to a level where the 3-5% choke is again effective.
An important point in this case is that the customer should not arbitrarily increase the choke size (above 5%) assuming it will continue to decrease the likelihood of nuisance tripping.
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.
Electrotek Concepts, Inc., Evaluation of Distribution Capacitor Switching Concerns, Final Report, EPRI TR-107332, October 1997.
RELATED STANDARDS IEEE Standard 1036-1992
GLOSSARY AND ACRONYMS ASD: Adjustable-Speed Drive PWM: Pulse Width Modulation MOV: Metal Oxide Varistor TVSS: Transient Voltage Surge Suppressors
Published by Marek KAŁUSKI, Marek MICHALAK, Mirosław PIETRANIK, Karolina SKRZYPEK, Monika SZAFRAŃSKA, National Institute of Telecommunications
Abstract. The paper presents some preliminary results of the in-situ measurements of disturbances that were observed in mine underground power network. The purpose of the measurements was to identify potential interfering signals that may occur in such and similar power networks and may be dangerous for electronic equipment powered from, or working in close proximity of such network.
Streszczenie. W artykule przedstawiono wstępne wyniki pomiarów in-situ zaburzeń obserwowanych w podziemnej sieci zasilającej kopalni. Celem pomiarów było zidentyfikowanie potencjalnych sygnałów zakłócających pracę innych urządzeń, które mogą wystąpić w takich i podobnych sieciach zasilających i mogą być niebezpieczne dla urządzeń elektronicznych zasilanych z takiej sieci lub pracujących w jej bezpośredniej bliskości (Zakłócenia w przemysłowych sieciach zasilających).
Keywords: power lines, disturbances, industrial environment, mines Słowa kluczowe: sieci zasilania, zaburzenia, środowisko przemysłowe, kopalnie
Comparison between disturbance and immunity levels
The level of disturbances observed in industrial environments and the required immunity of the devices expected to work in these environments are very often contradictory. Although it is believed that the fulfillment of the requirements according to standards concerning industrial environment (EN 61000-6-2: required levels of immunity and EN 61000-6-4: emission limits) should ensure compatible operation of all devices within a given environment, there are cases when this is not true. In some environments (for example underground mines) the problem of EMC was neglected for many years – there are many old machines that do not comply with standard industrial environment requirements and produce much bigger disturbances. Therefore there is significant risk, that devices introduced in the same environment will not be immune to these disturbances, even if they comply with EN 61000-6-2.
In current practice, many device manufacturers and future users of these devices present the opinion that meeting the aforementioned standard requirements is sufficient “to a peaceful sleep”. Well, it is not so!
It should be noted that the device expected to work in the industrial electromagnetic environment is tested in conditions much different from the environmental conditions in the destination of their workplace. In addition, tests are carried out “here and now” (in a short period of time), recording “continuous” disturbances, which in fact are present in the moment of measurements. In addition, one should note that in the real environment, there may be situations of simultaneous incidence of different disturbances, which present different characters, different ways of coupling etc., resulting in complex disturbance environment.
The EN 61000-6-2 [1] and EN 61000-6-4 [2] standards do not require observing the equipment in the long term, while there is a possibility of higher level disturbance appearance over a longer period of time. This particularly applies to immunity testing.
There are also significant problems resulting from the functional safety of devices and people safety in the environment where even rare events resulting from the disturbance (causing incorrect work of the device) may cause disastrous consequences. Occurrence of such situations when there is a possibility of high level disturbances appearance (occurring from time to time) must be related to functional safety and unanticipated consequences in real environment.
These considerations led the IEC to start work on finding new, more stringent requirements for the immunity of equipment working in special industrial environments. As a result, the new standard is being developed, EN 61000-6-7: “Electromagnetic compatibility (EMC) – Generic standards – immunity requirements for safety related systems and for equipment intended to perform safety-related functions (functional safety) in industrial environments”, for testing the immunity of the devices expected to work in the special industrial environments.
Power network is medium in which many electromagnetic events occur, for example switching on/off of high power devices can produce substantial problems and can provoke unwanted (and potentially dangerous) behaviour of equipment connected to the same power network. Most often these unwanted events are high-amplitude pulses of various duration. They can directly interfere with operations of various electronic control devices, or interfere with machine equipment, resulting in undesired reactions, that can even lead to very serious accidents.
Also sudden on/off operations of large inductive loads, which often occur in case of engines operating heavy lifts, turbines etc. can result in large pulses superimposed on the power waveform, with amplitudes exceeding significantly the peak value of nominal voltage in the power network. The fast transients visible on the slopes of voltage waveform are also observed with amplitudes of the order of few tens of Volts. In medium power and high-power networks the switching can result in the series of pulses of very short rise times (several ns), which can provoke substantial danger for microprocessor driven equipment.
The team of National Institute of Telecommunications, EMC Laboratory in Wroclaw conducted a series of power quality measurements in one of copper mines, both on and underground [4]. Measurements were done with power quality analyzer connected, for different periods of time, to power line connections of different mine machines (for example lifts or conveyor belt engines). Particular attention was paid to the on/off operation incidents and what phenomena connected with these operations can be recorded in terms of power quality.
Fig. 1 shows voltage changes in power line cables of ventilator engine of high effectiveness. Presented is max hold diagram of 40 sample measurements performed in 200 ms intervals. The amplitude of fast changes (in time period of about 100 μs) of voltage waveform exceeds 800 V.
The other phenomena typical for industrial power networks are strong distortions of power waveforms, and consequently the occurrence of harmonic distortions, see Fig. 2. Such disturbances are caused particularly by nonlinear equipment, such as for instance thyristor converters, often used in the lift machine circuits. The associated disturbances can particularly be seen in the lower frequency ranges.
Fig. 1. Voltage changes in power line cables.
Fig.2. Example of current waveform measurement for one of the phase lines of power inverters (1450 rotations/minute) and harmonic distortions. Symbols shown are according to standard EN 61000-4-30
During the tests in underground mines, the team of National Institute of Telecommunications collects information about the electromagnetic environment in the real operating conditions of the mining plant. These measurements are not performed on separated plants or mines for experimental studies, these tests have been done during copper exploitation in real life active mine. The most relevant research concerning the disturbances in electromagnetic environment in the mine turned out to be conducted disturbances – it is the result of the fact that, in most mines, power network is of the “soft” type to which devices, that generate significant transient conditions, are connected.
The measurement of conducted interference in LV industrial power network can only be performed using RF current probe. The use of Artificial Mains Network (typical for laboratory measurements of conducted interference) is impossible for industrial plant (with work currents of several hundreds of Amps with the voltages of 500/1000 Volts, for some industries even higher). There are no AMNs commercially available that can be used in such conditions. Furthermore, in most cases it is impossible to insert the AMN into the power line of a working mine or at least it causes great difficulty. Therefore it is another reason for current probe usage – the simplicity of measuring set-up and ease of use in real world industrial plant environment; there is no need to break the power line, which is necessary in case of AMN. The alternative could be the use of voltage probes, but it can also be difficult in real world industrial plant applications, because of the impossibility of definitive description of reference ground plane in in-situ conditions (especially in mine tunnels) and usually the mains cords are isolated, so there is a problem of how to connect the probe tip to the tested line.
APD measurements of conducted disturbances
Because of disturbance characteristics that can appear in industrial plants power networks, it is justified and worth to use APD (Amplitude Probability Distribution) measurements. As of today, the APD is usually recommended to be used for frequencies above 1 GHz, for wideband digital systems measurements [3]. However, it is worth to draw attention to the fact that nowadays digital systems appear also quite often at the frequencies below 1 GHz, for example BPLC systems in frequency band below 30 MHz. These systems are often used to control big machines or as industrial plants telecommunication systems.
APD can be very useful for disturbance characteristics description in cases, when those disturbances are random and short in time. In those cases one can, in realistic amount of time, evaluate parameters of the disturbances and compare them with current limits, set for measurements with a quasi-peak detector.
APD measurement technique is basically the evaluation of the probability of disturbance level being higher than allowed limit (for radiated or conducted emissions). The measurements are performed after determining the most critical frequency for the evaluated situation.
It is worth mentioning, that long time data gathering using APD allows to determine given disturbance occurrence percentage and its strength (level). Tests of this type are especially important for disturbance connected risks evaluation when safety is concerned.
Measurements in real industrial environment – mine power network
Presented below are some real world measurement results of conducted disturbances measured in real industrial power network. The figures show the electromagnetic disturbances in power networks in relation to current standards. Such relation shows how important it is to provide adequate protection against disturbances of devices in unique environments such as underground mines.
On figures 3 and 4 some real world measurements of conducted emissions in mine power lines are presented in comparison with existing limits for disturbances in industrial environments. As can be seen on those figures, those limits are significantly exceeded, therefore a simple conclusion that there is a need for further tests and analysis of disturbances occurring in those environments. One can ask the question if the immunity requirements are enough to give required protection for equipment introduced to such special environments, especially in case of equipment connected with safety (for example gas sensors and meters).
As it was mentioned before, the moments of on/off operations of high power machines in industrial environment can produce big currents in power lines (see figures 5 and 6). Those currents can present some serious interference for other equipment connected to the same power network. The biggest threat can exist for signal lines as well as control and safety equipment.
Fig.3. Comparison of some real world disturbances with existing limits. Measured in underground conveyor control room in a working copper mine.
Fig.4. Comparison of some real world disturbances with existing limits. Measured in underground conveyor control room in a working copper mine.
Fig.5. The current changer in power line. In 3rd minute – start of the transportation engine.
In existing mines it is also common to arrange different line (power, control and signal) parallel and there is some serious crosstalk between lines. The result of this can be the occurrence in one line of disturbances whose source is in a different line – when disturbances from power line occur in signal or control line they can potentially present significant danger, especially when some safety equipment is connected to these lines (for example gas sensors or meters).
Fig.6. The current changes in the moment of switching the conveyor on, power network analyzer probe connected before inverter.
Conclusion
The description of electromagnetic events given in this paper does not form the complete characterization of all phenomena. Its objective is to draw attention to some disturbing events, observed in the real world, that should be taken into account when evaluating their potential impact on the equipment working in underground mines. Further works are needed to describe some special industrial electromagnetic environments such as the one of underground mines. In 2010 the scientific consortium was formed by AGH University of Science and Technology, Institute of Innovative Technologies EMAG, National Institute of Telecommunications, Institute of Power Systems Automation Ltd. The purpose of this consortium is to prepare such a description for the electromagnetic environment in mines.
REFERENCES
[1] EN 61000-6-2 “Electromagnetic compatibility (EMC) – Part 6-2: Generic standards – Immunity for industrial environments” [2] EN 61000-6-4 “Electromagnetic compatibility (EMC) – Part 6-4: Generic standards – Emission standard for industrial environments” [3] CISPR/A/WG2/(Yamanaka-Shinozuka)2003-01, June 2003 Guidance for applying APD method to the compliance test and developing APD limits [4] Kałuski M., Michalak M., Pietranik M., Skrzypek K. i inni, prace statutowe Instytutu Łączności nr Z21/21300026/1009/06, Z21/21300018/1184/08, Instytut Łączności, 2006-2008
Authors: mgr inż. Marek Kałuski, E-mail: m.kaluski@itl.waw.pl; mgr inż. Marek Michalak, E-mail: m.michalak@itl.waw.pl; dr inż. Mirosław Pietranik, E-mail: m.pietranik@itl.waw.pl; mgr inż. Karolina Skrzypek, E-mail: k.skrzypek@itl.waw.pl; mgr inż. Monika Szafrańska, E-mail: m.szafranska@itl.waw.pl; National Institute of Telecommunications, EMC Department, ul. Swojczycka 38, 51-501 Wrocław.
Source & Publisher Item Identifier: PRZEGLĄD ELEKTROTECHNICZNY (Electrical Review), ISSN 0033-2097, R. 88 NR 9b/2012
Published by Electrotek Concepts, Inc., PQSoft Case Study: Transient Voltages at an Industrial Facility Capacitor Switching, Document ID: PQS0505, Date: June 30, 2005.
Abstract: This case study describes concerns for capacitor switching transients within customer facilities. Magnification of transients initiated at higher voltage capacitors and tripping of sensitive variable frequency drive (VFD) loads are evaluated in particular.
The results of measurements and simulations associated with capacitor switching on the utility system and the resulting impacts at an industrial process plant are presented.
These results illustrate the important effect of customer load characteristics on the transients experienced within the facilities.
INTRODUCTION
Capacitor switching is considered a normal system event on the utility system and the transients associated with these switching operations are generally not a problem for utility system equipment. However, the transients can be magnified within customer facilities or can cause misoperation of sensitive electronic equipment, such as variable frequency drives (VFDs).
These concerns were evaluated in detail for a manufacturing facility. The transient overvoltages were simulated using the Electromagnetic Transients Program (EMTP) and field tests were performed to verify the modeling and evaluate the impact on actual plant loads.
Magnification of capacitor switching transients is perhaps the most important concern because the transient voltages can be very high and the energy levels associated with these transients can cause failure of electronic equipment and protective devices. Even without magnification, normal capacitor switching transients can cause problems with sensitive electronic loads. VFDs, in particular, can experience nuisance tripping for transient voltage magnitudes as low as 1.2 per unit.
This case focuses on field tests and simulations performed to evaluate the impact of switching capacitor banks on the transmission system supplying a facility and switching new capacitor banks installed at the distribution substation for the facility.
SYSTEM DESCRIPTION
The facility is supplied from a 115kV transmission system as shown in the representative online diagram provided in Figure 1. The main station that supplies the industrial customer includes two switched 48 MVAr shunt capacitor banks.
The service to the facility is through two 22.4 MVA, 115/12.47 kV substation transformers located on the site. The plant loads are then supplied from service entrance stations that step down to 480 Volts. The electrical loads at the facility consist primarily of motors (pumps and fans) and electronic loads. Most of the manufacturing equipment requires 120/208 V power which is provided through local step-down transformers serving individual loads or combinations of loads through a distribution panel. The most sensitive production equipment is supplied from large scale centralized Uninterruptible Power Supply (UPS) systems. These systems are connected in an on-line full capacity, parallel redundant configuration.
Power factor correction capacitors are not used within the facility. Therefore, magnification at low voltage capacitors is not a problem. In order to maximize capacity of the substation, the utility installed power factor correction capacitors at the 12.47 kV buses. The proposed configuration consisted of 7200 kVAr (6×1200 kVAr) at bus #2 and 3600 kVAr (3×1200 kVAr) at bus # 3. The addition of the substation capacitors prompted the concern for switching transient voltages within the plant. Transients during switching of either the 48 MVAr, 115kV banks or during switching of the individual 1200 kVAr steps were evaluated.
A preliminary model for use with the EMTP was developed to evaluate the concerns for capacitor switching transients. The model was built to accurately represent the system source equivalents at the transmission buses, transformers stepping down to the distribution substations, capacitor banks, and equivalents for loads at the facility and nearby utility buses.
Figure 1 – Representative Oneline for Utility/Customer Capacitor Switching Interactions
A VFD was modeled in detail (Figure 2) to evaluate the impacts of the capacitor switching transients on a typical VFD load. The main concern for the VFDs is that they will experience nuisance tripping caused by an increase in the dc bus voltage within the drive during a capacitor switching transient. The particular drives being evaluated had relatively high dc overvoltage trip settings of 850 Volts. This setting can be as low as 760 Volts for some drives. Previous publications have shown that the nuisance tripping caused by dc overvoltages can be solved by installing an isolating reactor (choke) in series with the drive. The drives being evaluated at the facility did not have chokes but were separated from the main 480 Volt supply by relatively long cable lengths.
