Power Quality Blog

Power Quality Problems and New Solutions

Published by A. de Almeida, L. Moreira, J. Delgado, ISR – Department of Electrical and Computer Engineering, University of Coimbra, Pólo II, 3030-290 Coimbra (Portugal),

Phone: +351 239 796 218, fax: +351 239 406 672, E-mail: adealmeida@isr.uc.pt, licinio@isr.uc.pt, Jdelgado@elect.estv.ipv.pt.

Abstract: In this paper, the main Power Quality (PQ) problems are presented with their associated causes and consequences. The economic impacts associated with PQ are characterized. Finally, some solutions to mitigate the PQ problems are presented.

Key words: Power Quality, Power Quality problems, Power Quality costs, Power Quality solutions.

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. 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.

Along with technology advance, the organization of the worldwide economy has evolved towards globalisation and the profit margins of many activities tend to decrease. The increased sensitivity of the vast majority of processes (industrial, services and even residential) to PQ problems turns the availability of electric power with quality a crucial factor for competitiveness in every activity sector. The most critical areas are the continuous process industry and the information technology services. When a disturbance occurs, huge financial losses may happen, with the consequent loss of productivity and competitiveness.

Although many efforts have been taken by utilities, some consumers require a level of PQ higher than the level provided by modern electric networks. This implies that some measures must be taken in order to achieve higher levels of Power Quality.

2. Types of Power Quality Problems

The most common types of Power Quality problems are presented in Table I.

3. Power Quality Characterization

Even the most advanced transmission and distribution systems are not able to provide electrical energy with the desired level of reliability for the proper functioning of the loads in modern society. Modern T&D (transmission and distribution) systems are projected for 99,9 to 99,99% availability. This value is highly dependant of redundancy level of the network, which is different according to the geographical location and the voltage level (availability is higher at the HV network). In some remote sites, availability of T&D systems may be as low as 99%. Even with a 99,99% level there is an equivalent interruption time of 52 minutes per year.

The most demanding processes in the modern digital economy need electrical energy with 99.9999999% availability (9-nines reliability) to function properly.

Between 1992 and 1997, EPRI carried out a study in the US to characterize the average duration of disturbances. The result for a typical site, during the 6-year period is presented below.

Fig. 1 – Typical distribution of PQ disturbances by its duration for a typical facility in 6 years (1992-97) in the US [2].

Table I – Most common Power Quality problems [ 1], [4]

As it can be seen in Fig. 1., the vast majority of the disturbances registered (about 87%) lasted less than 1 second and only 12 have duration greater than 1 minute. It is clear that not all these disturbances cause equipment malfunctioning, but many types of sensitive equipment may be affected.

Another study of EPRI was undertaken, between 1993 and 1999, in order to characterize the PQ on Low Voltage (LV) distribution networks. This study concluded that 92% of disturbances in PQ were voltage sags with amplitude drops up to 50% and duration below 2 seconds. Fig. 2 shows the typical distribution of sags under 0.5 seconds and micro-interruptions.

Fig. 2 – Distribution of sag and micro-interruption in LV networks in US [3].

The situation in developed countries of Europe is very similar to the one observed in the US. Fig. 3 shows the characterization of PQ in an industrial area of the center of Portugal by monitoring of the supply in the period February 2002-January 2003.

Fig. 3 – Characterization of electrical energy supply disturbances in an industrial facility in Portugal.

4. Costs of Power Quality Problems

The costs of PQ problems are highly dependant of several factors, mainly the business area of activity. Other factors, like the sensitivity of the equipment used in the facilities and market conditions, among other, also influence the costs of PQ problems.

A. Power Quality Costs Evaluation

The costs related to a PQ disturbance can be divided in:

1) Direct costs. The costs that can be directly attributable to the disturbance. These costs include the damage in the equipment, loss of production, loss of raw material, salary costs during non-productive period and restart costs. Sometimes, during the non-productive period some savings are achieved, such as energy savings, which must be subtracted to the costs. Some disturbances do not imply production stoppage, but may have other costs associated, such as reduction of equipment efficiency and reduction of equipment lifetime.

2) Indirect costs. These costs are very hard to evaluate. Due to some disturbances and nonproductive periods, one company may not be able to accomplish the deadlines for some deliveries and loose future orders. Investments to prevent power quality problems may be considered an indirect cost.

3) Non-material inconvenience. Some inconveniences due to power disturbance cannot be expressed in money, such as not listening to the radio or watch TV. The only way to account these inconveniences is to establish an amount of money that the consumer is willing to pay to avoid this inconvenience [4], [5].

B. Estimates on Power Quality Costs

Several studies have been made to evaluate the costs of PQ problems for consumers. The assessment of an accurate value is nearly impossible; so all these studies are based on estimates. Some of these studies are presented below.

1) Business Week (1991). PQ costs were estimated on 26,000 million USD per year in the United States.
2) EPRI (1994). This study pointed 400,000 million USD per year for PQ costs in the United States.
3) US Department of Energy (1995). PQ costs were estimated on 150,000 million USD per year for United States.
4) Fortune Magazine (1998). Stated that PQ costs were around 10,000 million USD per year in United States.
5) E Source (2001). A study comprising continuous process industries, financial services and food processing in the United States, estimated the average annual costs of PQ problems on 60,000 to 80,000 USD per installation.
6) PQ costs in EU (2001). Overall PQ costs in industry and commerce, in European Union, are estimated in 10,000 million EUR per year [6].

The estimates of the various studies differ a lot, but all point to a common factor: the PQ costs are enormous.

C. Costs of Momentary Interruptions

An interruption is the PQ problem with the most perceivable impact on facilities. Table II summarizes the typical costs of momentary interruptions (1 minute) for different types of consumers. The costs presented are without major investments in technologies to achieve ride-through capabilities to cope with the interruption. These values are based on published services and Electrotek Concepts experiences with individual studies [5].

Table II – Typical costs of momentary interruptions (1 minute, in $/kW demand, for different types of industrial and services facilities.

As it can be seen, the industrial sector is the most affected by interruptions, especially the continuous process industry. In the services sector, communication and information processing is the most affected business area.

The costs of interruptions are also function of its duration. Fig. 4 depicts the costs of interruptions against its duration.

Fig. 4 – Costs of interruptions as function its duration [5].

5. Solutions for PQ Problems

The mitigation of PQ problems may take place at different levels: transmission, distribution and the end use equipment. As seen in Fig. 5, several measures can be taken at these levels.

Fig. 5 – Solutions for digital power [7]

6. 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.

7. 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.

Fig. 6 – Restoring technologies principle [1].

Recent technological advances in power electronics and storage technologies are turning the restoring technologies one of the premium solutions to mitigate PQ problems.

Fig. 7 – Working principle of an energy storage system.

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, supercapacitors and superconducting magnetic energy storage (SMES) present many advantages, electrochemical batteries still rule due to their low price and mature technology.

A. 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. Fig. 8 depicts the scheme of a flywheel, where the major advantages of this system are explained.

Fig. 8 – Flywheel [http://www.beaconpower.com]

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.

B. Supercapacitors

Supercapacitors (also known as ultracapacitors) 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 supercapacitor provides power during short duration interruptions or voltage sags.

Medium size supercapacitors (1 MJoule) are commercially available to implement ride through capability in small electronic equipment, but large supercapacitors are still in development, but may soon become a viable component of the energy storage field.

Fig. 9 – Electric double layer supercapacitor [http://www.esmacap.com]

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 [8].

C. 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.

Fig. 10 – SMES system [9].

Low temperature SMES cooled by liquid helium is commercially available. 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.

D. Comparison of Storage Systems

Fig. 11 shows a comparison of the different storage technology in terms of specific power and specific energy.

Fig. 11 – Specific power versus specific energy ranges for storage technologies [9].

Fig. 12 shows the specific costs of energy storage devices.

Fig. 12 – Specific costs of energy storage devices [10].

The high speed flywheel is in about the same cost range as the SMES and supercapacitors and about 5 times more expensive than a low speed flywheel due to its more complicated design and limited power rating. Electrochemical battery has a high degree of mature and a simple design. Below a storage time of 25 seconds the low speed flywheel can be more cost effective than the battery.

8. Distributed Resources – Distributed Generation

Distributed Generation (DG) units can be used to provide clean power to critical loads, isolating them from disturbances with origin in the grid. DG units can also be used as backup generators to assure energy supply to critical loads during sustained outages. Additionally DG units can be used for load management purposed to decrease the peak demand.

At present, reciprocating engine is the prevalent technology in DG market, but with technology advancements, other technologies are becoming more attractive, such as microturbines or fuel cells (Table III).

Table III – Evolution of DG technologies.

If DG units are to be used as back-up generation, a storage unit must be used to provide energy to the loads during the period between the origin of the disturbance and the start-up of the emergency generator.

The most common solution is the combination of electrochemical batteries UPS and a diesel genset. At present, the integration of a flywheel and a diesel genset in a single unit is also becoming a popular solution, offered by many manufacturers.

Fig. 13 – Scheme of a continuous power system, using a flywheel and a diesel genset [www.geindustrialsystems.com].

Fig. 14 – Dynamic UPS, by Hitec Power Protection, bv. [http://www.hitec-ups.com].