The two 48 MVAr, 115kV capacitor banks have 0.3 mH series reactors to limit back-to-back switching inrush currents. The banks are switched with conventional SF6 breakers.
The new 12.47 kV capacitor consist of individual 1200 kVAr steps and series reactors to limit the inrush current during the back-to-back switching. The individual 1200 kVAr steps are switched with motorized air-break switches.
Figure 2 – VFD Oneline Diagram
SIMULATION RESULTS
Simulations were performed using the model described to evaluate the expected transient voltages at the facility loads during capacitor switching at either 115 kV or 12.47 kV locations.
Energizing the 48 MVAr, 115kV Capacitor Banks
Switching of the 115 kV capacitor banks was being performed on a regular basis for some time prior to the addition of the 12.47 kV capacitors. This case results in a transient voltage at the facility that has been recorded by monitoring equipment on a number of occasions. Figure 3 is a typical case showing the transient voltage at a 480 Volt bus. These transients were not causing any problems at the facility.
The waveform in Figure 3 was used to estimate the damping being provided by the 115 kV transmission system and load equivalents. Figure 4 illustrates simulation results for the same case. The damping in the simulation case is somewhat less than the damping in the measured waveform that should result in conservative results from the simulations
Figure 3 – Measured and Simulated 480 Volt Bus Voltage during 115kV Capacitor Energizing
Energizing the 12.47 kV Capacitor Banks
This case involves the addition of power factor correction capacitor banks at the 12.47 kV buses. These capacitor banks are configured in 1200 kVAr steps. The first step to be energized results in the highest transient. The transients associated with energizing of subsequent steps are lower because the transient is shared with the other capacitors already in service. Figure 4-Figure 6 give the simulated transients at the 60 HP VFD load for energizing the first 1200 kVAr step. The transients are illustrated in per unit (pu). This is the actual voltage divided by the normal peak voltage magnitude. In Figure 4, the peak transient is approximately 1.3 pu, or about 880 Volts (L-L). The transient magnitude in the dc bus of the drive should be somewhat less than this due to current limiting impedances in the drive. Therefore, it is not likely that this transient would cause drive tripping.
Figure 4 – Simulated VFD Location – ac Input Current
Figure 5 – Simulated VFD Location – ac Input Current
Figure 6 – Simulated VFD Location – dc Bus Voltage
FIELD TEST RESULTS
It was clear from the initial simulations that the potential for nuisance tripping of VFDs or other electronic loads existed during capacitor switching operations. Field tests were performed to evaluate the actual transient voltages at the facility during switching of the new 1200 kVAr capacitor banks and during switching of the 48 MVAr, 115kV capacitor banks. These tests were scheduled during maintenance shut down at the facility to minimize the impacts if any nuisance tripping was encountered.
The test cases performed are summarized in Table 1 below. Basically, energizing operations for the three individual 1200 kVAr capacitor steps at the 12.47kV bus were evaluated first and then one of the 48 MVAr, 115kV banks was energized to evaluate the potential for magnification. Finally, the individual 1200 kVAr steps were de-energized and then put back into service.
For each of the test cases, the transient voltages were recorded at a number of locations. Two of the locations were selected to evaluate the impact of the transients at electronic loads. One site was a 60 HP VFD and the other load site was one of the main UPS installations. The 12.47kV bus was supplying these locations.
Table 1 – Summary of Field Measurements
.
SUMMARY
The simulations and measurements demonstrated the importance of plant loads in reducing capacitor switching transient voltages. Electronic loads and UPS systems can provide significant damping due to the capacitors and batteries that are effectively connected to the 480 volt bus through diodes or SCRs. These loads make up a significant portion of the total load at the facility. Nuisance tripping of VFDs and other sensitive electronic loads can be a very important concern in industrial plants with continuous processes. Careful evaluation of the capacitor switching transients using digital simulations can indicate potential problems. However, it may be necessary to perform actual field measurements in cases where the expected transients are near the device limits. For the system studied, damping provided by system loads prevents nuisance tripping of the VFDs.
REFERENCES
S. S. Mikhail and M. F. McGranaghan, “Evaluation of Switching Concerns Associated with 345 kV Shunt Capacitor Applications,” IEEE Transactions PAS, Vol. 106, No. 4, pp. 221-230, April 1986.
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.
G. Hensley, T. Singh, M. Samotyj, M. McGranaghan, and R. Zavadil, “Impact of Utility Switched Capacitors on Customer Systems, Part II – Adjustable Speed Drive Concerns,” IEEE Transactions PWRD, pp. 1623-1628, October 1991.
RELATED STANDARDS IEEE Std. 1036-1992
GLOSSARY AND ACRONYMS ASD: Adjustable-Speed Drive EMTP: Electromagnetic Transients Program MOV: Metal Oxide Varistor PWM: Pulse Width Modulation TVSS: Transient Voltage Surge Suppressor UPS: Uninterruptible Power Supply
Published by Andrzej SOWA, Bialystok University of Technology, Faculty of Electrical Engineering
Abstract: Correctness estimation of lightning protection solutions require definition of lightning current distribution in conductive installations entering the structure during direct strike to lightning protection system of this structure. It concerns particularly this part of lightning current, which flows in electric installation. Information about this current gives the possibility to estimate the levels of overvoltages on the equipment’s ports and appropriate choose of surge protective devices. The calculations of this current in low-voltage power systems connected to different types of structures calculations were performed based on the circuit theory approach.
Streszczenie. Poprawny dobór rozwiązań ochrony odgromowej wymaga posiadania informacji o podziale prądu piorunowego w przewodzących instalacjach dochodzących do obiektu podczas bezpośredniego wyładowania w urządzenie piorunochronne. Szczególnie istotne jest określenie prądów udarowych występujących w instalacji elektrycznej. Wykorzystując metody obwodowe wyznaczono prądy wpływające do przewodów instalacji elektrycznych kilku różnych typów obiektów budowlanych (Prądy piorunowe w instalacji elektrycznej).
Keywords: surge protective device, lightning current, lightning protection, low-voltage power system Słowa kluczowe: urządzenia ograniczające przepięcia, ochrona odgromowa, instalacja elektryczna
Introduction
During direct lightning strike to external lightning protection system (LPS) of structure the surge current flows on air-termination and down conductors into the earth and generate high potential which might lead to dangerous sparking between installations inside the structures.
To avoid such sparks all metal installations, low-voltage power system and date links at the entrance of the structure must be integrated into the equipotential bounding. In case of low-voltage power system LVPS, the protection against potential differences requires the surge protective devices SPD type 1. The arrangement of SPD should be places and montage in such manner, that their limited overvoltages to the levels which are required for low-voltage installation and for supply ports of devices. From the application point of view, it is interesting to evaluate the maximal values and shapes of surge currents in individual SPD during direct lightning strike to the LPS of structures.
It was characteristic of the different analyses and measurements that these currents were a large part of total lightning current (Table 1).
Table 1. Lightning currents distribution during direct strike to LPS
.
In this paper, the great attention has been paid to develop suitable models for evaluating the lightning current distribution within the conductors of LVPS and especially the stress of individual SPD.
Models description
In paper only the low-voltage side of distribution system has been considered. Analyses were done for the LVPS with one stage protection systems – the arrangements with voltage-switching SPDs type 1 inside the structure. In theoretical consideration the model of SPD was realized on the base of switch with additional resistor, when the switch is closed. The spark-over voltages of SPDs were 1500 V and 2500 V.
Furthermore the impedances, such as inductances and resistances of the SPDs connections also are included. The LVPS was connected to the distribution transformer located outside the structure. The earthing impedance of the transformer is represented by L3 I R3 [6]. The basic of LVPS has been converted into equivalent circuit diagram presented in Fig.1.
Fig.1. Circuit diagram for LPS with switching SPD type 1
In analysis the surge currents 100 kA and 150 kA (peak values) and shape 10/350μs were used for simulation the first lightning strokes.
This lightning current was described by typical equation [7]:
.
where: IML = peak values of current (100 kA or 150 kA), η = 0,930, 𝜏1 = 19 μs, 𝜏2 = 485 μs, t -time
Calculations were realized for LPS model of the trade hall with dimensions 48 m x 12 m x 12 m (case A). Down conductors were connected to simple earth electrodes type A with resistance RG = 6,4 Ω (Fig. 2).
Fig.2. Model of LPS and LVPS used in calculation (case A)
Additionally, the calculations were realized for LPS of 2 types of structures and the following conditions were considered:
Case B – office building (Fig. 3) • lightning protection level II, • base equal 20 m x 40 m and height h = 20 m, • the mesh side of the air-termination system on the roof was 10 m x 10 m, and the distance between down conductors 10 m, • conductors of LPS with radius 4 mm, • surge currents were injected to the corners of LPS, • earth termination system type A with earth electrode resistance RG = 10 Ω.
Case C – residential building (Fig. 3) • lightning protection level IV, • base 10 m x 20 m and maximal high 10 m, • four down conductors at each edge of the structure, • conductors of LPS with radius 4 mm, • earth termination system type A with earth electrode resistance RG = 21 Ω.
Calculation results
Theoretical calculations of lightning current distribution are based on the simple circuit theory approach. In models, the conductive elements of LPS have been represented with an equivalent π model taking into account resistance and self-inductance, the inductance coupling with another segments and capacitance to the ground.
In the proposed models the influence of current in lightning channel between striking point and cloud were not considered. In calculation The Electromagnetic Transient Program EMTP [8] was used.
Examples of computed currents (case A) that flow to the earthing system of transformer through the PEN and phases conductors are shown in figure 4.
Fig.3. Diagram of lightning protection systems (case B and C)
Fig.4. Calculated waveforms for currents in SPD (iSPD), PEN conductor (iPEN) and in earthing system of transformer (iST)
The overall division of lightning current is influenced by many factors. In calculations the following were considered:
• resistance R3 in range from 1 Ω to 20 Ω (Fig. 5 and 7). • distance d between transformer and SPDs from 10 m to 300 m (Fig. 6),
A reduction of the partials lightning currents in SPDs and PEN conductor can be achieved if the LVPS bounding bar is connected to additional grounding rod RAG. In calculations the values of RAG were the same like RG in LPS.
Fig.5. Maximal values of currents with increasing values of transformer grounding system (case A, d = 100 m, w = 2m), a) currents in SPD and in PEN conductors, b) total current in LVPS
Changes in surge current distribution caused by additional grounding rods are presented in Fig. 5.
Fig.6. Maximal values of currents in conductors of LVPS (case A, w = 2m) with increasing distance d between SPD and transformer
Fig.7. Maximal values of currents in conductors of LVPS with increasing values of transformer grounding system (d = 100 m, w = 5 m,, a) office building – case C, b) residential building – case C
In order to comparison the effect of LPS on current distributions in conductors of LVPS calculations were realized with the same conditions in analyzed structures (values of RG, R3 = 5 Ω, distances d = 100 m and w = 5 m). Some results are presented in table 2.
Table 2. Maximal values of currents in conductors of LVPS
.
Conclusions
In this paper the results of numerical calculations of lightning currents in low-voltage power systems supplying different types of buildings during direct strikes to LPS were presented.
The knowledge of these currents may be great importance for an accurate determination of adequate SPD system in low-voltage installations inside the structures, more accurately than it can be done using the procedures suggested by international standards.
Further studies will be performed to correct models of SPD, low-voltage installations and load inside the structures.
REFERENCES
[1] Rakov V. A., Uman M. M., Mata C. T., Rambo k. J., Stapleton M.V., Sutil R.R., Direct Lightning Strike to the Lightning Protective System of a Residential Building: Triggered- Lightning Experiments. Trans. on Power Delivery, vol. 17, No. 2, 2002, p.575-586 [2] IEC 62305-3, Protection against lightning – Part 3: Physical damage to structure and life hazard. [3] EN 61643-11, Surge protective devices connected to low-voltage systems. [4] DeCarlo A., Rakov V. A., Jerauld J.E., Schnetzer G. H., Uman M., Schoene J.: Distribution of Currents in the Lightning Protective System of a Residential Building – Part I : Triggered Lightning Experiments. Trans. on Power Delivery, vol. 23, No. 4, 2008, p.2439-2446. [5] Celli G., Ghiani E., Pilo F. A simulation tool for overvoltages brought inside a building through its grounding system. 26th International Conference on Lightning Protection, Cracow, Poland, 2002, 7b.2 [6] Birkl J., Zahlmann P.: Lightning currents in low-voltage power installation, 29th International Conference on Lightning Protection, Uppsala, Sweden, 2008, p. 10-1-1 – 10-1-25. [7] IEC 62305-1, Protection against lightning – Part 1: General principles. [8] ElectroMagnetic Transients Program (EMTP) Rule Book, http://www.eeug.org
Autor: dr hab. inż. Andrzej W. Sowa, prof. P.B., Politechnika Białostocka, Wydział Elektryczny, 15-351 Białystok, ul. Wiejska 45D, E-mail: Andrzej.sowa@ochrona.net.pl
Source & Publisher Item Identifier: PRZEGLĄD ELEKTROTECHNICZNY (Electrical Review), ISSN 0033-2097, R. 88 NR 9b/2012
Published by Jarosław WIATER, Białystok University of Technology
Abstract. Step voltage situation arise when it is possible for a person to make simultaneous contact with a part of an electrical system which is not live under normal conditions but has become live due to the passage of current for example lightning strike one. The purpose of this paper is to provide knowledge about the levels of step voltages around tree without lightning protective system during direct lightning stroke to it. Step voltage measurement results were presented for maple tree. High voltage surge generator were used as an excitation source (37kA, 8/26μs).
Streszczenie. Różnica potencjałów na powierzchni ziemi determinuje powstanie napięć krokowych. Bezpośrednie wyładowanie piorunowe w drzewo może spowodować ich powstanie. W tym artykule zaprezentowano wyniki pomiarów napięć krokowych, które powstały przy przepływie prądu udarowego przez Klon Zwyczajny. Jako źródło wymuszające wykorzystano wysokonapięciowy generator prądów udarowych – 37kA, 8/26μs. (Napięcia krokowe w pobliżu drzewa przy wymuszeniu w postaci prądu udarowego).
Keywords: step voltage, lightning, measurements, maple tree. Słowa kluczowe: napięcie krokowe, wyładowanie piorunowe, pomiary, klon zwyczajny.
Introduction
The direct lightning strike to tree not equipped with lightning protective system (LPS) can be dangerous for the people standing nearby it. Transient step voltages can arise on the ground surface due to surge current injected into the soil by the earth electrode which it this particular case can be tree trunk and tree roots. There isn’t much information about the life hazard caused by transient electric stress on human being.
As well known ventricular fibrillation is caused when an electrical stimulus of sufficient strength strikes the heart in the vulnerable period. This period which is represented in the electrocardiogram by the T-wave, is characterized by a non-homogeneity or differences in the refractoriness of the heart fibres. Only then can fibrillation be initiated, and it is self sustained by cyclic excitations. Fibrillation of the ventricles (the main pumping chambers) is accompanied by a loss of coordinated muscular contraction and the heart muscle quickly becomes exhausted and, if the condition is not soon corrected, an irreversible standstill of the heart occurs [1, 7].