9. Enhanced Interface Devices

Besides energy storage systems and DG, some other devices may be used to solve PQ problems. Using proper interface devices, one can isolate the loads from disturbances deriving from the grid.

A. Dynamic Voltage Restorer

A dynamic voltage restorer (DVR) acts like a voltage source connected in series with the load. The working principle of the most common DVRs is similar to Fig. 7. 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.

B. Transient Voltage Surge suppressors (TVSS)

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.

C. Constant Voltage Transformers

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.

Fig. 15 – 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.

D. Noise Filters

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 kHz range is considerable.

E. Isolation Transformers

Isolation transformers are used to isolate sensitive loads from transients and noise deriving from the mains. In some cases (Delta-Wye connection) isolation transformers keep harmonic currents generated by loads from getting upstream the transformer. The particularity of isolation transformers is a grounded shield made of nonmagnetic foil located between the primary and the secondary. Any noise or transient that come from the source in transmitted through the capacitance between the primary and the shield and on to the ground and does not reach the load.

Fig. 16 – Isolation transformer.

F. Static VAR Compensators

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).

G. Harmonic Filters

Harmonic filters are used to reduce undesirable harmonics. They can be divided in two groups: passive filters and active filters.

Fig. 17 – Harmonic filters [11].

Passive filters (Fig. 17 left) 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. If the system varies (change of harmonic components), passive filters may become ineffective and cause resonance.

Active filters (Fig. 17 right) 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.

10. Develop 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.

One major step in this direction was taken with the CBEMA curve (Fig 18), created by the Computer and Business Equipment Manufacturer’s Association. This standard specifies the minimum withstanding capability of computer equipment to voltage sags, microinterruptions and overvoltages.

Fig. 18 – CBEMA curve.

Fig. 19 – ITIC curve

This curve, although substituted recently by ITIC (Information Technology Industry Council) curve (Fig. 19), is still a reference in the area of PQ. When the voltage is within the limits determined by the shaded zone, the equipment should function normally. When the voltage is comprised on the zone below the permitted zone, the equipments may malfunction or stop. When the voltage is comprised in the upper prohibited zone, besides equipment malfunction, damage on the equipment may occur.

Other standardization organizations (IEC, CENELEC, IEEE, etc) 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.

Table IV – Most important parameters defined by European Norm 50160:2001.

11. 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.

Adding a capacitor with a larger capacity to power supplies, using cables with larger neutral conductors, derating transformers and adjusting undervoltage relays, are measures that could be taken by manufacturers to reduce the sensitivity of equipment to PQ problems.

12. Conclusions

The availability of electric power with high quality is crucial for the running of the modern society. If some sectors are satisfied with the quality of the power provided by utilities, some others are more demanding.

To avoid the huge losses related to PQ problems, the most demanding consumers must take action to prevent the problems. Among the various measures, selection of less sensitive equipment can play an important role. When even the most robust equipment is affected, then other measures must be taken, such as installation of restoring technologies, distributed generation or an interface device to prevent PQ problems.

References

[1] J. Delgado, “Gestão da Qualidade Total Aplicada ao Sector do Fornecimento da Energia Eléctrica”, Thesis submitted to fulfilment of the requirements for the degree of PhD. in Electrotechnical Engineering, Coimbra, September 2002.
[2] “The Two Seconds Problem”, American Superconductor and EPRI Research, March 1998.
[3] EPRI Power Delivery Group, “The Future of Power Delivery in the 21st Century”, 1999.
[4] M. Bollen, “Understanding Power Quality Problems – Voltage Sags and Interruptions”, IEEE Press Series on Power Engineering – John Wiley and Sons, Piscataway, USA (2000).
[5] M. McGranaghan, “Costs of Interruptions”, in proceedings of the Power Quality 2002 Conference, Rosemont, Illinois, pp 1-8, October 2002..
[6] D. Chapman, “Costs of Poor Power Quality”, Power Quality Application Guide – Copper Development Association, March 2001.
[7] EPRI, “Creating the Electricity Infrastructure for a Digital Society”, UIE-2000 Conference, Lisbon, 1-3, November 2000.
[8] http://www.esma-cap.com
[9] P. Ribeiro, B. Johnson, M. Crow, A. Arsoy, Y. Liu, “Energy Storage Systems for Advanced Power Applications”, Proceedings of the IEEE, vol 89, no.12, December 2001.
[10] H. Darrelmann, “Comparison of Alternative Short Time Storage Systems”, Piller, GmbH, Osterode, Germany.
[11] P. Ferracci, “ Power Quality”, Schneider Electric Cahier Technique no. 199, September 2000.

What is an SMPS and how does it generate harmonics?

Published by Mirus International Inc., [2010-01-08] MIRUS-FAQ001-B2, FAQ’s Harmonic Mitigating Transformers, 31 Sun Pac Blvd., Brampton, Ontario, Canada. L6S 5P6.

What is an SMPS and how does it generate harmonics?

The Switch-mode Power Supply (SMPS) is found in most power electronics today. Its reduced size and weight, better energy efficiency and lower cost make it far superior to the power supply technology it replaced.

Electronic devices need power supplies to convert the 120VAC receptacle voltage to the low voltage DC levels that they require. Older generation power supplies used large and heavy 60 Hz step-down transformers to convert the AC input voltage to lower values before rectification. The SMPS avoids the heavy 60 Hz step-down transformer by directly rectifying the 120VAC using an input diode bridge (Figure 4-1). The rectified voltage is then converted to lower voltages by much smaller and lighter switch-mode dc-to-dc converters using tiny transformers that operate at very high frequency. Consequently the SMPS is very small and light.

Figure 4-1: Typical circuit diagram of Switch-mode Power Supply

The SMPS is not without its downside, however. The operation of the diode bridge and accompanying smoothing capacitor is very non-linear in nature. That is, it draws current in non-sinusoidal pulses at the peak of the voltage waveform (see Figure 4-2). This non-sinusoidal current waveform is very rich in harmonic currents.

Figure 4-2: Pulsed current waveform and resultant voltage flat-topping of a typical Switch-mode Power Supply

Because the SMPS has become the standard computer power supply, they are found in large quantities in commercial buildings. Acting together, the multitude of SMPS units can badly distort what started out as a sine wave voltage waveform.

Twice per cycle every SMPS draws a pulse of current to recharge its capacitor to the peak value of the supply voltage. Between voltage peaks the capacitor discharges to support the load and the SMPS does not draw current from the utility. The supply voltage peak is flattened by the instantaneous voltage drops throughout the distribution system caused by the simultaneous current pulses drawn by the multiple SMPS units. The expected sine wave with a peak of 120 x √2 = 169.4V instead starts to resemble a square wave. The flattened voltage waveform contains a lowered fundamental voltage component plus 3^rd, 5^th, 7^th, 9^th and higher voltage harmonics.

Harmonics and Harmonic Mitigating Transformers (HMT’s) Questions and Answers

This document has been written to provide answers to the more frequently asked questions we have received regarding harmonics and the Harmonic Mitigating Transformer technology used to address them. This information will be of interest to both those experienced in harmonic mitigation techniques and those new to the problem of harmonics. For additional information visit our Website at www.mirusinternational.com.

Voltage Flicker Calculation Methodology

Published by Trishia Swayne, P.E., Leidos.

Why is Electrical Infrared Thermography Inspection Important?

Published by Carelabs (Carelabz), Website: carelabz.com

Infrared Scanning of electrical installations falls under classification of Predictive Maintenance Fault Finding. The value is that we are now able to predict an electrical fault before the element fails completely.

We are able to get this due to the heat build-up of any electrical element under stress and predict that it will fail while it is still functional and appears normal to the naked eye or any other test equipment. The heat signature identified with the use of an Infrared camera. The pictures are then analysed and put into an easy-to-follow report so that they can rectify before a breakdown occurs, preventing loss of production due to unplanned down time.

Unexpected breakdowns in electrical supply can inconvenient and costly. Infrared electrical thermography is a useful tool that can recognize stressed elements of your electrical installation before they break down or cause a fire. This gives you the opportunity to solve the problem as part of planned maintenance before it causes a serious problem.

Another result of failed or stressed electrical elements is the risk of fire; in fact the risk is more real than commonly realise. It is in this vein that insurance companies are increasingly calling for infrared electrical thermography surveys as a valuable risk assessment aid.

In the past this service was only available to very large companies and mining houses due to the cost determinant, but as with everything there have been massive advancements in the last few years and Thermo Scanning has now become a very profitable tool in the small to medium size business world.

Maintenance includes vibration analysis on machines, audio, ultrasonic and infrared thermography inspection on electrical systems. Thermography used to recognize equipment hot spots. This task is typically carried out using temperature sensing instruments like thermocouple sensors or other forms of thermometers. Limitation of this analysis is that this kind of instrument can give maintenance personnel only with temperature readings on certain spots but not overall electrical system.

Thermography inspection generally uses infrared instrumentation to scan and create a temperature profile of intended targets.

In a typical manufacturing plant, Infrared thermography inspections did on electrical systems such as electrical switchboards, high-voltage distribution equipment motors, corresponding controllers, transformers and other control panels.

Switchgear Thermography

A great deal of investment is presently being made installation of thermal viewing ports for switchgear. These ports allow infrared inspections to carry out without removing switchgear covers, thus it would avoid worker arc-flash exposure. Installation of permanent infrared sensors and continuous infrared monitors are also reasonable methods for recognizing potential thermal failures of critical equipment. The principle of outdoor switchgear assemblies is often compromised by defective strip heaters. The strip heaters increase the switchgear temperature slightly above ambient to prevent condensation during daily or seasonal temperature changes. Functionality of these strips heaters and their effectiveness to carry out this duty can decide by carrying out thermal imaging of the switchgear enclosures. In other words, and once again, absence of heating identifies a potential problem.

Benefits of Thermography Survey

A major insurance carrier estimates that nearly 25 percent of all electrical failures attributed to faulty electrical connections. Therefore, many insurance firms are the driving force behind requiring facilities to conduct annual infrared surveys. Infrared technology has evolved into one of the most effective technologies for preventing failures and added benefit of not requiring an outage to carry out, as it can done on raw. Several further benefits of infrared technology listed below:

Hot spots such as loose connections and bad contacts.

Under-rated cables overheating under existing demand.
Unbalanced loads.
Stressed earth leakage units, circuit breakers, conductors and other electrical elements.

Infrared Electrical Thermography Survey Benefits

An infrared electrical thermography survey can result in significant financial savings for the client by:

Reducing the risk of an electrical fire.
Reducing the risk of an unplanned electrical outage.
Identifying priorities for planned maintenance, resulting in your spend going where it needed most.

Determines if the elements and system have been properly installed and are not damaged
Reduces downtime
Reduces risk of equipment failure
Increases safety
Improves insurability
Reduces liability exposure of the designers and installers
Improves system performance
Determines elements and systems carry out properly and meet the design intent
Determines if elements and systems compliance with the project specifications and design
Reduces construction schedule delays
Saves money

Infrared Thermography Testing can be Done on

Detecting loose or corroded electrical connections
Detecting electrical unbalance and overloads
Inspecting bearings and Electrical motors
Inspecting steam systems IR Imaging helps better to recognise and report suspect elements
Enable the repair to done right the first time.
High resistance connections
Hot spots
Over loaded cables
Over loaded fuses or breakers
Imminent motor or conveyor bearing failure
Motor windings over heating
Overheating in distribution equipment
Phase load imbalance
Hot spots in high level lighting
Heat build-up in overcrowded trunking
Thermal insulation breakdown (hot or cold)
Thermal loss
Damp ingress

Source: https://carelabz.com/electrical-infrared-thermography-inspection-important/

Some Comments on Power Acceptability Curves

Published by

G. Heydt , Arizona State University Tempe, Arizona USA
R. Thallam, Salt River Project Phoenix, Arizona USA
M. Albu, Universitatea Politehnica Bucureşti Bucharest, Romania

CARIBBEAN COLLOQUIUM ON POWER QUALITY (CCPQ), JUNE 2003

Abstract

Power acceptability curves, also known as voltage vulnerability or sensitivity curves, have been used for over 30 years to characterize momentary events of low voltage in power distribution systems. In this paper, a summary of how the curves were developed is given, and some thoughts on the applicability of the curves are presented.

Index terms: CBEMA curve, voltage sags, power quality, power acceptability, voltage sensitivity.

I. Power acceptability

Many power quality indices relate to steady state phenomena, and relatively few relate to momentary events. However, many power quality engineers feel that bus voltage sags, a natural consequence of a highly interconnected transmission system, may be the most important type of power quality degradation, and therefore a useful measure of the severity of these events is desirable. One such metric is the power acceptability curve (or voltage sensitivity or voltage vulnerability curve) which is a graphic metric of the severity of bus voltage sags plotted versus the duration of these events. Table I shows some of the issues that might be captured by a power acceptability (sensitivity) metric.

The best known of the graphical metrics for bus voltage sensitivity is the Computer Business Equipment Manufacturing Association (CBEMA) curve which is a graphic depicting the severity of a distribution bus voltage sag, Δ V, versus its duration T. The Δ V-T plane is a two dimensional space in which the line Δ V = 0 represents the case that distribution voltage is at rated value, and the Δ V < 0 half-plane is the bus voltage sag region. Overvoltage and undervoltage events of very minimal impact (small | Δ V | ) are considered ‘acceptable’ in the sense that loads are not disrupted; further, very short duration events (small T) are considered acceptable. Thus the Δ V – T plane is divided into acceptable and unacceptable regions. Fig. (1) shows the CBEMA power acceptability curve. The CBEMA curve depicted in Fig. (1) has Δ V indicated as a percent of rated voltage, and T shown on a logarithmic scale in seconds.

Table I Some issues in voltage sag and overvoltage events in primary distribution systems

Fig. (1) The CBEMA power acceptability curve

References [1-3] discuss a fuzzy logic alternative to assess voltage – load sensitivity, testing of loads to CBEMA standards, and computer performance during voltage sags respectively. Bollen has discussed a classification system of voltage sags and their effects [4]. Ride through issues for adjustable speed drives appear in [5]. References [6] and [7] by Kyei and other researcher describe research into the ‘derivation’ of these curves by using data from appropriate models of loads.

It is evident that power acceptability curves have frailties in design and application. For example, very short duration events (e.g., less than a cycle in duration) have an ambiguity in the sense that the duration of the event may be difficult to identify, and the point on- wave of the disturbance may have significant impact on the load. Point-on-wave information is not depicted in the Δ V-T plane. Further, the three phase implications of a power acceptability curve as indicated above are not clear: should one utilize phase information in the Δ V-T plane, or the positive sequence of the distribution voltage? Or is the graph basically a single phase representation? Another commonly asked question relates to the equation of the loci shown in Fig. (1). The CBEMA curve was developed from experimental and historical data: that is, cases of load disruption of mainframe computers were plotted in the Δ V-T plane, and a separator was developed to identify the acceptable and unacceptable regions.

II. A power quality standard

In 1998, Ayyanar and others [7] suggested the concept of a standard to represent whether power distributed is acceptable or unacceptable. The essence of the concept is that one needs to write a concrete criterion upon which acceptability is decided. One ultimate criterion of power acceptability relates to the operating status of the industrial process.

The particular power quality criterion depends on the nature of the load. For example, simple incandescent lighting loads may have a very loose criterion for acceptability, while certain sensitive computer controls may have a much more restrictive criterion. The difficulty in the selection of a single suitable criterion is confounded by the many possible load types. For simplicity, consider the rectifier load type depicted in Fig. (2). Voltage sags occur due to faults in the transmission, subtransmission, and primary distribution system, and they appear as low voltage conditions at V_ac depicted in Fig. (2). If the sag is of short duration and shallow depth, the ultimate industrial process ‘rides through’ the disturbance. This means that although V_ac is depressed, V_dc does not experience a sufficient disturbance to affect the load. The concept of a voltage standard is introduced at this point: a voltage standard is a criterion for power acceptability based on a minimum acceptable DC voltage at the output of a rectifier below which proper operation of the load is disrupted.

As an example of a voltage standard consider the following: if V_dc drops below 87% of rated voltage, the load is lost, and the distribution power is deemed to be unacceptable. The term ‘standard’ used in this context refers to the ultimate criterion upon which a decision of acceptability of supply is made. The use of the term ‘standard’ is not meant to imply an industry wide standard such as an IEEE standard. Fig. (3) shows a simulation study suitable for quantifying the effect of sags on rectifier load performance.

Fig. (2) A rectifier load

Fig. (3) Simulation of a three phase rectifier load

III. Analytical synthesis of the CBEMA curve

The CBEMA curve was derived from experimental and historical data taken from mainframe computers. The best engineering interpretation of the CBEMA curve can be given in terms of a voltage standard applied to the DC bus voltage of a rectifier load. Consider the case of either a single phase full wave bridge rectifier or the three phase bridge counterpart. Let the load on the DC side be an RLC load. If the DC bus voltage under a faulted condition is plotted as a function of the sag duration, the resulting curve is depicted in Fig. (4). From Fig. (4), the locus of V_dc could be represented as a double exponential in the form,

V_dc(t) = A + Be^–^bt + C e^-ct.

Parameter A is the ultimate (t → ∞ ) voltage, V_end, of the rectifier output. For the single phase case, and for the balanced three phase case, A is simply the depth of the AC bus voltage sag.

Fig. (4) Locus of V_dc(t) under fault conditions (at t = 0) for a single phase bridge rectifier

For more complex cases, e.g. unbalanced sags, parameter A can similarly be identified as the ultimate DC circuit voltage if the sag were to persist indefinitely (this is readily calculable by steady state analysis of the given sag condition and the rectifier type). If three points are selected on the CBEMA curve to identify the RLC filter combination used in the rectifier types considered in the original CBEMA tests, one finds,

V_dc(t) = V_end + 0.288e^-1.06t + (0.712-V_end)e^-23.7t. (1)

As an example, let the voltage standard be V_dc ≥ 0.87. Then the V_dc excursion becomes unacceptable at T when V_dc = 0.87 in Equation (1). Solution for Vend in terms of t = T in this expression gives

This is the formula for the undervoltage limb of the CBEMA curve (V_end in per unit, T in seconds).