Measuring situation description
Step voltage level is strictly correlated with tree root system. Situation is similar to grounding system case. Most tree roots do not penetrate very deeply into the soil. Unless the topsoil is bare or unprotected, trees will concentrate most of their absorbing roots in the top 15 to 45 cm of soil, where water, nutrients, and oxygen can be found. Tree root systems cover more area than might be expected – usually extending out in an irregular pattern 2 to 3 times larger than the crown area. However, on a dry weight basis, the “root to shoot” ratio is around 20 to 80%, making the top four to five times heavier than the roots. The type of roots formed initially is specific to a given species with age the initial root form is often modified by the growing environment. Such thing as soil hard-pans, water tables, texture, structure, and degree of compaction all influence the mature root form. There are three basic classes of tree root systems:
a) tap root (for example: hickory, walnut, butternut, white oak, hornbeam), b) heart root (for example: red oak, honey locust, basswood, sycamore, pines), c) flat root presented on figure 1 (for example: birch, fir, spruce, sugar maple, cottonwood, silver maple, hackberry).
Fig. 1. Maple flat tree root system and transient step voltage measurement method.
In small park for the measurements purpose maple tree were selected. Maple tree should create worst case with respect to step voltage level because flat root system. On figure 2 were presented photo taken during measurements.
Fig. 2. Step voltage measurements in progress
The step potential is defined as the potential difference between a person’s outstretched feet, normally 1 meter apart – figure 1, without the person touching any earthed structure [4] – human worst.
For the purpose of circuit analysis, the human foot is usually represented as a conducting metallic disc and the contact resistance of shoes, socks, etc., is neglected [4]. Traditionally, the metallic disc representing the foot is taken as a circular plate with a radius of 0,08 m. A value of 1000Ω were used as a resistance of a human body from one foot to the other foot [4]. During the measurements voltage electrodes dug on 0,08m depth represents human foot.
Arrangement during the step voltage measurements presents figure 3. High voltage generator were connected to the maple tree by copper ring. Ring were installed on 1,3m height from ground level. Current loop were closed by four additional current electrodes putted on square traverse. High voltage generator produced 8,4/26,4μs current surge with Imax=37kA [8]. Current waveform presents figure 4. Generated surge current forces step voltages nearby the tree.
Fig. 3. Arrangement of step voltage measurements – front view
Fig. 4. High voltage surge generator output current
All recorded waveforms were presented on figure 5 and 6 for different time window. Maximal value depends on time moment. In first microseconds step voltage have got negative polarization and reaches 4kV for smallest foot distance. This voltage jump is strictly correlated with high voltage generator. At time t=0μs internal capacitors were connected to output by triggering mechanism. This forces high voltage level at nearby t=0μs time. It seems possible that voltage level during real lightning strike can reach millions volts. This phenomenon will be analyzed in future time [9-14].
At fourteen microsecond step voltage have got positive polarization and reaches 2,8kV for 100cm foot distance.
Fig. 5. Step voltage nearby tree – view for t∊<0;100>μs
Fig. 6. Step voltage nearby tree – view for t∊<0;18>μs
Conclusion
In order to ensure the safety of people at a open area, it is necessary to ensure that step potentials in and around the yard during lightning conditions are kept below set limits. These maximum permitted step potentials are addressed within various national and international electrical standards. The results of measurements of lightning transient step voltage distributions in an around a tree in case of direct lightning stroke to it were presented above. Observed step voltage exceed two times safe level according IEEE Std 80 2002 [3]. Main factor which determine step voltage distributions were different tree roots classes. Trees such as birch, fir, spruce, maple, cottonwood, silver maple, hackberry and they root system provides highest step voltage levels.
This work was co-funded by the European Union under the European Social Fund.
REFERENCES
[1] Biegelmeier G, Lee WR.: “New considerations on the threshold of ventricular fibrillation.” IEEE Proceedings Vol 127, No 2, Pt A, March 1980. [2] BS 7354 – 1990: Code of practice for Design of high-voltage open-terminal stations. [3] IEEE Std 80-2002: IEEE Guide for Safety in AC Substation. [4] Electricity Association Technical Specification 41-24: Guidelines for the Design, Installation, Testing and Maintenance of Main Earthing Systems in Substations. [5] PN-E-05115:2002. Instalacje elektroenergetyczne prądu przemiennego o napięciu wyższym niż 1kV. PKN, Warszawa 2002. [6] W.Simpson, A. TenWolde “Physical Properties and Moisture Relations of Wood”. http://www.fs.fed.us/. [7] http://ballengearry.com.au/papers/Step_and_Touch_Voltage_ update_for_2004_090804.pdf [8] Augustyniak L.: Surge voltage portable generator generating 1.2/50 mu s test waveshape of peak value up to 4 kV. Przeglad Elektotechniczny. V: 83, Issue: 9, pp. 37-38, 2007. [9] Markowska R.: Analysis of Lightning Electromagnetic Exposures in Building Electrical Installation with SPD. Przeglad Elektrotechniczny, V:86, Issue: 3, pp. 48-50, 2010. [10] Sowa A.: Coordination the solutions of lightning protection system with electromagnetic compatibility requirements. Przeglad Elektrotechniczny, V: 85, I:9, pp. 332-339, 2009. [11] Wiater J.: Lightning hazard minimization of the HV substation signal ports. Przeglad Elektrotechniczny, Volume: 86, Issue: 3, pp. 172-175, 2010. [12] Wiater J.: Analyzer and lightning voltage surge logger for data transmission lines. Przeglad Elektrotechniczny, Volume: 86, Issue: 11B, pp. 97-98, 2010. [13] Wiater J.: Remote earth localization for lightning surge condition on the high voltage substation. Przeglad Elektrotechniczny, Volume: 86, Issue: 3, pp. 96-97, 2010. [14] Wiater J.: Influence of Different Lightning Source Models on Current Distribution in the HV Substation. Przeglad Elektrotechniczny, Volume: 86, Issue: 3, pp. 94-95, 2010.
Authors: dr inż. Jarosław Wiater, Bialystok University of Technology, Department of Telecommunications and Electronic Equipment, ul. Wiejska 45d, 15-351 Białystok, Poland. E-mail: jaroslawwiater@we.pb.edu.pl
Source & Publisher Item Identifier: PRZEGLĄD ELEKTROTECHNICZNY (Electrical Review), ISSN 0033-2097, R. 88 NR 9b/2012
Published by Wiesław BROCIEK1, Robert WILANOWICZ2, Warsaw University of Technology (1) , Radom University of Technology (2)
Abstract. The paper presents a circuit model of three phase arc arrangement, taking into account the nonlinearity of the arc appearing In the particular phases as well as the windings vector groups of a furnace transformer. This model was designed on the ground of parameters, resulting from a voltage current characteristic of the arc. At modeling of changes of parameters of an arc we applied the random values of resistance. The values of higher harmonics of current and voltage for each considered case have been evaluated by using the simulation program MicroCap – 8. In the paper we have included the exemplary results of numerical calculations.
Streszczenie W artykule przedstawiono model obwodowy trójfazowego urządzenia łukowego z uwzględnieniem nieliniowości łuku w poszczególnych fazach oraz grupy połączeń transformatora piecowego. Model ten opracowano na podstawie parametrów wynikających z nieliniowej charakterystyki napięciowo – prądowej łuku. Do symulacji zmian parametrów łuku elektrycznego wykorzystano generator zmienny losowo. Wszystkie obliczenia zostały przeprowadzone w programie MicroCap – 8. Obliczono wartości wyższych harmonicznych prądu i napięcia w odbiorniku oraz po stronie pierwotnej i wtórnej transformatora piecowego. Zamieszczono przykładowe rezultaty obliczeń numerycznych. (Zniekształcenia napięcia i prądu w układzie zasilania pieca łukowego)
Keywords: nonlinear load of random parameters, higher harmonics of current and voltage. Słowa kluczowe: odbiornik nieliniowy o parametrach zmiennych losowo, wyższe harmoniczne prądu i napięcia
Introduction
One of the crucial parameters describing the quality of the electrical energy are these describing the distortion of the voltage an current from the ideal sinusoid. They may be measured by the higher harmonics, i.e. contents of higher harmonics and their values. Because of consequences for power system followed by the higher harmonics the obligatory standards define the upper limits of distortion, acceptable in the system. Determination of the distortion degree caused by the applied electrical equipment is the fundamental task in the qualification of the correct performance of this equipment.
The article present results of simulation tests concerning the cooperation of the nonlinear load (AC – arc furnace) with the power system are presented. The values of higher voltage and current harmonics and total harmonic distortion on the primary and secondary sides of arc furnace transformer 30/0.75 kV for each considered case have been evaluated by the experiments performed by using the simulation program MicroCap-8.
System description
Figure 1 presents the general scheme of power system supplying the nonlinear load i = f(u). Fig. 1 presents the simplified power supply diagram of electric steel works with arc-arrangements. A three phase arc arrangement consists of: an arc furnace, strong-current circuit and furnace transformer. Accurate description of the effect which could make possible to create a precise model of the arc arrangement. is rendered difficult. However, one should carry out investigations in this course in orders. to create a model of UHP arrangement which would reflect real operating conditioners.
Fig.1. The general structure of the simulated system
The model presented in Fig. 1 is composed of the following elements: supplying point 110kV of short-circuit power Szw= 500MVA, the main supplying point (PCC) 30kV of short circuit power Szw=200MVA, transformer 30/0.75kV, S=75MVA and nonlinear load i=f(u,t ) – arc furnace.
The parameters (RS, LS) of the model representing the supplying systems on the level 30kV have been determined using following formulas (for f=50 Hz) [3].
.
Fig.2.The circuit model of the analysed system at nonlinear load in 3 phases
To ensure the continuity of arc current each electrode is controlled by independent controller. As a result the electrodes currently move depending on iron charge resistance, charge shape and phase of technology process. Currents of three-phase line are random functions. Fig. 2 shows the system with three controlled sources modelling current-voltage arc characteristics and resistances modelling the state of arc stability of each electrode. It means that system to be analyzed is nonlinear and its parameters are random [2,3,4].
To the largest and most energy consuming nonlinear high power receivers belong siderurgical arc arrangements provided with transformers amounted to power of tens MVA. The characteristic feature for the coding of charge period consists in very rough and irregular mutability of power consumption by the furnace caused by nonlinear variations of the arc resistance as well as variations of the plasma physicochemical properties in the arc’s column. The arc furnace becomes then a nonlinear asymmetric receiver, and at low level of short-circuit power it gives occasion to interferences in the power supply network, thus making worse quality of electric power supplied from the heavy current system to other consumers. Such interferences include: fluctuations and asymmetry of voltage as well as deformation of voltage curve. As the electric arc is an element of nonlinear voltage-current characteristic, therefore, the furnace draws out of the network; a considerably deformed current and becomes a source of higher currents’ harmonics. Investigation of influence of siderurgical arc arrangements on the electric power system wants establishment of higher harmonics current distribution in this system. This makes it possible to determine voltage drops of higher harmonics as well as those harmonics of voltage in any node of the system. One of the determination procedures of voltage distortion degree consists in determination of the voltage distortion factor THDV determined by the following relation [1,3]
.
The admissible values of voltage distortion for 30kV are [4]
.
The admissible values of voltage distortion for 110kV are [4]
.
The represented model was designed on the ground of parameters resulting from the empirical determination method of the arc voltage-current characteristic [1,3].
The furnace transformer 30/0.75 kV of power S=75 MVA has 12 degrees of voltage control. Low secondary voltage of this transformer is its characteristic feature. The most advantageous scheme of connections of a two-winding furnace transformer is:
the primary winding with delta connection changed over voltagelesely into a star connected winding,
the secondary winding should be always with delta connection.
Connection of the furnace transformer secondary terminals into triangle shows the following superiority in comparison with connection into star:
the short-circuit current between electrodes is distributed into two phases of the transformer,
connection of secondary terminals into triangle makes it possible to realize a so-called three-phase bifilar circuit.
Fig.3. Voltage – current characteristics of an arc furnace in phase A
Fig. 3 presents the exemplary voltage – current characteristics of an arc in phase A. In numerical calculations we have considered the voltage-current characteristics of the real AC arc furnace. They are presented in Fig. 4.
Fig.4. Real voltage – current characteristics of an arc furnace in phase A
Fig.5. Instantaneous values of current In electrodes A, B, C
Fig.6. Instantaneous values of a) currents on the primary side of transformer, b) the voltage on the secondary side of transformer.
Fig.7. Deformation of currents in electrode A, a) rms values of harmonics, b) the percent distortion versus frequency, c) the waveform of the current
Fig.8. Deformation of currents in phase A, a) rms values of harmonics, b) the percent distortion versus frequency, c) the waveform of the current .
Fig.9. Deformation of voltages in electrode A, a) rms values of harmonics, b) the percent distortion versus frequency, c) the waveform of the voltages
Figures 5 and 6 presents instantaneous values of currents on the primary side of transformer, the voltage on the secondary side of transformer and current of the electrodes. Figures from 7 to 10 presents the distorted currents & voltages in the system of Fig. 3. Tables 1 and 2 presents the values of currents & voltages harmonics in the circuit of Fig. 3 at nonsymmetrical load.
Fig.10. Deformation of voltages in phase A, a) rms values of harmonics, b) the percent distortion versus frequency, c) the waveform of the voltages
As a results of simulation we hale got the currents and voltages of the values very close to that in real operation of the system. After determination of the currents and voltages we can easily the other parameters of the system including the rms values of harmonics, as well THD coefficient being the basic measures of the quality of electrical energy.
Table 1. The values of the current harmonics in the circuit of Fig. 3 at nonsymmetrical load.
.
Table 2. The values of the voltages harmonics in the circuit of Fig. 3 at nonsymmetrical load.
.
Conclusions
For simulation of the changes of parameters of the electric arc we have used the random number generator built In MicroCap program.
The performed experiments allow to determine the propagation of the higher harmonics of the voltage and current generated by the nonlinear load in the power system. The model can by easily extended to the other nonlinear loads, for example the arc furnace supplied from the real system.
The comparison of the shapes of the currents in the 3 phase real system and in our model has confirmed the accuracy of the proposed approach. The results of simulation and measurements allow to assess the level of generated higher harmonics by nonlinear loads and influence of these harmonics on power system.
The modeling and simulation AC arc furnace, is a handy and recommended tool for the analysis of the power system cooperating with the nonlinear load.
This research activity was financed by the National Science Centre.
REFERENCES
[1] Brociek W., Wilanowicz R. Electric power quality parameters in transformer stations supplying nonlinear load, Przegląd Elektrotechniczny, 11/2003, pp.861 – 864. [2] Ozgun O.Abur A.Flicker study using a novel arc furnace model, IEEE Transactions on power delivery, vol 17 no.4.10, 2002 [3] Brociek W., Wilanowicz R., Siwek K. Determination of the electric power in transformer station supplying nonlinear load: experimental and numerical study, V CPEE, Jazłowiec 2003, pp.141-144. [4] Wang Y.F., Jiang J.G. A novel chaotic model of electric arc furnace for power quality studies, Proc.of Int. Conf. on Electrical Machines and Systems 2007, Oct. 8-11, Seoul, Korea.
Authors: dr inż. Wiesław Brociek, Warsaw University of Technology, Institute of Theory of Electrical Engineering, Measurement and Information Systems, E-mail: brociek@iem.pw.edu.pl, dr inż. Robert Wilanowicz, Radom University of Technology.