IV. Some practical considerations

Application of the CBEMA curve or most other power quality ‘standards’ require certain practical considerations. Among these non-ideal considerations are:

The meaning of Δ V for short term events, especially when represented in root-mean square (RMS) values
Three phase considerations
Non ideal sags (e.g., the sag is –10% for the first few cycles, followed by –15% for the next few cycles – or even less ideal conditions in which the sag has no well defined value
Repeating events (e.g., one event, followed by restoration of normal operating conditions, followed by another event)
Point-on wave issues (see Section 5)
Multiple loads each with different sensitivity to bus voltage magnitude

Some of these issues are more easily considered than others. However, the rectifier and 87% V_dc interpretation given above do apply in all the cited practical cases. That is, at least in theory, a given non ideal, and perhaps three phase case, could be simulated utilizing a rectifier load with a DC circuit filter of the type cited above in connection with the ‘derivation of the CBEMA curve’. The three phase case is most easily considered as follows: Fig. (4) shows a power acceptability curve for a three phase rectifier. The case considered here is that of a phase A to ground fault using an 87% V_dc voltage standard. The procedure for the development of the power acceptability curve is similar to the one employed in deriving Equation (1). The unbalanced rectifier is analyzed simply, and V_dc(t) in this case is given as

V_dc(t) = V_end + 0.159e^-0.158t + (0.841-V_end)e^-4.63t . (2)

In Equation (2), the time constants were obtained using an LC filter on the DC side of a three phase, six-pulse bridge rectifier. The values of the LC were chosen to agree with the filter design used in the single phase case mentioned in connection with the derivation of Equation (1). That is, the CBEMA curve was found to correspond to the single phase rectifier case plus filter F. If filter F is used as a filter in the three phase case, Equation (2) results. Select a voltage standard of V_dc ≥ 0.87 When substituted into Equation (2) gives a formula for the power acceptability curve shown in Fig. (5) as

Other unbalanced faults are analyzed similarly.

The issue of short term representation of Δ V in terms of RMS values was considered in [8]. In many power quality studies, waveforms are characterized through a RMS value,

where f(t) is a time signal and T is either the period of the time signal or a suitably long time.

Fig. (5) Power acceptability curve for a three phase rectifier load with a phase-ground fault at phase A, 87% V_dc voltage standard

For the periodic case, when T is an integer multiple of the period of f(t), and t₀ is a fixed point on the wave, the RMS value is termed a synchronous RMS (s-RMS). The s-RMS operation maps a time signal to a single point and can be visualized as an information concentrator. It is a simple matter to demonstrate that the s-RMS quantifies the Joule effect of a sinusoidal voltage or current. Reference [9] contains a discussion of applications and calculation procedures. Fig. (6) shows an example of a short term voltage sag for which the following key parameters are noted:

T_w is the length of the observation time window
T_s is the duration of the change in signal’s amplitude
T is the period of the signal, assumed as with sinusoidal variation
T₀ is the moment of the amplitude change (considering that the observation window starts at t = 0)
r is the magnitude of amplitude change (in p.u.; the reference value is the amplitude at t < t₀). Note that r ≤ 1 and r ≥ 0 for voltage sags, r < 0 for swells.
ϕ is the phase at t = 0.

Fig. (6) Model of a voltage sag signal

In power quality studies, the effects on consumers are often quantified in terms of the deviation of secondary distribution voltage RMS values. However when sag events are of short duration, the RMS values may have a problematic interpretation. There are many hardware and software algorithms which compute RMS values, and it becomes advisable to identify the hidden possible errors in calculation and interpretation. Note that the RMS operator is nonlinear, but working with F²_rms and f²(t) gives the linear formulation,

If the RMS operator is continuously carried out over a windowed time T, using past samples from the input signal g(t), a moving average finite impulse response filtering is performed,

where r_T(t) is a rectangular pulse which is zero everywhere except in the interval [t–T, t] where it is unity. In the Fourier domain

The notation (*) denotes frequency domain convolution. Equation (4) indicates that there is a frequency response interpretation to the RMS operator. References [10,11] further discuss factors relating to the calculation of the RMS value.

The problem of repeated events is considered in [12]. The concept of repeated events is problematic because a second event, following closely after a first event, could have greater impact than an isolated event that is identical to the cited second event. For example, a momentary sag occurring at t = 0, for six cycles, followed by a second event at t = 0.15 s (60 Hz system) of duration six cycles might be analyzed; in such a case, the analysis of the second event of six cycles is quite different from an analysis performed of an isolated, non-repeated event of identical duration and sag depth. Heydt [12] suggests that there is a recovery time for which a system must progress in order to render an event in isolation from previous events. The concept of a recovery time is very similar to that of the ‘derivation of the CBEMA curve’ given above: that is, the recovery time of a sag can be plotted in the form of isopleths on a Δ V-T plane. The alternative, if the information is available, is to simulate the double (or triple, or multiple) event using a circuit as indicated in Figures (2) and (3).

The issues of multiple loads can be depicted as Fig. (7). For such a configuration, the CBEMA curve for each load may be calculated, tailoring the curve as needed. When the resultant CBEMA curves are drawn on a common Δ V-T plane, the inner area contains the acceptable region, and the outer area is the unacceptable region as shown in Fig. (8). The area(s) between the inner and outer regions represent power acceptable to some loads, and unacceptable to others.

Fig. (7) Multiple loads at a point of common coupling (PCC)

Fig. (8) Power acceptability region for the case of multiple loads

V. Point on wave issues

A momentary interruption of voltage or momentary sag in voltage magnitude may initiate at any point in the sinusoidal cycle as indicated in Fig. (9). For a linear load at unity power factor, the load current will be identical in phase to the indicated voltage. The energy transfer from the source to the load depends generally on θ_o as well as the duration of the sag. Consider a total outage of supply voltage. Integrating v(t)i(t) over θ_o to θ_o + θ where θ is the duration of the sag represented in radians assuming 60 Hz (or 50 Hz as appropriate), one finds that the energy that should have been delivered during the sag (and is now unserved due to the outage) is W,

For this simple formula, the rms supply voltage and current are both 1.0 per unit. Note that for values of θ that correspond to less than a half cycle (i.e., θ < π ), the CBEMA curve dictates that power delivery is ‘acceptable’. For longer duration outages, W depends not only on the duration of the outage θ , but also the point on wave θ_o at the initiation of the sag.

The more general case of a linear load with power factor cos(ϕ ) is more involved since the instantaneous power is a double frequency sine wave whose DC offset (i.e., the average power) is proportional to cos(ϕ ) . The unserved energy on total outage is

Collins and others have discussed the practical implications of the point on wave of the initiation of a voltage sag, including laboratory verified phenomena [13]. For long outages (largeθ , e.g., much larger than three cycles or 6π radians), the term in Equations (3) and (4) that is proportional to θ dominates, and the unserved energy is no longer greatly dependent on the point on wave at the sag initiation.

Fig. (9) Point on wave initiation of a voltage sag event

VI. A single index to show compliance with CBEMA

In most areas of engineering, it is important to use indices to measure or quantify the quality of performance. Power acceptability curves graphically depict power quality; but is there an index that can be used to assess “acceptability” or “unacceptability”? Consider Fig. (10) in this matter. Point P represents an event Δ V = Δ V_p and T = T_p (shown as ‘unacceptable’ in Fig. (10)). As an index of power acceptability, it is proposed to vary the threshold V_T until the power acceptability curve passes through P. This is shown as dashed lines in Fig. (10). Then, one sets V_T to V_Tp ,

Fig. (10) Graphic interpretation of an index of power acceptability for an event P

Consider the index V_Tp / VT . If V_Tp / V_T ≥ 1, the point P represents an acceptable event. It is a simple matter to show that the theoretical maximum of the index V_Tp / V_T is 1/V_T . Introduce the notation I_pa for the new index,

I_pa = V_Tp / V_T.

If one uses the notation T_x as the maximum time for which acceptable power is attained upon a total outage (i.e., Δ V = -1),

This is an index of power acceptability for the event P. When the index is greater than unity, one is in the acceptable power region, and when the index is below unity, one is in the unacceptable region. At unity itself, the event is exactly on the CBEMA curve.

VII. Recommendations and concluding comments

In this paper, the CBEMA curve was revisited and the curve was analytically synthesized using a new concept, the voltage standard. The standard refers to an ultimate criterion that power is unacceptable if the DC voltage of a certain rectifier load drops below 87% of rated value. A double exponential equation describing the CBEMA curve is developed. This provides a useful method to consider the effect of unbalanced voltage sags and to develop CBEMA-like curves for other types of loads. A scalar index of compliance termed I_pa has been illustrated. This index is based on the CBEMA curve compliance.

Additional practical considerations relating to power acceptability include:

The meaning of Δ V for short term events, especially when represented in root-mean square (RMS) values
Three phase considerations
Non ideal sags
Repeating events
The energy served to a load during a sag as a function of the point-on-wave of the initiation of the event
Multiple loads each with different sensitivity to bus voltage magnitude.

It appears that the main advantage of the CBEMA curve is the ease in application, and also in the familiarity of the concept by most power engineers.

Although accuracy of the curve in predicting true acceptability – unacceptability of the power supply may not be a strong point of CBEMA technology, at least some problematic issues of its application may be resolved using the concept of a voltage standard.