Source & Publisher Item Identifier: PRZEGLĄD ELEKTROTECHNICZNY (Electrical Review), ISSN 0033-2097, R. 87 NR 7/2011
Published by Liu Liqun, Liu Chunxia, Taiyuan University of Science & Technology
Abstract. This paper gives the techno-economic analysis of a wind-PV-diesel system with storage battery backup for an off-grid power station specially located in Dongwangsha, studying data from a particular site in littoral China. The simulation and optimization results indicated the most optimized sizing of hybrid system. Moreover, the sensitivity analysis is discussed at a diesel price of 2$/L and annual average wind speed of 4.41 m/s and interest rate of 6%. The proposed hybrid system is more cost effective and environmental friendly than the diesel only system.
Streszczenie. W artykule przestawiono techniczno-ekonomiczną analizę systemu energetycznego składającego się z urządzeń wiatrowych, fotowoltaicznych i silnikiem Diesla oraz baterii magazynującej na przykładzie sieci w miejscowości Dongwagsha w Chinach. Rezultaty symulacji i optymalizacji pokazały, że system hybrydowy jest znacznie korzystniejszy niż tylko oparty na silniku Diesla. (Techniczno-ekonomiczna analiza systemu hybrydowego z wykorzystaniem źródeł odnawialnych – na przykładzie Chin)
Keywords: Renewable energy; Techno-economic analysis; Hybrid power supply system. Słowa kluczowe: energia odnawialna, analiza ekonomiczna, hybrydowy system energetyczny
Introduction
It is well known that China is the largest developing country and the second largest energy consumption country in the world. The total consumption amount of coal and oil are more than 2.74 and 0.36 billion tons in 2008, respectively, and natural gas is about 80.7 billion m3 [1]. Abundant energy consumption brought a lot of pollutants and a large amount of emissions. For example, the SO2 emission from 2000 in China is more than 20 million tons, which ranks the first in the world [2]. The CO2 emission is more than 4.5 billion tons, which ranks the second in the world [3]. The total pollution loss accounts for 10% of Chinese Gross Domestic Product [4]. Chinese primary energy supply structure is very inappropriate [5]. Chinese central government and local governments have waked up to the problem to realize the sustainable development of country in future. Renewable Energy Law has been established in February 28, 2005. The development and application of renewable energy has been regarded in order to improve the inappropriate energy structure. The renewable sources in China are abundant, such as wind, solar, and biomass energy, etc. For example, the total amount of wind power in China is more than 3.2 billion kilowatts at 10 meter height, and the amount which can be effectively utilized is about one billion kilowatts. As a conclusion, the prospect of renewable energy in China is beautiful in foresee future.
The conventional power grid in China can not supply the total end consumers with enough power. In this period of time, millions of off-grid consumers have to use the diesel generator in order to receive the supply of power. Moreover, some industry equipments located in remote areas which apart from the power grid, such as communication base station, island power system, and radar station, and so on. The generating system uses the renewable sources can afford steady electricity supply. For example, some standalone PV power stations have been established in remote areas to improve the power quality of off -grid consumer which have enhanced the standard of living of ordinary people. However, a common drawback is existed in the stand-alone wind energy and solar energy generating power system, and the output electric power is unpredictable which change with the changing of weather, such as insolation, temp, and wind speed, etc. Fortunately, the hybrid wind-solar system can partially overcome the problems which integrate multi-fold resources in a proper combination, and the output quality of power is improved. But the price of wind power generator and PV is costly at present and the initial capital of hybrid only wind-solar system is big. The expensive levelized cost of energy (COE) can not be accepted by the ordinary user. The conventional diesel only power system consumes a lot of diesel and discharge a mass of greenhouse gases. Fossil resource came in increasing price, and the hybrid generating system is suggested to offer steady and reliable and cheap power supply for the off-grid user or the industry equipment as compare with the diesel only generating system. Certainly, the techno-economic analysis of hybrid system is necessary to minimize the initial capital, COE, operating & maintain cost (O&M), net present cost (NPC), diesel consumption and unmet electric rate at suffice for consumer’ need.
Many literatures have analyzed the feasibility of renewable resource generating system by using HOMER or Hybrid2 or RETscreen [6-14]. Unfortunately, the renewable power supply system in China did not consider the optimal analysis of components of renewable resources power system at present. The article presents the feasibility analyses of renewable power station to optimal configure and reduce the initial capital and NPC. In order to arouse the regard of designer, an established power station is used to compare with the proposed optimal system. In this context, the present study carries out a techno-economic analysis by using HOMER software of the USA National renewable Energy Laboratory (NREL) and the data of National Aeronautics and Space Administration (NASA) to optimize configure of a hybrid wind-PV-diesel system with storage battery backup for an off-grid power station which located in Dongwangsha, Chongming islands [15-16].
Site and Meteorological data and Electrical load
Dongwangsha located in Chongming island of Shanghai where apart from the national electrical grid. The village gets power through diesel generating power plant in the past. The diesel only system is difficult to ensure the continuous electricity supply during breakdown and scheduled shutdown of diesel units. A hybrid wind-PV-battery power station has been established in the site that is capable of meeting the load. The geographical coordinates of the data established project site were 31o31′ N latitude, 121o57′ E longitude and 1 meter altitude above mean sea level. The existing meteorological data of wind speeds are measured at 40 meters height according to an established wind speed weather station located in project site, but without accurate data of solar insolation clearness Index, earth skin temperature, relative humidity, and wind speed at 10 meters altitude above the surface of the earth.
The meteorology data from NASA is used to the proposed hybrid system. The monthly average daily total global solar radiation (GSR) ( kW / m2 / d ) and clearness index are shown in Fig.1 (a), and the data is gained via internet by using HOMER based on the latitude and longitude. The scaled annual average value of GSR is 3.95kWh/m2 / d . The highest values of GSR are gained during the months of May to August with a maximum of 5.559kWh/m2 / d . The lowest values are gained during the months of December to January with a minimum of 2.183kWh /m2 / d .According to solar radiation, Fig.1 (b) shows that the hourly available PV power output throughout the year.
Fig.1. Meteorological data of solar insolation and available power output at the site
The monthly average daily wind speed (m/s ) at the 10 meter above the surface of the earth was collected from NASA as shown in Fig.2 (a), and the scaled annual average value of wind speed is 4.41m/s. The highest values of wind speed are observed during the months of January to February with a maximum of 4.91m/s . The lowest values are observed during the months of May with a minimum of 3.97m/s. The hourly available power output throughout the year according to wind speed is shown in Fig.2 (b). Moreover, the annual average wind speed can not same, and the considered wind speed in this paper are 4, 4.41, and 5m/s .
Fig.2. Meteorological data and available wind speed at the site
Fig.3. Variation of Load demands of proposed project.
The hybrid wind-solar only system with one wind turbine of rated power 5kW and PV with rated power 34.3 kW and 990 batteries with rated capacities 100Ah and rated voltage 2V have been established in the Dongwangsha. The initial capital is about 251,397$. The typical hourly electrical consumption data used in this project was measured and described in literature [17]. The average value per day of load was 139kWh, and the maximum hourly power of the load was 35kW, and the daily load factor varied from 0.074 to 0.629. Fig.3 (a) shows the typical hourly load demand. The highest values of electric consumption appeared between 18:00 hours and 22:00 hours with a maximum power of 22 kW , and the lowest value of load demand appeared between 22:00 to 16:00 of the following day with a minimum power of 2.6kW , and the rated voltage of load is 220V. As shown in Fig.3 (b), the hourly available electric consumption throughout the year is varied with season changing.
Hybrid wind-PV-diesel-battery power station
A hybrid wind-PV-diesel-battery power station is used to discuss the possible optimal configure of power station in Dongwangsha, and the proposed autonomous hybrid system consists of one diesel generator, some PV arrays, some storage batteries, some wind power generators, and a power converter. The hybrid power with battery backup is used to maintain regular supply during the conspicuous bad weather and breakdown and scheduled shutdown of diesel unit. The hybrid power system optimization software HOMER developed by NREL has been used in this proposed power station, which is a computer model to assist in the design of micro power systems and to facilitate the comparison of power generating technologies across a wide range of applications. The physical behavior and lifecycle cost and total cost of installing and operating the system over its life span of a project is described. HOMER allows the modeller to compare with many different design options based on their technical and economic merits. It also assists in understanding and quantifying the effects of uncertainty or changes in the sensitivities [16].
Solar PV modules are connected in series and parallel string in order to produce enough electric power according to the voltage and current demand of load. The initial capital and the replacement cost of PV are 4400$ and 4400$ per kilowatt ($/kW) in China, respectively. The PV sizes are considered to be 0, 10, 15, 20 and 25 kW. Operation and maintenance cost of PV array is 20$/kW per year. PV array were considered as fixed and the slope degree is 31.5. Working lifetime of PV panels are taken as 20 years and don’t consider the tracking system. The effect of temp for PV output power is considered in the article. Temp coefficient of power is -0.5%/oC , and nominal operating cell temp is 47oC and output power efficiency is 13% at standard test conditions.
The horizontal-axes wind turbine series of Guangdong Shangneng wind power equipment ltd. (GSWPEI) are considered for the proposed power station design. SN-3000WL type wind electric generator is considered for this project via compare with the cost of per watt of other types. The initial capital and the replacement cost of wind turbine are 2100$ and 2000$ per kilowatt, respectively. Five different wind turbines quantities (0, 20, 24, 28 and 32) are taken in the hybrid system. Operation and maintenance cost of wind turbine is considered to be 20$ per year, while the working lifetime of wind turbines and the hub height are taken as 15 years and 15 meters, respectively. The output power of SN-3000WL type wind electric generator during various wind speed can be seen from literature [18].
A battery band is used as a back up system to maintain the electric consumption at bad weather. SN150-12 type battery is considered for the proposed project which is produced by GSWPEI, and the nominal capacity and nominal voltage of each battery are 150 Ah and 12V, respectively. Round trip efficiency and minimum state of charge are taken as 85% and 30%, respectively. The float life, maximum charge rate and maximum charge current are 10 years, 1A/Ah and 18A, respectively. The lifetime throughput and suggested value are 1075 and 2343 kWh.
The initial capital and the replacement cost each battery is considered to be 110$ and 100$, respectively. Operation and maintenance cost is 2$ per year. The battery quantities are considered to be 0, 48, 120, 192, 264, and 336. The initial capital and the replacement cost of diesel generator is taken as 220$/kW and 200$/kW, respectively. The diesel generator sizes are considered to be 0, 15, 20, 30 and 35 kW. Operation and maintenance cost is 0.04$/hr per year. Operating lifetime hours are 15000 hours and minimum load rate is 30%. Furthermore, the carbon monoxide and unburned hydrocarbons and particulate matter and nitrogen oxides of fuel are 6.5, 0.72, and 0.49, and 58 gram per liter (g/L), respectively. And proportion of fuel sulfur converted is 2.2. The current diesel price per liter in China is about 1$. The price of diesel is used for sensitivity analysis and five discrete values (0.5, 0.8, 1, 1.2, and 2 $/L) were considered.
The power converter is needed to maintain power flow between the AC and DC components. The initial capital and the replacement cost of power converter is about 195$ and 195$ per kilowatt, respectively. Six different sizes of power converter (0, 25, 30, 35, 40 and 45 kW) are taken in the model. Operation and maintenance cost is taken as 0$ per year. The lifetime is 15 years. Inverter and the rectifier efficiency is 90% and 85%, respectively.
The annual real interest rate is taken as 6%, and the interest rates are considered to be 4%, 6%, and 8%. The project lifetime is 25 years. Dispatch strategy uses cycle charging. Apply set point state of charge is 80%. Operating reserve includes the percent of load and percent of renewable output, and the hourly load as percent of load is 10%, and solar power output and wind power output is 25% and 50%, respectively.
Optimal results and discussion
According to the above input, there are 202,500 possible system configurations which comprised of 45 sensitivities and 4500 simulations for each sensitivity run. Table.1 displays the values of each optimization variable to simulate all possible system configurations. HP pavilion ze2000 notebook PC, AMD Sempron 2800+ CPU, with 1.59 GHz speed, 768 MB took 1 hour, 31 minutes and 12 second to calculate the possible configurations.
Table.1. Probable of system configurations
Source: Authors’ new contribution to this paper
Table.2 summarizes the optimization results for a wind speed value of 4.41m/s (annual average wind speed at the site), interest rate of 6%, and the diesel price in China, which currently equals 1$/L. The suggested optimal hybrid wind-diesel-battery (WDB) power station consists of 32 wind power turbine with rated power of 3kW, 15kW diesel generator, 336 batteries, and 40 kW sized power converter. The proposed hybrid system was found to have an initial capital of 115,260$ with an annual operating cost of 7,002$, total NPC of 204,775$, COE of 0.316$/kWh, renewable energy fraction of 0.955, the diesel consumption of 1,314 L, and working hours of 282 hours. The suggested system of HOMER decreases initial capital of 136,137$ (251,397$-115,260$) as compare with the established hybrid wind-PV-battery system, and the NPC of suggested system of HOMER is less than the initial capital of hybrid system in literature [17]. The only hybrid wind-PV-battery system is considered for the power station, which can be seen in the third row from Table.2. The system with 10 kW PV, 32 wind turbine with rated power of 3kW, 336 batteries, and 35 kW converter is suggested. The hybrid renewable energy only system increases an initial capital of 39,725$, total NPC of 25,766$, and the COE of 0.039 $/kWh while the operating cost decreases 1,091$ per year to compare with the suggested hybrid WDB system. The renewable fraction is 1. The suggested hybrid renewable energy only system of HOMER decreases an initial capital of 96,412$ (251,397$-154,985$) as compare with the established hybrid wind-PV-battery system in literature [17], and the NPC of suggested system of HOMER is less than the initial capital of hybrid system. The diesel only generating system was found in nine row of Fig.4, which consists of a diesel generator with rated power 35 kW. Which increases an operating cost of 60,796$ per year ($/yr), total NPC of 669,615$, and the COE of 1.032 $/kWh while the initial capital decreases 107,560$ as compare with the suggested hybrid wind-diesel-battery system. The renewable fraction is 0. The diesel consumption increases 50,343L, and the operating hours increases 8478 hours (hrs).
The cash flow summary details of different components for the suggested hybrid WDB system can be seen from Table.3, such as total NPC, initial capital, replacement and operation, fuel and salvage. The suggested hybrid system was able to meet the power requirement of load with 95% wind power and 5% diesel generator power. The AC primary load consumption is 50,709 kWh per year (kWh/yr) and the excess electricity quantity is 23,455 kWh/yr which account for 27.2% of total power production, and unmet electric load quantity is 25.7 kWh/yr which account for 0.1% of total power production, and the capacity shortage is 32.8 kWh/yr which account for 0.1% of total power production. The monthly average electric production is shown in Fig.4. The wind power can meet the power demand during the months of January to April and June and November to December. The diesel generator is operated in May and July and August and September and October to meet the unmet electric load demand.
Table.2. Probable optimal configurations of system
Source: Authors’ new contribution to this paper
Table.3. Cost summary of component
Source: Authors’ new contribution to this paper.
Fig.4. Monthly average electric production of suggested system
Fig.5. Charge state and monthly storage operating capacity.