Acknowledgements

The authors gratefully acknowledge the support of the Power Systems Engineering Research Center (PSerc) and SRP. Most of the work represented here came from Mr. John Kyei of the California ISO, Folsom CA. Dr. Raja Ayyanar of Arizona State University originated the concept of the voltage standard, and the authors acknowledge his contribution. Dr. Albu acknowledges the support of the Fullbright Fellowship.

References

[1] B. Bonatto, T. Niimura, H. Dommel, “A fuzzy logic application to represent load sensitivity to voltage sags,” Proceedings International Conference on Harmonics and Quality of Power, October, 1998, pp. 60-64.
[2] E. Collins, R. Morgan, “A three phase sag generator for testing industrial equipment,” IEEE Transactions on Power Delivery, v. 11, No. 1, January, 1996, pp. 526 – 532.
[3] D. Koval, “Computer performance degradation due to their susceptibility to power supply disturbances,” Conference Record, IEEE Industry Applications Society Annual Meeting, October, 1989, v. 2, pp. 1754 – 1760.
[4] M. Bollen, L. Zhang, “Analysis of voltage tolerance of AC adjustable-speed drives for three-phase balanced and unbalanced sags,” IEEE Transactions on Industry Applications, v. 36, No. 3, May-June 2000, pp. 904 – 910.
[5] E. Collins, A. Mansoor, “Effects of voltage sags on AC motor drives,” Proceedings of the IEEE Technical Conference on the Textile, Fiber and Film Industry, 1997, pp. 9 – 16.
[6] J. Kyei, “Analysis and design of power acceptability curves for industrial loads,” MSEE Thesis, Arizona State University, Tempe AZ, December 2001.
[7] J. Kyei, R. Ayyanar, G. Heydt, R. Thallam, J. Blevins, “The design of power acceptability curves,” accepted for publication, IEEE Trans. on Power Delivery, 2003.
[8] M. Albu, G. Heydt, “On the use of RMS values in power quality assessment,” accepted for publication, IEEE Trans. on Power Delivery, 2003.
[9] S. Kuo, B. Lee, Real-Time Digital Signal Processing. Implementations, Applications, and Experiments with the TMS320C55X, John Wiley and Sons, New York, 2001.
[10] S. Herraiz-Jaramillo, G. Heydt, E. O’Neill-Carrillo, “Power quality indices for aperiodic voltages and currents,” IEEE Transactions on Power Delivery, v. 15, No. 2, April 2000, pp. 784 790.
[11] N. Tunaboylu, E. Collins, P. Chaney, “Voltage disturbance evaluation using the missing voltage technique,” Proceedings of the 8th International Conference on Harmonics and Quality of Power 1998, pp. 577 – 582.
[12] G. T. Heydt, Computer Analysis Methods for Power Systems, Second edition, Stars in a Circle Publications, Scottsdale, AZ, 1996.
[13] E. R. Collins, M. A. Bridgwood, “The impact of power system disturbances on AC-coil contactors,” Proceedings of the IEEE Technical Conference on the Textile, Fiber and Film Industry, 1997, pp. 2-6.

Why do non-linear loads have low power factors and why is it important to have a high power factor?

Published by Mirus International Inc., [2010-01-08] MIRUS-FAQ001-B2, FAQ’s Harmonic Mitigating Transformers, 31 Sun Pac Blvd., Brampton, Ontario, Canada. L6S 5P6.

Power factor is a measure of how effectively a specific load consumes electricity to produce work. The higher the power factor, the more work produced for a given voltage and current. Figure 3-1 shows the power vector relationships for both linear and non-linear loads. Power factor is always measured as the ratio between real power in kilowatts (kW) and apparent power in kilovoltamperes (kVA).

**Figure 3- 1: Power factor relationship for Linear and Non-linear loads**

For linear loads, the apparent power in kVA (S = V•I) is the vector sum of the reactive power in kVAR (Q) and the real power in kW (P). The power factor is P/S = CosΦ, where Φ is the angle between S and P. This angle is the same as the displacement angle between the voltage and the current for linear loads. For a given amount of current, increasing the displacement angle will increase Q, decrease P, and lower the PF. Inductive loads such as induction motors cause their current to lag the voltage, capacitors cause their current to lead the voltage, and purely resistive loads draw their current in-phase with the voltage. For circuits with strictly linear loads (a rare situation) simple capacitor banks may be added to the system to improve a lagging power factor due to induction motors or other lagging loads.

For non-linear loads, the harmonic currents they draw produce no useful work and therefore are reactive in nature. The power vector relationship becomes 3 dimensional with distortion reactive power, H, combining with both Q and P to produce the apparent power which the power system must deliver. Power factor remains the ratio of kW to kVA but the kVA now has a harmonic component as well. True power factor becomes the combination of displacement power factor and distortion power factor. For most typical non-linear loads, the displacement power factor will be near unity. True power factor however, is normally very low because of the distortion component. For example, the displacement power factor of a personal computer will be near unity but its total power factor is often in the 0.65 – 0.7 range. The best way to improve a poor power factor caused by non-linear loads is to remove the harmonic currents.

Most Utilities charge their customers for energy supplied in kilowatt-hours during the billing period plus a demand charge for that period. The demand charge is based upon the peak load during the period. The demand charge is applied by the utility because it must provide equipment large enough for the peak kVA demand even though the customer’s power demand may be much lower. If the power factor during the peak period (usually a 10 minute sliding window) is lower than required by the utility (usually 0.9 or 0.95), the utility may also apply a low PF penalty charge as part of the demand charge portion of the bill.

Suppose the peak demand was 800kW with apparent power consumption of 1000kVA (a PF of 0.8). If a power factor penalty was applied at 0.9, the Utility would charge the customer as if his demand was 0.9 x 1000kVA = 900kW even though his peak was really 800kW, a penalty of 100kW. Improving the power factor to 0.85 at 1000kVA demand would lower the penalty to just 50kW. For power factors of 0.9 to 1.0, there would be no penalty and the demand charge would be based upon the actual peak kW. The demand charge is often a substantial part of the customer’s overall power bill, so it is worthwhile to maintain good power factor during peak loading and reducing the harmonic current as drawn by the loads can help achieve this.

References:

1. Roger C. Dugan, Electrical Power Systems Quality, McGraw-Hill, New York NY, 1996, pp. 130-133
2.H. Rissik, The Fundamental Theory of Arc Convertors, Chapman and Hall, London, 1939, pp 85-97

Harmonics and Harmonic Mitigating Transformers (HMT’s) Questions and Answers

A Technical Investigation of Voltage Sag

Published by ¹Vima P. Mali, ²R. L. Chakrasali, ¹K. S. Aprameya, ¹Electrical and Electronics Engineering Research Centre, University B.D.T. College of Engineering, Davanagere, India. ²Department of Electrical and Electronics Engineering, SDM College of Engineering and Technology, Dharwad, India.

Research Paper Source: American Journal of Engineering Research (AJER) 2015. American Journal of Engineering Research (AJER) e-ISSN: 2320-0847 p-ISSN : 2320-0936 Volume-4, Issue-10, pp-60-68 www.ajer.org

Abstract: Voltage sag is regarded as one of the most harmful power quality disturbances due to its costly impact on sensitive loads. The vast majority of the problems occurring across the utility, transmission and industrial sides are voltage sags. The source of sag can be difficult to locate, since it occurs either inside or outside facilities. So, this paper analyses some aspects of voltage sag such as the cost of voltage sag, their characteristics, types of voltage sag, its occurrence, percentage of sag present in power system, acceptable level of voltage sag curve, voltage sag indices, its economical impact, ways to mitigate the voltage sag and finally few devices used to mitigate voltage sag.

Keywords: Voltage sag, impact, types, occurrence etc.

I. INTRODUCTION

The name power quality has become one of the most productive concepts in the power industry since late 1980s. Power quality is the “Degree to which both the utilization and delivery of electric power affects the performance of electrical equipment” [1].Power quality is decided by magnitude of voltage and frequency. Voltage quality problem is divided into under voltage, overvoltage, interruption, voltage sag, voltage swell and so on, and frequency quality problem could be classified into frequency variations, transient, harmonics, etc. [2].

Voltage sag or Voltage dip the two terms are equivalent. According to the IEEE defined standard (IEEE Std. 1159, 1995), voltage sag is the decrease of rms value of voltage from 0.1 to 0.9 per unit (pu), for a duration of 0.5 cycle to 1 minute. The International Electrotechnical Commission, IEC, has the following definition for a dip (IEC 61000-2-1, 1990). “A voltage dip is a sudden reduction of the voltage at a point in the electrical system, followed by a voltage recovery after a short period of time, from half a cycle to a few seconds”. Voltage sags are present in power systems, but only during the past decades customers are becoming more sensitive to the inconvenience caused [3].

Voltage sag can cause serious problems to sensitive loads, because these loads often drop off-line due to voltage sag. As a result, some industrial facilities experience production outage that results in economic losses [4, 5, 6]. In several processes such as semiconductor manufacturing or food processing plants, the voltage dip of very short duration can cost a substantial amount of money [7]. Voltage dip is the main power quality problem for the semiconductor and continuous manufacturing industries, and also to the hotels and telecom sectors [8].