The monthly storage operating capacity (SOC) of batteries is shown in Fig.5. The nominal capacity and usable nominal capacity and lifetime throughput are 605 and 423 and 361,200 kWh, respectively. And the autonomy is 73.1 hours (hrs). The battery wear cost and average energy cost are 0.101 and 0.010 $/kWh, respectively. The input energy and output energy and storage depletion and losses and annual throughput are 43,005 and 36,874 and 133 and 5,998 and 39,996 kWh/yr, respectively. The expected life of batteries is 9.03 years. The running characteristic of converter is shown in Table.4. The suggested hybrid wind-diesel-battery power system with 95% wind power could avoid addition of greenhouse gases emissions, such as carbon dioxide, carbon monoxide, unburned hydrocarbons, nitrogen oxides, sulfur dioxide, and particulate matter. The emissions of proposed hybrid system decrease the carbon dioxide of 132,530 (136,031-3,461) kilogram (kg), the carbon monoxide of 327.46 (336-8.54) kg, unburned hydrocarbons of 36.254 (37.2-0.946) kg, particulate matter of 24.656 (25.3-0.644) kg, sulfur dioxide of 266.05 (273-6.95) kg, and nitrogen oxides of 2919.8 (2,996-76.2) kg as compare with the diesel only generating system.
Table.4. Characteristic of converter
Source: Authors’ new contribution to this paper.
Table.5. Probable sensitivity variables
Source: Authors’ new contribution to this paper.
Fig.6. Effect of sensitivities in NPC.
As mention above, there are 45 sensitivity variables which are presented in Table.5. The sensitivity variables will range with the changing of external circumstance, such as meteorological and economical domain. The changing of diesel prices and average values of annual wind speed and different annual real interest rates are considered in this section, which affects the NPC and COE and O&M. For example, the effects of sensitivity variables for NPC are shown in Fig.6. The NPC will increase with the increasing of diesel price and decreasing of annual average wind speed and interest rate. The NPC will decrease with the decreasing of diesel price and increasing of annual average wind speed and interest rate. And the changing of annual average wind speed has the biggest effect for total NPC.
Fig.7. Sensitivity analysis for possible optimal configure.
Fig.7 exhibits the sensitivity analysis results in terms of interest rate and annual average wind speeds for maximum annual capacity shortage of 0%. There are three values for the interest rate and three values for the annual average wind speed were specified. The axes of the graph correspond to these two sensitivity variables. The diamonds in the Figure indicate these sensitivity cases, and the colour of each diamond indicates the optimal system type for that sensitivity case [16]. At the range of annual average wind speed from 4.2 to 4.4 m/s and the interest rate less than 5.5% and the fixed diesel price of 2 $/L, for example, the optimal system type was hybrid wind-PV–battery system. At the annual average wind speed more than 4.4 m/s, the optimal system type was hybrid wind-diesel-battery system. As a conclusion, the design of optimal system must consider the meteorological data of site and external economical condition.
Conclusion
In this article, a hybrid system comprising of wind turbines, PV modules, diesel generator, and storage batteries, is discussed to explore the possibility of utilizing power of the wind and solar to reduce the dependence on fossil fuel for power generation to meet the electric requirement of Dongwangsha located in the seaside of the Chongming islands, Shanghai. The most economical power system consists of 32 wind power turbines with rated power of 3kW, 15kW diesel generator, 336 batteries, and 40kW sized power converter at annual average wind speed of 4.41m/s and diesel price of 1 $/L and interest rate of 6%. The initial capital and total NPC is 115,260$ and 204,775$, respectively. The operating cost is 7,002$ per year and COE is 0.316$/kWh and renewable energy fraction is 0.955 and the diesel consumption is 1,314 L and working hours is 282 hrs. When the power station uses the hybrid renewable energy only system, the economical power system consist of 10kW PV, 32 wind power turbines with rated power of 3kW, 336 batteries, and 35 kW converter. The initial capital and total NPC is 154,985$ and 230,541$, respectively. The operating cost is 5911$ per year and COE is 0.355$/kWh and renewable energy fraction is 1. These suggested systems are more environmental friendly than the conventional diesel only system and the greenhouse gases emission is less than the diesel only system. As a conclusion, the techno-economic analysis is very important to select the optimal configure of hybrid power system.
Acknowledgments: this work was supported by the Program for the Industrialization of the High and New Technology of Shanxi province (NO: 2010016), Youth Science Foundation of Shanxi province (NO: 2011021014-2).
REFERENCES
[1] National Bureau of Statistics of China, 2008. Statistics report of country economy and society development in 2009. http://www.stats.gov.cn/tjgb/ndtjgb/qgndtjgb/t20090226_402540710.htm . [2] CCTV, Strongly promote the development of pollution reduction in transition, 2009. http://hd.cctv.com/20090706/107332.shtml. [3] CCTV, A reporter asked the Ministry of Foreign Affairs denied that China’s carbon dioxide emissions ranks the first, 2007. http://news.cctv.com/china/20070621/108885.shtml . [4] CCTV, The Great Divide of environmental protection: the loss due to pollution in China each year 10% of GDP, 2007. http://finance.cctv.com/20070319/100794.shtml. [5] Chinese Development and Innovation Committee (CDIC), 2008. http://finance.sina.com.cn/roll/20080409/02202131646.shtml. [6] Celik, A.N., The system performance of autonomous photovoltaic-wind hybrid energy systems using synthetically generated weather data, Renewable Energy, 27(2002), No. 1, 107-121. [7] Diaf S., Diaf D., and Belhamel M., et al, A methodology for optimal sizing of autonomous hybrid PV/wind system, Energy Policy, 35(2007), no. 11, 5708-5718. [8] Iqbal M.T., Pre-feasibility study of a wind-diesel system for St. Brendan’s, Newfoundland, Wind Engineering, 27(2003), no.1, 39-51. [9] Khan M.J., Iqbal M.T., Pre-feasibility study of stand-alone hybrid energy systems for applications in Newfoundland, Renewable Energy, 30(2005), no. 6, 835-854. [10] Mills A., Al-Hallaj S., Simulation of hydrogen-based hybrid systems using Hybrid2, International Journal of Hydrogen Energy, 29(2004), no. 10, 991-999. [11] Thompson S., Duggirala B., The feasibility of renewable energies at an off-grid community in Canada, Renewable and Sustainable Energy Reviews, 13(2009), no.9, 2740-2745. [12] Tina G., Gagliano S., and Raiti S., Hybrid solar/wind power system probabilistic modelling for long-term performance assessment, Solar Energy, 80(2006), no.5, 578-588. [13] Yang H., Zhou W., and Lu L., et al, Optimal sizing method for stand-alone hybrid solar-wind system with LPSP technology by using genetic algorithm, Solar Energy, 82(2008), no. 4, 354-367. [14] Yang H., Lu L., and Zhou W., A novel optimization sizing model for hybrid solar-wind power generation system, Solar Energy, 81(2007), no. 1, 76-84. [15] National Aeronautics and Space Administration (NASA), http://eosweb.larc.nasa.gov/ [16] USA National renewable Energy Laboratory (NREL), http://homerenergy.com/ [17] Gu C., Research for simulation and optimization of solar/wind hybrid power station, Shanghai Jiaotong University Master thesis, Shanghai, 55-61, 2004. [18] Guangdong Shangneng wind power equipment ltd., http://www.sunningpower.com/
Authors: Assistant prof. L.Q. Liu, college of electronic and Information engineering, Taiyuan University of Science & Technology, Waliu road 66, Wanbolin district, Taiyuan, China, Email: llqd2004@163.com; Assistant prof. C.X. Liu, College of computer science & technology, Taiyuan University of Science & Technology, Waliu road 66, Wanbolin district, Taiyuan, China, Email: lcx456@163.com.
Source & Publisher Item Identifier: PRZEGLĄD ELEKTROTECHNICZNY (Electrical Review), ISSN 0033-2097, R. 88 NR 7a/2012
Published by Electrotek Concepts, Inc., PQSoft Case Study: PWM Drive Motor Failures due to Transient Voltages, Document ID: PQS0404, Date: June 30, 2004.
Abstract: In an effort to improve the efficiency of many industrial processes, equipment has been retrofitted with adjustable speed drives (ASDs). The ASDs allow for better speed control, soft starting of motors, and increased efficiency of the overall process operation. Unfortunately, there can be some drawbacks when using ASDs. While the effects of ASDs on the power system are well known, many engineers and system integrators are unaware of the effects that an ASD can have on the motor that it drives. This case study addresses one of the adverse affects; motor winding failure due to over voltages.
INTRODUCTION
In general, ac motor ASDs can be divided into two basic categories according to the working principle of the drive circuitry:
Phase controlled front-end rectifiers, output current source inverter (CSI)
Uncontrolled diode-bridge rectifier front-end, dc link, and voltage source inverter (VSI)
Until the late 1980’s, the inverters of large drives were Thyristor based with either forced-commutation or load-commutation. For CSI drives based on Thyristor or GTO devices, the inverter switching frequency was limited to several hundred Hz. This switching frequency implies that these devices have relatively high commutation losses and need a relatively long commutation period. Consequently, motors supplied by CSI drives will have less chance of seeing fast-front voltages and therefore, are not discussed.
Drives for most small and medium sized induction motors may utilize voltage source inverters (VSI) to provide variable frequency ac output. The common drive structure adopted by industry consists of an uncontrolled diode-bridge rectifier, dc link, and PWM VSI inverter as shown in Figure 1. The dc link for the VSI type drive is basically a ripple smoothing capacitor. The inverter output waveform is generated by a series of step-like functions. Stray parameters of the circuit and commutation of switching devices from one phase to another prevent an ideal step-change in the output voltage. Steep-front waveform generation is one of the inherent characteristics of a high frequency operation voltage source inverter.
Figure 1 – Oneline Diagram Showing Power System and PWM Circuit
Both frequency and magnitude of the output voltage are adjusted by controlling the inverter’s operation. State of the art VSIs are based on IGBT technology. With IGBT devices, the inverter operates with a switching frequency ranging from tens of Hz to tens of thousands of Hz. Figure 2 illustrates the typical output voltage of a PWM drive. The switching frequency of the most commonly used PWM drives is in the range of 1000 Hz to 5000 Hz. The rise times of the pulses for IGBT VSIs can be on the order of 10μs to 0.1μs.
Figure 2 – Measured Line-to-Line Output Voltage of a Typical PWM ASD
THE PROBLEM
The problem occurs on the output of the ASD at the drive terminals. The high switching frequency of IGBTs allows sophisticated PWM schemes to be implemented. One of the advantages of the high switching frequency inverter is the reduction of low order harmonics, which results in the reduction of the output filter requirements. However, this benefit can only be achieved under certain circuit conditions. Under some particular conditions, the fast changing voltage resulting from high frequency switching operation of IGBT VSIs can create severe insulation problems for an induction motor.
Machine insulation integrity is affected by the rate of change of voltage as well as the over voltage magnitude. A voltage with a high rate of change tends to be distributed along a motor’s winding 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 long lengths of cable. Usually, the characteristic impedance of the motor can be ten to one hundred times that of the characteristic impedance of the cable connecting the drive to the motor.
The most harmful effect of the PWM ASD 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. Figure 3 illustrates one pulse transition as measured at the output of the ASD. Figure 4 illustrates several pulse transitions as measured at the terminals of an induction motor being supplied by an ASD. Notice the overshoot at the beginning and end of each pulse.
Figure 3 – Voltage Measured at Output of an ASD Feeding an Induction Motor.
Figure 4 – Voltage Measured at Motor Terminals of a Motor fed by an ASD.
THE SOLUTION
There are many potential mitigation techniques which might be employed to solve the over voltage problem at the motor terminals. With the proven EMTP simulation model, effectiveness of two promising solution methods were explored. These two methods are:
Install a line choke, in series with the connection cable, on the output of the PWM drive.
Install a capacitor, in parallel with the motor, at the motor terminals.
To evaluate the choke solution, a 5% (on the base of the 25 hp motor) inductor was inserted between the drive output and the connecting cable at the drive output terminals. The resulting phase-to-phase voltages at the drive output terminals and at the motor terminals are plotted together in. This ac output choke failed to control motor terminal over voltage as illustrated in Figure 5.
Figure 5 – Phase-to-Phase Voltage at Drive Output and Motor Terminals
The choke inductance did help to reduce the rate of change of the voltage seen by the motor. However, the choke created an extra circuit mesh which formed its current loop through the inverter source, choke inductance and cable equivalent capacitance.
The idea of installing capacitance at the motor terminal is initially drawn from the concept of matching the cable surge impedance with the characteristic impedance seen at the motor terminals. However, a precise matching of the impedance may require an amount of capacitance which is undesired for overall system consideration. Therefore, a compromise solution is to add some amount of capacitance to reduce the motor terminal characteristic impedance, and to introduce the proper level of damping to control the voltage overshoot. Based on this concept, three basic damping circuits can be used. They are:
An over damped circuit
A critically damped circuit
An under damped circuit
Each circuit includes a resistance in series with a capacitor. This series combination will be in parallel with the motor’s windings. The EMTP simulations showed that the best results were obtained when a critically damped circuit was employed at a properly selected resonant frequency. This method assures that the pulse at the end of the cable more closely matches the pulse at the beginning of the cable.
Knowing the switching frequency of the PWM drive was 3000 Hz and based on practical experience, it was decided that a good resonant frequency for the damping circuit would be five times the switching frequency or 15,000 Hz. Using the inductance of the cable (0.027mH for 150 ft. of # 8 AWG) the value of the capacitance was calculated to be 4.2μF and the damping resistance was selected to be 5.5 ohms. This circuit’s damping resistance was calculated based on 15,000 Hz.
Figure 6 – Simulated Phase-to-Phase Voltage at PWM Output Terminals with Damping Circuit
Figure 6 illustrates the effect that the introduced RC circuit has on the voltage at the motor terminals. The negligible over shoot in the voltage waveform illustrates that the circuit is slightly under damped. However, this approach does seem to have some merit. The high voltage reflection at the motor terminals is well under control and the over voltage is less than 1.2 per-unit. But, even more importantly, the steep front of the voltage pulse has been greatly reduced.
SUMMARY
When retrofitting induction motors with PWM ASDs, care must be taken in the length of the cable feeding the motor. Cases have been reported where a cable length as short as fifty feet caused a transients problem at the motor terminals. Conversely, there have been cases where the cable length was over 200 feet without adverse effects. If the problem is detected, an effective solution is to parallel an RC branch right at the motor terminals. Parameter selection for this RC branch is based on the drive circuit information and should be determined on case-by-case basis.
ASD manufacturers are now working with motor manufacturers to match drive duty motors to their drives. The ASD and motor come as a complete package. In fact, some motor manufacturers will not honor warranties for motors that are driven by PWM ASDs. And, in the case of new installations, require that the drive and motor be purchased as a package. The induction motors are designed to withstand the severe duties imposed on them by the high switching frequencies of the PWM drives.
REFERENCES
[1] Melhorn, C. J., Le Tang, “Transient Effects of PWM Drives on Induction Motors,” IEEE Transactions on Industry Applications, Volume 33, Number 4, July/August 1997. [2] Persson, E., “Transient effects in application of PWM inverters to induction motors,” presented at the IEEE/IAS 1991 Pulp and Paper Industry Conf., Montreal, P.Q., Canada, June 3-7, 1991, Paper PID 91-28.
GLOSSARY AND ACRONYMS ASD: Adjustable-Speed Drive PWM: Pulse Width Modulation VSI: Voltage Source Inverter
Published by Alena OTCENASOVA, Juraj ALTUS, Petr HECKO, Marek ROCH, Slovakia, University of Zilina, Faculty of Electrical Engineering, Department of Power Electrical Systems
Abstract. Worsened quality of supplied as well as demanded electricity causes in reality a large financial loss. The main quality parameters monitored today are voltage dips and interruptions. In this context there are relevant statistics of measurement results and especially the possibility of their improvement by using Dynamic Voltage Restorer, for which we have proposed a possible method of control. The proposed regulation DVR is based on Park transformation of immediate values of voltage in network and its feedback transformation. The method is shown and verified on simulation model.
Streszczenie. Pogorszenie jakości dostarczanej jak również zapotrzebowanej energii elektrycznej powoduje powstawanie dużych strat finansowych. Podstawowymi parametrami jakościowymi monitorowanymi obecnie są spadki napięcia i przerwy w zasilaniu. W tym kontekście istnieją adekwatn statystyki pomiarowe, a w szczególności możliwości ich poprawy poprzez użycie dynamicznego układu odtwarzania napięcia (DVR), dla którego zaproponowaliśmy metodę sterowania. Proponowana metoda sterowania układem DVR bazuje na transformacji Parka wartości chwilowych napięcia w sieci i ich transformacji odwrotnej. Metoda została przedstawiona i zweryfikowana na modelu symulacyjnym. (Pomiary parametrów napięcia w praktyce i możliwości poprawy jakości napięcia)
Keywords: power quality, electrical network, voltage dips and interruptions, dynamic voltage restorer, controlling of DVR. Słowa kluczowe: jakość energii, sieć elektryczna, spadki napięcia i przerwy w zasilaniu, dynamiczny układ odtwarzania napięcia, sterowanie układem DVR.
Introduction
Problems with power quality are always hot topic. The quality of electricity is influenced by many factors and to keep the parameters within the required limits is in many cases difficult. Worsened quality of electricity is often caused by customers by the nature of theirs operation. As they are in many cases supplied together with other customers from the point of common coupling it can lead to a situation where one customer’s device may retroactively influence also other customers. Likewise, negative influences can come from a distribution and from the transmission system too.
Study [1] focused on the problems with power quality prepared by the David Chapman from the Copper Development Association confirms that supplied electricity, whose quality does not satisfy relevant standards, causes huge economic losses. During resolving study David Chapman calculated concrete values of losses and says that the problems with power quality cost industry and business in the European Union around 10 billion euro per year.
Quality parameters of voltage
In general, power quality is evaluated according to the quality of electrical voltage. Basic characteristics of the quality parameters are given in the STN EN 50160 „Voltage characteristics of electricity supplied from the public distribution networks” [2]. This standard applies to low and medium voltage supply and is generally valid. It is applicable to electricity networks in European industrial areas, as well as the electricity network supplying the „two families isolated in the desert”.
This leads to various quality requirements. For many customers is the quality of voltage low even if it satisfies the requirements for quality according to STN EN 50160 and electricity is unusable (for example, the frequency of interruptions and voltage drops). Therefore stricter standards exist, for example STN EN 61000-2-2 and STN EN 61000-2-4 [3]. It is important that the customer has agreed properties of energy supplied according to the standards that match the type of operation with suppliers of electricity.
Standards that characterize quality parameters of electricity and specify the limits of parameters serve to ensure the functionality of devices supplied from this network, as well as devices that are connected to the network later [4].
Problems with quality of electricity in operating practice
Power Grid Company [5] deals mainly with power quality measurement, delivery devices that increase the quality of electricity supply and distribution activities of components that are necessary for the production of compensation units, of low power electronics and semiconductors. Fig. 1 shows the statistical processing of various problems in industrial networks. These data are measurement results done by Power Grid Company [5] mainly in Slovakia and minor share in the Czech Republic.
Fig.1. Statistics of the most frequent problems measured by the Power Grid Company
From the qualitative parameters listed in Fig. 1 dips and interruptions of supply voltage cause the highest economic losses [6]. Voltage dips and interruptions are especially dangerous for a group of customers, which are called the sensitive customers. For these customers even short time voltage dip may have the same economic impact as a long-term interruption of power supply. Examples of industries that are the most sensitive to the quality of power supply, including the costs to be paid on one fault (in dollars), are shown in Fig. 2 [6].
Fig.2. The cost of failure Voltage in the industry [6]
Dips (sags) and interruptions of voltage
Sags and interruptions belong among the quality attributes that can be easily identified by today’s network analysers. However solving problems is difficult, especially financially. As this can cause failure, damage to engine and also equipment, which results in considerable financial losses, increased attention is devoted to solving related issues. Their occurrence and frequency is random and are usually caused by external factors (weather, animals, vegetation, traffic accidents …). Statistics of interruptions and voltage drops of voltage (Fig.3) shows that in case of electricity the largest group is voltage dips from 10 to 30 % with duration from 10 to 100 ms.
Fig.3. Statistical processing of voltage dips [5]
In the operation, that deals with production cable harnesses for cars (cutting machine, foaming machine, ultrasonic welder, transporters, lighting, air conditioning, heating) there have been taken comparative measurements of power quality of network on 22 kV and 400 V sides of transformer in the substation MV on shorting terminal for invoice measurement.
At the same time short-term measurements were performed on the electronic board of the individual machines – foaming machine, ultrasonic welding, UPS, cutting machine. The problems presented by the client: power interruptions of electricity supply network – both short and long, voltage dips in the network – the consequent failure of production facilities. More dips, but also interruption in the L3 phase was recorded (Fig. 4) during network analysis.
The measurement was made simultaneously on the LV system and MV system, and therefore dips to low and middle voltage side can be compared. Comparison shows that dips and interruptions on LV side are always caused by dips and interruptions in the MV system.
Power Grid Company offers to solve the problem by installation of “Active voltage conditioner (AVC)”. AVC can eliminate a few seconds duration voltage sags and interruptions by defined way. The company offers several different AVC, which correspond to the different requirements for electric power.
Fig.4. Waveform of dip and interruption of voltage in phase L3 on MV side
AVC parameters are based on customer’s requirements on elimination of the dip length and its level and of course on desired power. The price of the equipment is highly dependent on AVC specifications. There is a possibility of AVC installation, which eliminates voltage failure for example 30 s for power several MV.A. Another possibility is for example use of industrial uninterruptible power supplies (UPS) where price is considerably higher.
The company can install UPS to machines that are most sensitive to power disturbances and therefore cause maximum economic damage. Another possibility is to supply the company from another power line which will supply company by energy in the case of primary source failure.
Another option, which will be discussed in the next part, is to compensate voltages dips and interruptions by Dynamic Voltage Restorer (DVR).
The principle of operation the DVR
Development of such compensation devices is connected with the development of power semiconductor devices. Basic connection of the DVR is in Fig. 5.
Fig.5. The principle of operation the DVR
The DVR is connected in series between the supply network and sensitive load. The voltage dips compensator is composed of single direct energy storage device, inverter, control circuit and a serial transformer. If the voltage dip appears on the supply side, the DVR will respond to this decline, and injects voltage into the network, which is needed to compensate accrued voltage dip. The result of this process is then constant voltage amplitude, which is to terminals of sensitive customer [6].
DVR control algorithms
The most important part in terms of proper function of the DVR is the correct control algorithm. The main function of the control algorithm is to keep required voltage value of sensitive loads in case of failure. Means of proceedings of real applied voltage compensators are “Know – how” of companies that install these devices. Various institutions such as universities, research institutes and specialized companies they are still exploring and testing new voltage compensators control algorithms. In general, it is difficult to design control algorithm, which could be generally applied and would also accept the economic aspects. Therefore, there are several control methods, and their application should be considered individually for each specific project.
There are several basic methods that are used in the control of compensators for dips and interruptions. These are the methods Pre-sag, In-phase and the newest method of Minimal Energy Control (controlling for the minimum energy) [7].
Model DVR based on the Park transformation
In terms of solving the problem of voltage dips, we have proposed a model DVR (Fig. 7). The control algorithm is based on the Park transformation.
Park’s transformation is the transformation connected with the rotor axis in theory of machines. It is also known as the transformation from abc, respectively αβ to dq0 components. Graphical presentation of Park transformation is shown in Fig. 6 [9].
Fig.6. Graphical presentation of Park transformation [9]
Park’s transformation is derived from the equations of Clarke transformation. When we derive equations as a base we use the equations for the calculation general stator variable [9]. Equations (1) and (2) represent mathematical description of the Park transformation:
.
The inverse Park’s transformation can be expressed by a similar procedure. For actual utilization in this paper there are presented only the final equations of the inverse Park transformation (6), (7) and (8).
Equations of Clarke and Park transformation are presented in different literatures in different modifications, while the principle remains the same. Simulation program Matlab/Simulink, which is used to simulate control DVR, calculates Park transformation using equations (3), (4) and (5):
.
Inverse (return) Park transformation is in MatLab described by equations (6), (7) and (8):
.
These equations are implemented into transformation blocks in Matlab/Simulink and are verified using computer simulations in the following part of article.
The principle of model operation DVR
The voltage that is measured in per units at the terminals appliance is transformed into dq0 components. We follow the theory that if the three-phase system is symmetrical, zero or q component does not develop after the Park transformation. If there is the asymmetry (for example the voltage dips in one phase) q component develops as well as the zero component. We compare this transformed value of voltage on appliance with dq0 constants, which represents a symmetrical system. If deviation occurs, the PI controllers regulate the deviation to zero. Then Park’s transformation is applied retroactively, and the resulting voltage is supplied into the PWM generator, which then sends impulses to the inverter, which creates the required voltage. Energy storage supplies inverter by power and it is created by direct ideal source. This voltage is then filtered by LC filter and through serial transformer injected into the network.
Verification of functions of the model DVR
To check correct DVR functioning there were simulated different disturbances and the response of monitored DVR. The DVR is designed and tested on the level of 22 kV, where it is normally installed.
For example, we present DVR reaction on the interphase short circuit that was simulated on the 110 kV voltage level at the time from 0.1 to 0.2 s. The resulting waveforms are shown in Fig. 8 and Fig. 9. In Fig. 8 is the voltage waveform displayed before DVR and the voltage waveform on the terminals of appliance is on the Fig. 9.
Fig.7. The model DVR in Matlab/simulink
Fig.8. The voltage waveform in supply network
Fig.9. The voltage waveform on the terminals of appliance
From the measured curves we can see that the customer has on its terminal constant value of voltage even if voltage failure occurs in the power supply network. It shows that DVR model fulfills its primary function and the proposed control algorithm works as we require from him.
Conclusion
In the article are described some results of measurements of the quality parameters of electricity with focus on voltage sags and interruptions. These disturbances cause significant financial losses for manufacturing companies. Therefore, there have been described ways how to prevent financial losses.
One of such methods is to install the Dynamic Voltage Restorer, which is closer specified in the article. The function of this voltage compensator has been verified by computer simulation and results confirm its proper operation. Installing the DVR is only theoretical with respect to its purchase price and the final choice of appropriate compensation is on the managers of the company.
Acknowledgment
This work was supported by European regional development fund (ERDF), name of project: Centre of excellence of power electronics systems and materials for their components, No.: OPVaV-2008/2.1/01-SORO, ITMS 26220120003.
REFERENCES
[1] Chapman, D., The cost of poor power quality. Power quality application guide. London, UK, 2001 [2] STN EN 50160 (33 0121). 2011. Voltage characteristics of electricity supplied by public electricity networks [3] STN EN 61000-2-4 (33 3432). 2003. Electromagnetic compatibility (EMC). Part 2-4: Environment – Compatibility levels in industrial plants for low-frequency conducted disturbances [4] Altus, J.,Novák, M.; Otčenášová, A.; Pokorný , M. , Electromagnetic compatibility of power systems, EDIS, publisher University of Zilina, SK, 2004, ISBN 80-8070-227-6 [5] http://www.power-grid.eu, + database Power Grid Company [6] Kuo, Y. Z.: Dynamic voltage restorer for sensitive equipment. Arlington, Texas. Dissertation thesis, 2007 [7] Dobrucký, B., Šul, R., Špánik, P., Zabezpečenie napájania kontinuálnych procesov s využitím zariadení FACTS. AT&P journal, Nov. 2005, pp. 79-81. [8] Hečko P. , Compensation voltage dips and interruptions in power station 110/25 kV, Žilina, Slovakia, Dissertation thesis, 2010 [9] Š t ěpina, J., Souměrné složky v teorii točivých elektrických strojů. Academia, Praha, 1969
Authors: prof. Ing. Juraj Altus, PhD., University of Zilina, Faculty of Electrical Engineering, Department of Power Electrical Systems, Univerzitná 1, 010 26 Žilina, Slovakia, E-mail: Juraj.Altus@fel.uniza.sk; doc. Ing. Alena Otčenášová, PhD., University of Zilina, Department of Power Electrical Systems, Alena.Otcenasova@fel.uniza.sk; Ing. Petr Hečko, E-mail: petrhecko@gmail.com.
Source & Publisher Item Identifier: PRZEGLĄD ELEKTROTECHNICZNY (Electrical Review), ISSN 0033-2097, R. 88 NR 9a/2012
Published by P. M. Balasubramaniam1 and S. U. Prabha2* 1Department of Electrical and Electronics Engineering, Sri Shakthi Institute of Engineering and Technology, Coimbatore, India 2Department of Electrical and Electronics Engineering, Sri Ramakrishna Engineering College, Coimbatore, India *Corresponding author. E-mail: baluanujayen@gmail.com
Abstract Classically, the aim of the electric power system is to generate electrical energy and to deliver this energy to the end-user equipment at an acceptable voltage. As nonlinear loads draw harmonic and reactive power components of current from ac mains, the quality of power deteriorates. This paper presents a review of the main power quality (PQ) problems with their associated causes and solutions with codes and standards. This paper concludes with some solutions to mitigate the Power Quality problems are presented.
Key words: IEEE 519, Total Harmonic Distortion, Point of Common Coupling, Total Demand Distortion
1. Introduction
Power quality (PQ) related issues are of most concern nowadays. The widespread use of electronic equipment, such as information technology equipment, power electronics such as adjustable speed drives (ASD), programmable logic controllers (PLC), energy-efficient lighting, led to a complete change of electric loads nature [1-3]. These loads are simultaneously the major causers and the major victims of power quality problems. Due to their non-linearity, all these loads cause disturbances in the voltage waveform.
Although many efforts have been taken by utilities, some consumers require a level of power quality higher than the level provided by modern electric networks [4]. This implies that some measures must be taken in order to achieve higher levels of power quality in Figure 1.
Figure 1. Sources of the review materials.
This paper provides an overview of major power quality issues, solutions and related standards based on an extensive number of publications. The referred publications are mainly extracted from IEEE transactions, IEEE Magazines, IEEE Proceedings, IEE/IET (Institution of Electrical Engineers, currently Institution of Engineering and Technology) Proceedings as well as a few, yet very important Conferences and patent documents on power quality and power quality improvement modules.
The review will be particularly useful for: 1) power system designers and researchers engaged in design, optimization, and quality-enhancement activities in today’s competitive environment; 2) practising engineers who would like to enrich their educational background about the system interaction aspects of power system for any application; and 3) undergraduate and postgraduate students who wish to integrate power quality issues and solutions with modern computing practices.
The paper starts with a short introduction to power quality issues (see section 2). In section 3, the solutions reviewed from various sources for power quality are given. A detailed literature review on codes and standards related to power quality are illustrated in sections 4. Finally, section 5 presents the summary and conclusions.
2. Types of Power Quality Issues
The most common types of power quality problems are presented in Table 1.
Table 1. Most common power quality issues [5-15]
.