International Joint Working Group (JWG) C4.1110 sponsored by CIGRE, CIRED and UIE has addressed a number of aspects of the immunity of equipment and installations against voltage dips and also identified areas were additional work is required. The work took place between 2006 and 2009 and resulted in a technical brochure distributed via CIGRE and UIE [9]. Voltage sag on a power grid can affect facilities within a 100-mile radius. According to Electric Power Research Institute the voltage sag causes 92% of distribution & transmission power quality problems.

A typical electric customer in the U.S experiences 40 to 60 sag events per year with those events resulting in the voltage dropping to between 60 to 90% and lasting several cycles to more than a second. The large majority of faults on a utility system are single line-to-ground faults (SLGF). Three phase faults can be more severe, but much less common. System wide, an urban customer on average may see 1 or 2 interruptions a year whereas the same customer may experience over 20 voltage sag occurrences a year depending on how many circuits are fed from the substation.

II. COST OF VOLTAGE SAG

Voltage sag lasting for a few cycles result in losses of several million dollars includes:

a. Repairs cost.
b. Increased buffer inventories.
c. Product quality issues affect brand name, fame of the industry and even the country.
d. Customer dissatisfaction due to huge loss in business.
e. Penalties and disposal fees.
f. Product-related losses, such as loss of product/materials, hampered production capacity, disposal charges, and increased inventory requirements.
g. Labor-related losses, such as idle employees, overtime, cleanup and repair.
h. Ancillary costs, such as damaged equipment, lost opportunity cost and penalties due to shipping delays.

III. CHARACTERISTICS OF VOLTAGE SAG

The magnitude of voltage and the frequency are the parameters that specify the voltage sag.

a. Magnitude:

Figure 1. Voltage sag characteristics

The sag magnitude is the minimum of rms voltage and refers to the retained voltage or to the drop of the voltage (IEEE P1564). Thus, a 70% sag in a 230-V system indicates the voltage dropped to 161 V. One could be tricked into thinking that 70% sag refers to a drop of 70%, thus a remaining voltage of 30% [10].The most common approach to obtain the sag magnitude is to use rms voltage. There are other alternatives, e.g. rms voltage of fundamental component and peak voltage [11,12].The rms voltage is calculated over one cycle using equation 1

The rms value using one half cycle is given by equation 2

Where N is the number of samples per cycles, V(i) is the instantaneous sampled voltage and k is the instant when the rms voltage is estimated.

b. Duration: Sag duration is commonly determined by the speed of the fault clearing time. The voltage sag duration is nothing but the period of time in which the voltage is lower than the stated limit; normally sag duration is less than 1 second (IEEE Std. 493, 1997). According to IEEE Std. 1159, 1995 voltage sag has been classified into three types based on their duration i) Instantaneous (0.5-30cycle) ii) Momentary (30 cycles-3sec) iii) Temporary (3sec-1min). For measurements in the three-phases systems the three rms voltages have to be considered to determine duration of the sag. The voltage sag starts when at least one of the rms voltages drops below the sag-starting threshold. The sag ends when all three voltages have recovered above the sag-ending threshold.

c. Unbalance of Sag: In the power system the faults are classified as symmetrical (balanced) and unsymmetrical (unbalanced) depending on the type of fault. If three phase fault occurs, the sag will be symmetrical but if the fault is single phase, double phase or double phase to ground faults the sag in three phases will not be symmetrical.

d. Phase-Angle Jump: A short circuit in a power system not only causes voltage sag, but also changes the phase angle of the voltage leading to phase-angle jump. The phase-angle jump is visible in a time-domain plot of the sag as a shift in voltage zero-crossing between the pre-event and the during-event voltage. If source and feeder impedance have equal X/R ratio, there will be no phase-angle jump in the voltage at the Point of Common Coupling. This is the case for faults in transmission systems, but normally not for faults in distribution systems. The distribution systems may have phase-angle jumps up to a few tens of degrees.

For unsymmetrical faults, the analysis becomes much more complicated. A consequence of unsymmetrical faults (single-phase, phase-to-phase, two-phase-to-ground) is that single-phase load experiences a phase-angle jump even for equal X = R ratio of feeder and source impedance. From the measured voltage wave shape, the phase angle of the voltage during the event must be compared with the phase angle of the voltage before the event.

IV. TYPES OF VOLTAGE SAG

Based on the phases affected during the sag, the voltage sag has been classified into three types:

a. Single Phase Sags: The frequently occurring voltage sags are single phase events which are basically due to a phase to ground fault occurring somewhere on the system. On other feeders from the same substation this phase to ground fault appears as single phase voltage sag. Typical causes are lightning strikes, tree branches, animal contact etc. It is common to see single phase voltage sags to 30% of nominal voltage or less in industrial plants.

b. Phase to Phase Sags: The two phase or phase to phase sags are caused by tree branches, adverse weather, animals or vehicle collision with utility poles. These types of sags typically appear on other feeders from the same substation.

c. Three Phase Sags: These sags are caused by switching or tripping of a 3 phase circuit breaker, switch or recloser which will create three phase voltage sag on other lines fed from the same substation. Symmetrical 3 phase sags arise from starting large motors and they account for less than 20% of all sag events and are usually confined to an industrial plant or its immediate neighbors.

V. OCCURRENCE OF VOLTAGE SAG

Voltage sag occurs at almost all locations in the power system and avoiding them is only practically possible up to a certain extent. Voltage sag is caused by faults on the system, transformer energizing, or heavy load switching. Reducing the number and severity of voltage sag experienced by a customer, beyond what is normally considered as good engineering practice, can be very expensive [11].

Utility side voltage sag occurs due to operation of reclosers & circuit breakers, equipment fails( due to overloading, cable faults), bad weather (thunderstorms and lightning strikes cause a significant number of voltage sags), animals & birds(squirrels, raccoons and snakes occasionally find their way onto power lines or transformers and can cause a short circuit or either phase to phase or phase to ground), Vehicles occasionally collide with utility poles (causing lines to touch, protective devices trip and voltage sags occur), Construction activity( Digging foundations for new building construction can result in damage to underground power lines and create voltage sags).

Salt spray builds up on power line insulators over time in coastal areas, even many miles inland, can cause flash over especially in stormy weather. Dust in arid inland areas can cause similar problems. As circuit protector devices operate voltage sags appear on other feeders. If electrical equipment fails due to overloading, cable faults etc., protective equipment will operate at the sub-station and voltage sags will be seen on other feeder lines across the utility system.

Industrial side voltage sags occurs within an industrial facility (due to factory equipment, office equipment, air conditioning & elevator drive motors) or a group of facilities by the starting of large electric motors either individually or in groups. The large current inrush on starting can cause voltage sags in the local or adjacent areas even if the utility line voltage remains at a constant nominal value. Starting a large load, such as an electric motor or resistive heater, typically draw 150% to 500% of their operating current as they come up to speed. Resistive heaters typically draw 150% of their rated current until they warm up. Even 80% of all power quality problems occur in a company’s distribution and grounding/bonding systems.

Electronic process controls, sensors, computer controls, PLC’s and variable speed drives, conventional electrical relays are all to some degree susceptible to voltage sags. In many cases one or more of these devices may trip if there is a voltage sag to less than 90% of nominal voltage even if the duration is only for one or two cycles i.e. less than 100 milliseconds. The time to restart production after such an unplanned stoppage can typically be measured in minutes, hours or even days. Costs per event can be many tens of thousands of dollars. Voltage sag cannot be eliminated fully so, Industrial customers who have invested heavily in production equipment which is susceptible to voltage sags must take responsibility for their own solutions to voltage sags or lose some benefit from their investment.

VI. PERCENTAGE OF SAG PRESENT IN POWER SYSTEM

The most common types of voltage abnormalities are: harmonics, voltage sags, voltage swells and short interruptions. Among these, voltage sags account for the highest percentage of occurrences in equipment interruptions, as shown in Figure 1. The figure 1 indicates that voltage sags account for the highest percentage of equipment interruptions, i.e., 31%. Voltage sags are also major power quality problem that contributes to nuisance tripping and malfunction of sensitive equipment in industrial processes and Table 1 below gives causes of voltage sag on distribution system based on number of voltage sag occurrences and its percentage.

Figure 1 Power quality disturbances [28]

Table 1. Causes of voltage sag on distribution system

VII. CLASSIFICATION OF VOLTAGE SAG

There are two methods for classification the three phase voltage sags i) ABC Classification (First method) ii) Symmetrical Components (Second method). Due to simplicity, first method is more used than the symmetrical components classification. However, this classification is based on a simplified model of the network and it is not recommended to use for the classification of voltage sags obtained from measured instantaneous voltages.

In the first classification, in 1997, Bollen has proposed a four type’s classification for voltage sags (A, B, C, D) based on type of fault which generates the sag [13]. This classification isn’t so good for voltage sags generated by 2PN (2 phase to neutral) faults [14, 15]. So, Bollen has proposed a new by adding another three (E, F, G) types of voltage sags. Types of voltage sag are:

Table 2. The phasor diagram and equations

The pre-event voltage in phase A is denoted as E1, recalling to the equivalence between phase A voltage and positive sequence voltage in a balanced system. The voltage in the phase that has experienced the sag or between the phases that has experienced the sag is indicated as V. In table 1 sag types are shown considering phase A as the reference phase. It means that another set can be derived for phase B or C are set as the reference phase. This classification is the base for international standard IEC 61000-4-11[18], because it makes possible generation of the seven types of voltage sag.

VIII. ACCEPTABLE LEVEL OF VOLTAGE SAG CURVE

This is generally determined by power quality curves, a plot of voltage magnitude versus time. Power quality curves represent the intensity and duration of voltage disturbances. The Computer and Business Equipment Manufacturers’ Association (CBEMA), and Semiconductor Equipment and Materials Institute (SEMI) have published information defining what levels of poor power quality, specifically voltage sag, equipment should be able to tolerate. Other power quality curves in common use today were developed by the American National Standards Institute (ANSI) and the Information Technology Industry Council (ITIC).

The ANSI curves plot the deviation from nominal voltage as a percentage of nominal voltage compared to the duration or the maximum length of time the voltage is permitted to reach. For example, the limit for voltage occurrences greater than 1 second duration might be ± 10%. The ITIC and CBEMA curves also plot voltage with respect to duration, but as a percentage of absolute voltage. Electronic equipment can typically withstand high voltages provided they last for less than 1 millisecond in duration, but voltages greater than +10% or -20% for between 0.5 seconds and 10 seconds duration are to likely create problems.

ITIC also shows that computer equipment should be able to ride through short-duration voltage sags, if the voltage doesn’t go below 70%. For sags of longer duration, voltages below 80% could affect the equipment. Even SEMI F47 semiconductor industry standard specifies an improved voltage sag ride-through for process tools. It requires a ride-through down to 50% voltage for 200 milliseconds, which will significantly reduce the number of voltage sags that may cause process disruptions in semiconductor plants. These curves are merely guidelines, and some electronic equipment may require higher power quality conditions than those represented in these standards.

IX. VOLTAGE SAG INDICES

PQ indices are key issue to indicate the different performance experienced at the transmission, sub-transmission, substation and distribution circuit levels. There are various ways of presenting voltage sag performance [16].

a.SARFI (System Average Rms Variation Frequency Index)
b.SIARFI (System Instantaneous Average Rms Variation Frequency Index)
c.SMARFI (System Momentary Average Rms Variation Frequency Index)

The most common index use is the SARFI. This index represents the average number of voltage sags experienced by a end user each year with a specified characteristic. For SARFI_X, the index would include all of the voltage dips where the minimum voltage was less than X(where X is a number between 0 and 100) gives the number of events with a duration between 10 milliseconds and 60 seconds and a retained voltage less than X%. SARFI_70 gives the number of events with retained voltage less than 70% [17, 18]. Standard voltage thresholds are 140, 120, 110, 90, 80, 70, 50, and 10 % of nominal.

X. ECONOMICAL IMPACT OF VOLTAGE SAG

The cost associated with the voltage sag is more compared to other power quality issues:

a. The cost to North American industry of production stoppages caused by voltage sags now exceeds U$250 billion per annum [9].

b. In South Africa, a recent study showed that major industries suffer annual losses of more than 200 US$ million due to voltage sag problems [11].

c. A study in United States (U.S.), the total damage by voltage sag may amount to 400 Billion Dollars [12].

d. Manufacturing facilities have cost ranging up to millions of dollars attributed to a single disruption of the process whereas the cost to commercial customer (e.g., banks, data center, customer service centers, etc.) can be just as high if not higher [14].

e. In automotive industry, four-cycle voltage sag can lost over 700,000 US$ in the following 72 minutes due to shut down of process and required rework from malfunction of programmable controllers and drive systems working in a real-time process environment [19].

f. One automaker estimated that the total losses from momentary voltage sag at all its plants runs to about $10M a year [1].

g. Manufacturing facilities have costs ranging from Rs.4, 00,000 to millions of rupees associated with a single interruption to the process. Momentary interruptions or voltages sags lasting less than 100 ms can have the same impact as in outage lasting many minutes [20].

h. If an interruption costs Rs.16, 00,000, the total costs associated with voltage sags and interruptions would be Rs.2, 70, 40,000/-year. (The total cost is appro. 17 times the cost of an interruption) [21].

The table 3 shows some industries and their loss per event due to voltage sag and table 4 shows Cost of Momentary interruption due to voltage sag.

Table 3. Industries and their loss per event

Sl.No	Industry	Loss per event (US$)
1.	Semiconductor industry	2,500,000
2.	Credit card processing	250,000
3.	Equipment manufacturing	100,000
4.	Automobile industry	75,000
5.	Chemical industry	50,000
6.	Paper Manufacturing	30,000

Table 4. Shows industries & their Cost of Momentary interruption [22]

		Cost of Momentary interruption	Cost of Momentary interruption
Sl.No	Industry	Minimum	Maximum
1.	Semiconductor Manufacturing	$20.0	$60.0
2.	Pharmaceutical	$5.0	$50.0
3.	Electronics	$8.0	$12.0
4.	Communications, Information Processing	$1.0	$10.0
5.	Automobile manufacturing	$5.0	$7.5
6.	Food processing	$3.0	$5.0
7.	Glass	$4.0	$6.0
8.	Petrochemical	$3.0	$5.0
9.	Textile	$2.0	$4.0
10.	Rubber & plastics	$3.0	$4.5
11.	Metal fabrication	$2.0	$4.0
12.	Mining	$2.0	$4.0
13.	Hospitals, Banks, Civil services	$2.0	$3.0
14.	Paper	$1.5	$2.5
15.	Printing (News Paper)	$1.0	$2.0
16.	Restaurants, bars, hotels	$0.5	$1.0
17.	Commercial shops	$0.1	$0.5

XI. MITIGATION OF VOLTAGE SAG

There are several ways to mitigate the voltage sag:

a. From Fault to Trip: The equipment trip is the main cause of voltage sag, if there are no equipment trips due to short-circuit fault, there is no voltage sag problem. Due to short circuit at the fault position, the voltage drops to zero, or to a very low value. This zero voltage is changed into an event of a certain magnitude and duration at the interface between the equipment and the power system. If the fault takes place in a radial part of the system, the protection intervention clearing the fault will also lead to an interruption. If there is sufficient redundancy present, the short circuit will only lead to voltage sag. If the resulting event exceeds a certain severity, it will cause an equipment trip. The equipment trip due to short circuit fault can be minimized by:

Reducing the fault-clearing time.
Changing the system such that short-circuits faults result in less severe events at the equipment terminals or at the customer interface.
Connecting mitigation equipment between the sensitive equipment and the supply.
Improving the immunity of the equipment.

b. Reducing the Number of Faults: Short circuits cannot be entirely eliminated. The majority of failures are due to faults on one or two distribution lines. Below mentioned fault mitigation measures may be expensive, especially for transmission systems but their costs have to be weighed against the consequences of the equipment trips. The actions taken are:

Replacing overhead lines with cables.
The use of insulated conductors on overhead lines.
Regular tree cutting in the area of the transmission line and fencing against animal.
Shielding overhead conductors with additional shield wires and by increasing insulation level.
Increased frequency of overhaul and periodic maintenance, cleaning insulators etc.

c. Reducing the Fault-Clearing Time: To minimize the fault clearing time several types of fault current limiters(able to clear a fault within one half-cycle) are in use for low and medium voltages system i.e. few tens to kilovolts, but actually they do not clear the fault, they only reduce the current magnitude within one or two cycles. Reducing the fault clearing time of any event does not reduce the number of events occurring, but can reduce the severity of fault impact. Recently introduced static circuit breaker has the same characteristics as fault current limiters. Fault-clearing time is not only the time needed to open the breaker, but also the time needed for the protection to make a decision.

d. Changing the Power System: The cost associated with changing the supply system may be high, especially for transmission and sub transmission voltage levels. But in case of industrial systems, the design stage will outweigh the cost. Some other ways to mitigate the voltage sags are:

By installing a generator near the sensitive load. The generators will keep up the voltage during remote sag.
Split buses or substations in the supply path to limit the number of feeders in the exposed area.
Determine the frequency, depth & duration of the voltage sag. Collection of data is essential if the optimal solution is to be determined.
In order to provide a cost effective solution to voltage sag problems, it is necessary to determine which equipment is more subjected to voltage sag.

XII. INSTALLING VOLTAGE SAG MITIGATING EQUIPMENT

There are number of mitigating devices used to mitigate the voltage sag:

a. Device Voltage Restorer (DVR): DVR uses modern power electronic components to insert a series voltage source between the supply and the load. The voltage source compensates for the voltage drop due to the sag. The DVR is a series connected facts device to protect sensitive loads from supply side disturbances; it can also act as a series active filter, isolating the source from harmonics generated by loads. This is often the best solution when voltage sags are the dominant concern. DVR is also used for protecting individual loads or group of loads.

b. Uniform Power Quality Conditioner (UPQC): is the integration of series and shunt active filters, connected back to back on the dc side and share a common DC capacitor. The series connected UPQC is responsible for mitigation supply side disturbances such as voltage sags, flickers, voltage unbalance and harmonics. The shunt component is responsible for mitigating the current quality problems caused by consumer: poor power factor, load harmonic currents, load unbalance etc. It can perform the function of both DSTATCOM and DVR [23].

c. Uninterruptable Power Supply (UPS): Utilize batteries to store energy that is converted to a usable form during outage or voltage sag. This is the most commonly used device to protect low-power equipment (computers, etc.) against voltage sags and interruptions. During the sag or interruption, the power supply is taken over by an internal battery. The battery can supply the load for, typically, between 15 and 30 minutes.

d. Motor-Generator Sets (M-G Sets): It usually utilizes flywheels for energy storage. They completely decouple the load from the electric power system. Rotational energy in the flywheel provides voltage regulation and voltage support during under voltage conditions. M-G sets have relatively high efficiency and low initial capital cost. They are only suitable for industrial environment due to noise and the maintenance required compare to office environment.

e. Ferro resonant, Constant Voltage Transformers (CVTs): can be used to improve voltage sag ride through capability. CVTs are especially attractive for constant, low power loads, variable loads, especially with high inrush currents, present more of a problem for CVTs because of the tuned circuit on the output. CVTs are basically 1:1 transformers which are excited high on their saturation curves, thereby providing output voltage which is not significantly affected by input voltage variations.

f. Static transfer switch: A static transfer switch switches the load from the supply with the sag to another supply within a few milliseconds. This limits the duration of sag to less than one half cycle, assuming that a suitable alternate supply is available.