Table 1. Continued
.
3. Solutions for Power Quality Issues
The mitigation of PQ problems may take place at different levels: transmission, distribution and the end-use equipment. Several measures can be taken at these levels are they are described in the following sections.
3.1 Grid Adequacy
Many PQ problems have origin in the transmission or distribution grid. Thus, a proper transmission and distribution grid, with adequate planning and maintenance, is essential to minimize the occurrence of PQ problems.
3.2 Unified Power Quality Conditioner (UPQC)
UPQC allows the alleviation of voltage and current disturbances that could affect sensitive electrical loads while compensating the load reactive power [16-19]. UPQC consists of combined series and shunt active power filters [20]. The main function of UPQC includes [21-25]:
(i) Reactive power compensation. (ii) Voltage regulation. (iii) Compensation for voltage sags and swells. (iv) Unbalance compensation for current and voltage (for 3-phase systems). (v) Neutral current compensation (for 3-phase 4-wire systems).
3.3 Distributed Resources – Energy Storage Systems
Interest in the use of distributed energy resources (DER) has increased substantially over the last few years because of their potential to provide increased reliability. These resources include distributed generation and energy storage systems. Energy storage systems, also known as restoring technologies, are used to provide the electric loads with ride-through capability in poor PQ environment. Recent technological advances in power electronics and storage technologies are turning the restoring technologies one of the premium solutions to mitigate PQ problems [26].
The first energy storage technology used in the field of PQ, yet the most used today, is electrochemical battery. Although new technologies, such as flywheels, Super-capacitors and Superconducting Magnetic Energy Storage (SMES) present many advantages, electrochemical batteries still rule due to their low price and mature technology [27].
3.4 Flywheels
A flywheel is an electromechanical device that couples a rotating electric machine (motor/generator) with a rotating mass to store energy for short durations. The motor/generator draws power provided by the grid to keep the rotor of the flywheel spinning. During a power disturbance, the kinetic energy stored in the rotor is transformed to DC electric energy by the generator, and the energy is delivered at a constant frequency and voltage through an inverter and a control system.
Traditional flywheel rotors are usually constructed of steel and are limited to a spin rate of a few thousand revolutions per minute (RPM). Advanced flywheels constructed from carbon fibre materials and magnetic bearings can spin in vacuum at speeds up to 40,000 to 60,000 RPM. The stored energy is proportional to the moment of inertia and to the square of the rotational speed. High speed flywheels can store much more energy than the conventional flywheels.
The flywheel provides power during a period between the loss of utility supplied power and either the return of utility power or the start of a back-up power system (i.e., diesel generator). Flywheels typically provide 1-100 seconds of ride-through time, and back-up generators are able to get online within 5-20 seconds.
3.5 Super-capacitors
Super-capacitors (also known as ultra-capacitors) are dc energy sources and must be interfaced to the electric grid with a static power conditioner, providing energy output at the grid frequency. A super-capacitor provides power during short duration interruptions or voltage sags. Medium size super-capacitors are commercially available to implement ride through capability in small electronic equipment, but large super-capacitors are still in development, but may soon become a viable component of the energy storage field [28,29].
Capacitance is very large because the distance between the plates is very small (several angstroms), and because the area of conductor surface (for instance of the activated carbon) reaches 1500-2000 m2/g (16000-21500 ft2/g). Thus, the energy stored by such capacitors may reach 50-60 J/g [30-38].
3.6 Superconducting Magnetic Energy Storage (SMES)
A magnetic field is created by circulating a DC current in a closed coil of superconducting wire. The path of the coil circulating current can be opened with a solid-state switch, which is modulated on and off. Due to the high inductance of the coil, when the switch is off (open), the magnetic coil behaves as a current source and will force current into the power converter which will charge to some voltage level. Proper modulation of the solid-state switch can hold the voltage within the proper operating range of the inverter, which converts the DC voltage into AC power. Atypical SMES system.
Low temperature SMES cooled by liquid helium is commercially available [39,40]. High temperature SMES cooled by liquid nitrogen is still in the development stage and may become a viable commercial energy storage source in the future due to its potentially lower costs. SMES systems are large and generally used for short durations, such as utility switching events.
3.7 Enhanced Interface Devices
Using proper interface devices, one can isolate the loads from disturbances deriving from the grid. The common interface devices are dynamic voltage restorer (DVR), transient voltage surge suppressors (TVSS), constant voltage transformers (CVT), Noise Filters, Active Power Filters, Isolation Transformers, Static VAR Compensators (SVC), Harmonic Filters and reactor for power quality improvement.
A dynamic voltage restorer (DVR) acts like a voltage source connected in series with the load. The output voltage of the DVR is kept approximately constant voltage at the load terminals by using a step-up transformer and/or stored energy to inject active and reactive power in the output supply trough a voltage converter.
Transient voltage surge suppressors are used as interface between the power source and sensitive loads, so that the transient voltage is clamped by the TVSS before it reaches the load. TVSSs usually contain a component with a nonlinear resistance (a metal oxide varistor or a zener diode) that limits excessive line voltage and conduct any excess impulse energy to ground.
Constant voltage transformers (CVT) were one of the first PQ solutions used to mitigate the effects of voltage sags and transients. To maintain the voltage constant, they use two principles that are normally avoided: resonance and core saturation. Atypical constant voltage transformer. When the resonance occurs, the current will increase to a point that causes the saturation of the magnetic core of the transformer. If the magnetic core is saturated, then the magnetic flux will remain roughly constant and the transformer will produce an approximately constant voltage output. If not properly used, a CVT will originate more PQ problems than the ones mitigated. It can produce transients, harmonics (voltage wave clipped on the top and sides) and it is inefficient (about 80% at full load). Its application is becoming uncommon due to technological advances in other areas.
Noise filters are used to avoid unwanted frequency current or voltage signals (noise) from reaching sensitive equipment. This can be accomplished by using a combination of capacitors and inductances that creates a low impedance path to the fundamental frequency and high impedance to higher frequencies, that is, a low-pass filter. They should be used when noise with frequency in the kilo Hertz range is considerable.
Static VAR compensators (SVR) use a combination of capacitors and reactors to regulate the voltage quickly. Solid-state switches control the insertion of the capacitors and reactors at the right magnitude to prevent the voltage from fluctuating. The main application of SVR is the voltage regulation in high voltage and the elimination of flicker caused by large loads (such as induction furnaces).
Harmonic filters are used to reduce undesirable harmonics. They can be divided in two groups: passive filters and active filters [41-43]. Passive filters consist in a low impedance path to the frequencies of the harmonics to be attenuated using passive components (inductors, capacitors and resistors). Several passive filters connected in parallel may be necessary to eliminate several harmonic components [44]. If the system varies (change of harmonic components), passive filters may become ineffective and cause resonance.
Active filters analyse the current consumed by the load and create a current that cancel the harmonic current generated by the loads. Active filters were expensive in the past, but they are now becoming cost effective compensating for unknown or changing harmonics [45].
Reactor for power quality improvement is a patented technology [46-49]. The device is installed on a power line, for removing noise that flows into the power line, characterized by: a reactor having a first coil and a second coil wound on a core disposed between the input end and the output ends of the power line, wherein in the reactor the first end of the first coil is connected to the input end of a first power line, the second end of the first coil is connected to the output end of the first power line, and the first end of the second coil is connected to the output end of the second power line, and the second end of the second coil is connected to the input end of the second power line.
3.8 Make End-use Devices Less Sensitive
Designing the equipment to be less sensitive to disturbances is usually the most cost effective measure to prevent PQ problems. Some manufacturers of end-use equipment are now recognising this problem, but the competitive market means that manufacturers should reduce costs and only respond to customers’ requirements. The exception is the ASD market, where manufacturers are actively promoting products with enhanced ride-through capabilities.
Some of the energy meters have the capability for power quality detection, monitoring, reporting, recording and communication in a revenue accuracy electrical power meter is disclosed [50]. Transient events are detected by monitoring the Wave shape of the electrical power and comparing deviations to a known threshold. Sags and Swells are detected by computing root mean square value over a rolling Window and comparing the computed value with a known threshold. Harmonic frequencies and symmetrical components are quantified by a known algorithm and compared with a known threshold [51]. Adding a capacitor with a larger capacity to power supplies, using cables with larger neutral conductors, derating transformers and adjusting under voltage relays, are measures that could be taken by manufacturers to reduce the sensitivity of equipment to PQ problems.
4. Codes and Standards
Some measures have been taken to regulate the minimum PQ level that utilities have to provide to consumers and the immunity level that equipment should have to operate properly when the power supplied is within the standards. Standardization organizations like IEC, CENELEC, and IEEE have developed a set of standards with the same purposes. In Europe, the most relevant standards in PQ are the EN 50160 (by CENELEC) and IEC 61000. IEEE power quality standards do not have such a structured and comprehensive set as compared to IEC [52]. Nonetheless, the IEEE standards give more practical and some theoretical background on the phenomena, which makes it a very useful reference. Some of the IEEE power quality standards are described in the ensuing sections.
4.1 IEEE 519
Power system problems that were associated with harmonics began to be of general concern in the 1970s, when two independent developments took place. The first was the oil embargo, which led to price increases in electricity and the move to save energy. Industrial consumers and utilities began to apply power factor improvement capacitors. The move to power factor improvement resulted in a significant increase in the number of capacitors connected to power systems. American standards regarding harmonics have been laid out by the IEEE in the 519 Standard: IEEE Recommended Practices and Requirements for Harmonic Control in Electric Power Systems. There is a combined effect of all nonlinear loads on utility systems that have a limited capability to absorb harmonic current. Further, utilities are charged with the responsibility to provide a high quality supply in terms of voltage level and waveform. IEEE 519 recognizes not only the absolute level of harmonics produced by an individual source but also their size relative to the supply network. It should be noted that IEEE 519 is limited to being a collection of Recommended Practices that serve as a guide to both suppliers and consumers of electrical energy. Where problems exist, because of excessive harmonic current injection or excessive voltage distortion, it is incumbent upon supplier and consumer to resolve the issues within a mutually acceptable framework [53].
4.2 IEEE 519 Standard for Harmonic Voltage Limits
According to IEEE 519 Table 2 shows that, harmonic voltage distortion on power system 69 kV and below is limited to 5% Total Harmonic Distortion with each individual harmonic limited 3% [54].
Table 2. Harmonic voltage distortion limits
.
4.3 IEEE 519 Standard for Harmonic Current Limits
General distribution systems [GDS 120V-69,000 V]: Current distortion limits are for odd harmonics. Even harmonics are limited to 25% of the odd Harmonic limits. For all power generation equipment, distortion limits are those with ISC/IL < 20. ISC is the maximum short circuit current at the point of coupling “PCC”. IL is the maximum fundamental frequency 15-or 30- minutes load current at PCC. TDD is the total demand distortion (= THD normalized by IL are shown in Table 3).
Table 3. Harmonic current distortion limits
.
General sub-transmission systems [GSTS 69 kV-161 kV]: The current harmonic distortion limits apply to limits of harmonics that loads should draw from the utility at the PCC. Note that the harmonic limits differ based on the ISC/IL rating, where ISC is the maximum short circuit current at the PCC, and I is the maximum demand load current at the PCC.
ISC is the available short circuit current at the point of common coupling. The ISC is determined by the size, impedance, and voltage of the service feeding the PCC. IL is the maximum demand load current (fundamental frequency component) measured at the PCC are shown in Table 4. It is suggested that existing facilities measure this over a period of time and average it. Those creating new designs should calculate the IL using anticipated peak operation of the facility. The point of common coupling with the consumer/utility interface is the closest point on the utility side of the customer service where another utility service customer is or could be supplied. The ownership of any apparatus such as a transformer that the utility might provide in the customers system is immaterial to the definition of PCC. This definition has been approved by IEEE working group.
Table 4. Maximum harmonic current distortion level
.
4.4 IEEE Standard 142-1991, Recommended Practice for Grounding of Industrial and Commercial Power Systems [55]
This standard presents a thorough investigation of the problems of grounding and the methods for solving these problems. There is a separate chapter for grounding sensitive equipment.
4.5 IEEE Standard 446-1987, Recommended Practice for Emergency and Standby Power Systems for Industrial and Commercial Applications
This standard is recommended engineering practices for the selection and application of emergency and standby power systems. It provides facility designers, operators and owners with guidelines for assuring uninterrupted power, virtually free of frequency excursions and voltage dips, surges, and transients.
4.6 IEEE Standard 1100-1999, Recommended Practice for Powering and Grounding Sensitive Electronic Equipment
Recommended design is installation, and maintenance practices for electrical power and grounding (including both power-related and signal-related noise control) of sensitive electronic processing equipment used in commercial and industrial applications.
4.7 IEEE Standard 1346-1998 Recommended Practice for Evaluating Electric Power System Compatibility with Electronic Process Equipment
A standard methodology for the technical and financial analysis of voltage sag compatibility between process equipment and electric power systems is recommended. The methodology presented is intended to be used as a planning tool to quantify the voltage sag environment and process sensitivity.
4.8 IEEE Standards Related to Voltage Sag and Reliability
The distribution voltage quality standard i.e. IEEE Standard P1564 gives the recommended indices and procedures for characterizing voltage sag performance and comparing performance across different systems. A new IEC Standard 61000-2-8 titled “Environment – Voltage Dips and Short Interruptions” has come recently. This standard warrants considerable discussion within the IEEE to avoid conflicting methods of characterizing system performance in different parts of the world.
4.9 IEEE Standards Related to Flicker
Developments in voltage flicker standards demonstrate how the industry can successfully coordinate IEEE and IEC activities. IEC Standard 61000-4-15 defines the measurement procedure and monitor requirements for characterizing flicker. The IEEE flicker task force working on Standard P1453 is set to adopt the IEC standard as its own.
4.10 Standards Related to Custom Power
IEEE Standard P1409 is currently developing an application guide for custom power technologies to provide enhanced power quality on the distribution system. This is an important area for many utilities that may want to offer enhanced power quality services.
4.11 Standards Related to Distributed Generation
The new IEEE Standard P1547 provides guidelines for interconnecting distributed generation with the power system.
4.12 420-2013 – IEEE Standard for the Design and Qualification of Class 1E Control Boards, Panels and Racks Used in Nuclear Power Generating Stations
This standard specifies the design requirements for new and/or modified Class 1E control boards, panels, and racks and establishes the methods to verify that these requirements have been satisfied. Methods for meeting the separation criteria contained in IEEE Std 384 are addressed. Qualification is also included to address the overall requirements of IEEE Std 323 and recommendations of IEEE Std 344.
4.13 IEEE Standard 384-2008 – IEEE Standard Criteria for Independence of Class 1E Equipment and Circuits
The independence requirements of the circuits and equipment comprising or associated with Class 1E systems are described. Criteria for the independence that can be achieved by physical separation and electrical isolation of circuits and equipment that are redundant are set forth. The determination of what is to be considered redundant is not addressed.
4.14 IEEE Standard C57.18.10-1998 – IEEE Standard Practices and Requirements for Semiconductor Power Rectifier Transformers
Practices and requirements for semiconductor power rectifier transformers for dedicated loads rated single-phase 300 kW and above and three-phase 500 kW and above are included. Static precipitators, high-voltage converters for DC power transmission, and other nonlinear loads are excluded. Service conditions, both usual and unusual, are specified, or other standards are referenced as appropriate. Routine tests are specified. An informative annex provides several examples of load loss calculations for transformers when subjected to non-sinusoidal currents, based on calculations provided in the standard.
4.15 IEEE Standard C57.21-1990 -IEEE Standard Requirements, Terminology and Test Code for Shunt Reactors Rated Over 500 kVA
All oil-immersed or dry-type, single-phase or three phase, outdoor or indoor shunt reactors rated over 500 kVA are covered. Terminology and general requirements are stated, and the basis for rating shunt reactors is set forth. Routine, design, and other tests are described, and methods for performing them are given. Losses and impedance, temperature rise, dielectric tests, and insulation levels are covered. Construction requirements for oil immersed reactors and construction and installation requirements for dry-type reactors are presented.
5. Summary and Conclusions
This paper gives a comprehensive review by critical analyzing about power quality problems, issues, related international standards, and the solutions. The correct solutions are also discussed which can be remedy for power quality problems generated in different phenomena. Coordination with existing industry practices and international harmonic standards is also considered in this paper. To overcome the negative impact of poor power quality on equipment and businesses, suitable power quality equipment can be invested. Identifying the right solution remains the first step. Many power quality problems are easily identified once a good description of the problems is obtained. Unfortunately, the tensions caused by power problems often result in vague or overly dramatic descriptions of the problem.
A power quality audit can help determine the causes of your problems and provide a well designed plan to correct them. The power quality audit checks the facility’s wiring and grounding to ensure that it is adequate for your applications and up to code. The auditor normally will check the quality of the ac voltage itself, and consider the impact of the utility’s power system. Many businesses and organizations rely on computer systems and other electrical equipment to carry out the mission critical functions, but they aren’t safeguarding against the dangers of an unreliable power supply. It is time utilities as well as businesses engage in more proactive approach to power quality treats by engaging in power quality analysis.
References
[1] Bollen, M., “Understanding Power Quality Problems – Voltage Sags and Interruptions”, IEEE Press Series on Power Engineering – John Wiley and Sons, Piscataway, USA(2000). doi: 10.1109/9780470546840.ch4 [2] Rajakumar, P., et al., Review on Power Quality Issues, IRACST – Engineering Science and Technology: An International Journal (ESTIJ), ISSN: 2250-3498, Vol.2, No. 1 (2012). [3] Delgado, J., Gestão da Qualidade Total Aplicada ao Sector do Fornecimento da Energia Eléctrica, Ph. D. Dissertation Electrotechnical Engineering, Universidade de Coimbra, Portugal September (2002). [4] Choi, W. Y., Kwon, J., Kim, E. H., Lee, J. J. and Kwon, B. H., “Bridgeless Boost Rectifier with Low Conduction Losses and Reduced Diode Reverse Recovery Problems,” IEEE Trans. Ind. Electron., Vol. 54, No. 2, pp. 769 – 780 (2007). doi: 10.1109/TIE.2007.891991 [5] Lin, T., Domijan, A., Jr. and Chu, F., “A Survey of Techniques for Power Quality Monitoring,” Int. J. Power Energy Syst., Vol. 25, No. 3, pp. 167-172 (2012). [6] Chen, G., Chen, Y. and Smedley, K. M., “Three-phase Four-leg Active Power Quality Conditioner without References Calculation,” Proc. Appl. Power Electron. Conf., pp. 587-593 (2004). doi: 10.1109/APEC.2004. 1295866 [7] López, Y.-K., de Vicuña, L. G., Castilla, M., Matas, J. and López, M., “Sliding-mode-control Design of a High-power-factor Buck-boost-rectifier,” IEEE Trans. Ind. Electron., Vol. 46, pp. 604-612 (1999). [8] Mollov, S. V. and Forsyth, A. J., “Analysis, Design and Resonant Current Control for a 1-MHz High – powerfactor Rectifier,” IEEE Trans. Ind. Electron., Vol. 46, pp. 620-627 (1999). doi: 10.1109/41.767070 [9] García, A. J., Cobos, J. A., Prieto, R., Alou, P. and Uceda, J., “An Alternative to Supply DC Voltages with High Power Factor,” IEEE Trans. Ind. Electron., Vol.46, pp. 703-709 (1999). doi: 10.1109/41.778219 [10] Lee, J.-Y., Moon, G.-W. and Youn, M.-J., “Design of a Power-factor-Correction Converter Based on Halfbridge Topology,” IEEE Trans. Ind. Electron., Vol. 46, pp. 710-723 (1999). doi: 10.1109/41.778222 [11] Madigan, M. T., Erickson, R.W. and Ismail, E. H., “Integrated High-quality Rectifier-regulators,” IEEE Trans. Ind. Electron., Vol. 46, pp. 749-758 (1999). doi: 10.1109/41.778229 [12] Wu, T.-F. and Chen, Y.-K., “Analysis and Design of an Isolated Single Stage Converter Achieving Power factor Correction and Fast Regulation,” IEEE Trans. Ind. Electron., Vol. 46, pp. 759-767 (1999). doi: 10.1109/41.778230 [13] Hsieh, G.-C. andWang, C.-M., “ZCS-PWM Full-wave Boost Rectifier with Unity Power Factor and Low Conduction Losses,” IEEE Trans. Ind. Electron., Vol. 46, pp. 768-779 (1999). doi: 10.1109/41.778234 [14] Tseng, C.-J. and Chen, C.-L., “ANovel ZVT PWMCúk Power-factor Corrector,” IEEE Trans. Ind. Electron., Vol. 46, pp. 780-787 (1999). doi: 10.1109/41.778240 [15] Ferracci, P., “Power Quality,” Schneider Electric Cahier Technique, No. 199, September (2012). [16] Zhu, P., Li, X., Kang, Y. and Chen, J., “A Novel Control Scheme in 2-phase Unified Power Quality Conditioner,” in Proc. 29th Annu. Conf. IEEE Ind. Electron. Soc., pp. 16917-1622 (2003). doi:10.1109/TIE.2014.2314055 [17] Ghosh, A., Jindal, A. K. and Joshi, A., “Modified Power Quality Conditioner for Voltage Regulation of Critical Load Bus,” Proc. Power Eng. Soc. Gen. Meet., pp. 471-476 (2004). doi: 10.1109/PES.2004.1372840 [18] Cheng, Y. and Philippe, L., “Advanced Control Methods for the 3-phase Unified Power Quality Conditioner,” Proc. Power Electron. Spec. Conf., pp. 4263-4267 (2004). doi: 10.1109/PESC.2004.1354754 [19] Tlusty, J. and Valouch, V., “Effectiveness of Unified Power Quality Conditioner for Flicker Mitigation,” Proc. 4th Int. Power Electron. Motion Control Conf. , pp. 902-907 (2004). [20] Khadkikar, V., Agarwal, P., Chandra, A., Barry, A. and Nguyen, T., “A Simple New Control Technique for Unified Power Quality Conditioner (UPQC),” Proc. 11th Int. Conf. Harmonics Quality Power, pp. 289-293 (2004). doi: 10.1109/ICHQP.2004.1409369 [21] Esfandiari, A., Parniani, M. and Mokhtari, H., “Mitigation of Electric Arc Furnace Disturbances Using the Unified Power Quality Conditioner,” Proc. 30th Annu. Conf. Ind. Electron. Soc., pp. 1469-1474 (2004). doi: 10.1109/IECON.2004.1431795 [22] Sepulveda, C. A., Espinoza, J. R., Moran, L. A. and Ortega, R., “Analysis and Design of a Linear Control Strategy for Three-phase UPQCs,” Proc. 30th Annu. Conf. IEEE Ind. Electron. Soc., Vol. 3, pp. 3060-3065 (2004). doi: 10.1109/IECON.2004.1432300 [23] Ng, F., Wong, M. C. and Han, Y. D., “Analysis and Control of UPQC and its DC-link Power by Use of pq-r Instantaneous Power Theory,” Proc. Power Electron. Syst. Appl., pp. 43-53 (2004). doi: 10.1109/TPEL. 2004.826499 [24] Tey, L. H., So, P. L. and Chu, Y. C., “Unified Power Quality Conditioner for Improving Power Quality Using ANN with Hysteresis Control,” Proc. Int. Conf. Power Syst. Technol., pp. 1441-1446 (2004). doi: 10. 1109/ICPST.2004.1460229 [25] Vinod Khadkikar, Enhancing Electric Power Quality Using UPQC: AComprehensive Overview, IEEE Transactions on Power Electronics, Vol. 27, No. 5 (2012). doi: 10.1109/TPEL.2011.2172001 [26] Ribeiro, P., Johnson, B., Crow, M., Arsoy, A. and Liu, Y., “Energy Storage Systems for Advanced Power Applications,” Proceedings of the IEEE, Vol. 89, No. 12, (2001). doi: 10.1109/5.975900 [27] Kwon, J. M., Choi, W. Y. and Kwon, B. H., “Cost-effective Boost Converter with Reverse-recovery Reduction and Power Factor Correction,” IEEE Trans. Ind. Electron., Vol. 55, No. 1, pp. 471-473 (2008). doi: 10.1109/TIE.2007.896526 [28] Tofoli, F. L., Coelho, E. A. A., de Freitas, L. C., Farias, V. J. and Vieira, J. B. Jr., “Proposal of a Soft-switching Single-phase Three-level Rectifier,” IEEE Trans. Ind. Electron., Vol. 55, No. 1, pp. 107-113 (2008). doi: 10. 1109/TIE.2007.896052 [29] IEEE Recommended Practice for Monitoring Electric Power Quality, IEEE Std. 1159-2009 (2009). [30] Ghosh, A., Jindal, A. K. and Joshi, A., “Inverter Control Using Output Feedback for Power Compensating Devices,” Proc. Convergent Technol. Conf. , pp. 48-52 (2003). doi: 10.1109/TENCON.2003.1273212 [31] Rodríguez, E., Abud, D. and Arau, J., “ANovel Singlestage Single-phase DC Uninterruptible Power Supply with Power-factor Correction,” IEEE Trans. Ind. Electron., Vol. 46, pp. 1137-1147 (1999). doi: 10.1109/41.808002 [32] Alonso, J. M., Calleja, A. J., López, E., Ribas, J. and Scades, M. R., “A Novel Single-stage Constant-wattage High-power-factor Electronic Ballast,” IEEE Trans. Ind. Electron., Vol. 46, pp. 1148-1158 (1999). doi: 10.1109/41.808004 [33] Pires, V. F. and Silva, J. F., “Half-bridge Single-phase Buck-boost Type AC-DC Converter with Sliding Mode Control of the Input Source Current,” Proc. IEE-Elect. Power Applicat., Vol. 147, No. 1, pp. 61-67 (2000). doi: 10.1049/ip-epa:20000020 [34] Siu, K.-W. and Lee, Y.-S., “A Novel High-efficiency Flyback Power-factor Correction Circuit with Regenerative Clamping and Soft Switching,” IEEE Trans. Circuits Syst. I, Vol. 47, pp. 350-356 (2000). doi: 10.1109/81.841917 [35] Matsui, K., Yamamoto, I., Hirose, S., Ando, K. and Kobayashi, T., “Utility-Interactive Photovoltaic Power Conditioning Systems with Forward Converter for Domestic Applications,” Proc. IEE-Elect. Power Applicat., Vol. 147, No. 3, pp. 199-205 (2000). doi: 10.1049/ipepa:20000374 [36] Buso, S., Spiazzi, G. and Tagliavia, D., “Simplified Control Technique for High-power-factor Flyback Cuk and Sepic Rectifiers Operating in CCM,” IEEE Trans. Ind. Applicat., Vol. 36, pp. 1413-1418 (2000). doi: 10.1109/28.871291 [37] Anderson, G. K. and Blaabjerg, F., “Current Programmed Control of a Single Phase Two-switch Buckboost Power Factor Correction Circuit,” Proc. IEEE APEC’01, pp. 350-356 (2001). doi: 10.1109/APEC. 2001.911671 [38] Chen, J., Maksimovic, D. and Erickson, R., “A New Low-stress Buck-boost Converter for Universal-input PFC Applications,” Proc. IEEE APEC’01, pp. 343-349 (2001). [39] Amaro, N. and Ceballos, J. M., “A Fast Algorithm for Initial Design of HTS Coils for SMES Applications,” IEEE Transactions on Applied Superconductivity, Vol.23, No. 3 (2013). doi: 10.1109/TASC.2012.2231912 [40] Chen, X. Y. and Jin, J. X., “Development of SMES Technology and its Applications in Power Grid,” Proc. Int. Conf. Appl. Supercond. Electromagn. Dev., pp.260-269 (2011). doi: 10.1109/ASEMD.2011.6145115 [41] Singh, B., Al-Haddad, K. and Chandra, A., “A Review of Active Filters for Power Quality Improvement,” IEEE Transactions on Industrial Electronics, Vol. 46, No. 5 (1999). doi: 10.1109/41.793345 [42] Singh, B., Al-Haddad, K. and Chandra, A., “A Review of Active Filters for Power Quality Improvement,” IEEE Trans. Ind. Electron., Vol. 46, No. 5, pp. 960-971 (2012). doi: 10.1109/41.793345 [43] Active Filters: Technical Document, 2100/1100 Series, Mitsubishi Electric Corp., Tokyo, Japan, pp. 1-36 (1989). [44] Kikuchi, A. H., “Active Power Filters,” in Toshiba GTRModule (IGBT) Application Notes, Toshiba Corp., Tokyo, Japan, pp. 44-45 (1992). [45] Gyugyi, L. and Strycula, E., “ActiveAC Power Filters,” in Conf. Rec. IEEE-IAS Annu. Meeting, pp. 529-535 (1976). [46] Inventor: KIM, Seon Ho Daejeon Metropolitan City 301-030 (KR), Device For Improving Power Quality, European Patent Application Number: 11809896.1, Date of Filing: 22.07.2011. [47] US Patent N0.: US 6,615,147 B1, Date of Patent: Sep.2, Inventors: Rene T. J. Onker, British Columbia, Invention: Revenue Meter with Power Quality Features (2003). [48] US Patent No.: US 8,326,576 B2, Date of Patent: Dec. 4, Inventor: Man-0n Pun, Cambridge, MA(U S), Invention: Detecting Power Quality Events in Power Distribution Networks (2012). [49] US Patent No.: WO 2011124223 A3, Date of Patent: Nov 29, Inventor(s): Lucian Asiminoaei, Sergej Kalaschnikow, Invention: Power Quality Improvement by Active Filter (2012). [50] Revenue Meter with Power Quality Features, Patent N0.: US 6,615,147 B1, Date of Patent: Sep. 2, Inventor: Rene T. J. Onker, British Columbia (2003). [51] Routimo, M., Salo, M. and Tuusa, H., “Improving the Active Power Filter Performance with a Prediction Based Reference Generation,” Nordic Workshop on Power and Industrial Electronics, Norpie (2004). [52] IEEE Standard 446-1987, “IEEE Recommended Practice for Emergency and Standby Power Systems for Industrial and Commercial Applications,” (IEEE Orange Book). doi: 10.1049/pe:19890012 [53] IEEE Std 1250-1995, “IEEE Guide for Service to Equipment Sensitive to Momentary Voltage Disturbances,” Art 5.1.1, Computers. [54] IEEE, “IEEE Guide for Service to Equipment Sensitive to Momentary Voltage Disturbances,” IEEE Std. 1250-1995. [55] IEEE Recommended Practice for Grounding of Industrial and Commercial Power Systems, 142-1991 (1992). doi: 10.1049/pe:19890012
Power Quality Blog 