XIII. CONCLUSION

Voltage sag is an avoidable natural phenomenon in a power system; faults in the system are the main reason for the voltage sag. The issues related to voltage sag are gaining importance because a small power outage has a great economical impact especially on the industrial consumers. A longer interruption harms practically all operations of modern society sensitive equipment. So it is necessary to have awareness regarding damages caused by voltage sag by analyzing their characteristics, types, its places of occurrence, percentage of damages caused by the presence of sag, acceptable level of sag curve and its indices. Lastly by taking following some strict measures and by installing mitigating equipment the voltage sag can be avoided up to certain extent.

REFERENCES

[1] Alexander Eigels Emanuel, John A. McNeill “Electric Power Quality”. Annu. Rev. Energy Environ 1997.22:263-303.
[2] Bong-Seok Kang et al; “A Study of the Impact of Voltage Sags and Temporary Interruptions on 3-Phase Induction Motors”, Dept.of Electrical Engineering, Soongsil University, Sangdo-dong, Dongjak-Gu, Seoul, 156-743, Korea.
[3] Pirjo Heine and Matti Lehtonen, “Voltage sag distributions caused by power systems faults,” IEEE Transactions on Power Systems, vol.18, No.4, pp.1367-1373, November 2003.
[4] Ward, D.J. (2001).Power quality and the security of electricity supply. Proceedings of the IEEE, Vol. 89,Issue: 12, Dec. 2001, pp: 1830 –1836.
[5] M. Firouzi et al; “Proposed New Structure for Fault Current Limiting and Power Quality Improving Functions”, International Conference on Renewable Energies and Power Quality (ICREPQ’10) Granada (Spain), 23rd to 25th March, 2010.
[6] Ward, D.J. (2001).Power quality and the security of electricity supply. Proceedings of the IEEE, Vol. 89,Issue: 12, Dec. 2001, pp: 1830 –1836.
[7] P.Zanchetta et al, “New power quality assessment criteria for supply systems under unbalanced and nonsinusoidal conditions”, IEEE trans.on Power Delivery, vol.19, no.3, July.2004pp1284-1290.
[8] Sharmistha Bhattacharyya et al; “Consequences of Poor Power Quality – An Overview”, Technical University of Eindhoven The Netherlands.
[9] CIGRE/CIRED/UIE JWG C4.110, Voltage dip immunity of equipment and installations, CIGRE Technical Brochure 412, published in 2010.
[10] Halpin, S.M. “Power Quality” The Electric Power Engineering Handbook Ed. L.L. Grigsby Boca Raton: CRC Press LLC, 2001.
[11] Suresh Kamble et al; “Characteristics Analysis of Voltage Sag in Distribution System using RMS Voltage Method”, ACEEE Int. J. on Electrical and Power Engineering, Vol. 03, No. 01, Feb 2012.
[12] Suresh Kamble et al; “Characterization of Voltage Sag Due To Balanced and Unbalanced Faults in Distribution Systems” International Journal Of Electrical Engineering. Volume 3, Issue 1, January- June (2012), pp. 197-209.
[13] M.H.J. Bollen, “Characterization of Voltage Sag Experienced by Three-Phase Adjustable-Speed Drives”, IEEE Transactions on Power Delivery, Vol.12, No.4, pp.1666-1671, Oct. 1997.
[14] Math H. J. Bollen, “Understanding Power Quality problems: Voltage Sags and Interruptions”, IEEE Press, 2000.
[15] Florin MOLNAR-MATEI et al; “New Method for Voltage Sags Characteristics Detection in Electrical Networks”, 978-1-4244-5794-6/10/$26.00 ©2010 IEEE.
[16] J. Wang, S. Chen, and T. T. Lie, “System voltage sag performance estimation,” IEEE Transactions on Power Delivery, vol. 20, pp. 1738-1747, April 2005.
[17] David B. Vannoy et all; “Roadmap for Power-Quality Standards Development” , Ieee Transactions On Industry Applications, Vol. 43, No. 2, March/April 2007.
[18] L.P. Singh et al; “A New Approach for Analysing Voltage Sag Severity Based on Power Quality Indices”, International Journal on Emerging Technologies 4(1): 1-5(2013).
[19] Nita R. Patne et al; “Factor Affecting Characteristic of Voltage Sag Due to Fault in the Power System”, Serbian Journal Of Electrical Engineering Vol. 5, No. 1, May 2008, 171-182.
[20] S. GUPTA, “Power Quality: Problems, Effects And Economic Impacts” International Journal of Electrical and Electronics Engineering ( IJEEE) Vol.1, Issue 1 Aug 2012 83-91 © IASET.
[21] M.Mc Granaghan et al, “Economic evaluation of power quality”, IEEE Power engineering review, Vol.22, no.2, Feb.2002, pp8-12.
[22] Recommended Practice for Monitoring Electric Power Quality, 1995. IEEE Std. 1159, IEEE.
[23] L. Dinesh et al; “Simulation of Unified Power Quality Conditioner for Power Quality Improvement Using Fuzzy Logic and Neural Networks” Innovative Systems Design and Engineering ,Vol 3, No 3, 2012

Do different types of non-linear loads generate different harmonics?

Published by Mirus International Inc., [2010-01-08] MIRUS-FAQ001-B2, FAQ’s Harmonic Mitigating Transformers, 31 Sun Pac Blvd., Brampton, Ontario, Canada. L6S 5P6.

By far the majority of today’s non-linear loads are rectifiers with DC smoothing capacitors. These rectifiers typically come in 3 types – (i) single phase, line-to-neutral, (ii) single phase, phase-to-phase and (iii) three-phase.

Single-phase line-to-neutral rectifier loads, such as switch-mode power supplies in computer equipment, generate current harmonics 3^rd, 5^th, 7^th, 9^thand higher. The 3^rd will be the most predominant and typically the most troublesome. 3^rd, 9^th and other odd multiples of the 3^rd harmonic are often referred to as triplen harmonics and because they add arithmetically in the neutral are also considered zero sequence currents. Line-to-neutral non-linear loads can be found in computer data centers, telecom rooms, broadcasting studios, schools, financial institutions, etc.

208V single-phase rectifier loads can also produce 3^rd, 5^th, 7^th, 9^th and higher harmonic currents but if they are reasonably balanced across the 3 phases, the amplitude of 3^rdand 9^th will be small. Because they are connected line-line, these loads cannot contribute to the neutral current. The largest current and voltage harmonics will generally be the 5^thfollowed by the 7^th. Typical single phase, 208V rectifier loads include the switch-mode power supplies in computer equipment and peripherals.

Three-phase rectifier loads are inherently balanced and therefore generally produce very little 3^rd and 9^thharmonic currents unless their voltage supply is unbalanced. Their principle harmonics are the 5^th and 7^th with 11^th and 13^th also present. They cannot produce neutral current because they are not connected to the neutral conductor. The rectifiers of variable speed drives and Uninterruptible Power Supplies (UPS) are typical examples of three-phase rectifier loads.

**Figure 2-1:** Typical non-linear load harmonic spectrums and waveforms

Harmonics and Harmonic Mitigating Transformers (HMT’s) Questions and Answers

What is Load Flow Analysis / Power Flow Analysis and Why is it Done?

Published by Carelabs (Carelabz), Website: carelabz.com

Load Flow Analysis / Power Flow Analysis

Power systems across the world subjected to great demands owing to expansions in the networks. Rapid development of a nation in every sphere is interlinked with its power transmission capacity. This can be done by adding new lines and by upgrading existing ones by adding new devices like FACTS. A stable power transmission network ensures prosperity.

Voltage instability in any network may lead to system collapse, when the bus voltage drops to such a level from which it cannot recover. In such a situation, complete system blackouts may take place. Hence voltage stability analysis is very important for successful process and planning of power system and for decreasing system losses. In this context, Load Flow or Power Flow Study and Analysis has been found useful by researchers in Voltage Stability Studies and Contingency Analysis.

Voltage Stability

Voltage stability is capacity of a power system to manage acceptable voltages at all buses in the system under normal conditions and after subjected to Voltage instability results in voltage collapse. Voltage collapse is the process by which the voltage falls to a low, unacceptable value as a result of an avalanche of events accompanying voltage instability. Voltage failure usually appears in power systems which usually heavy loaded failure and has reactive power shortages. In recent years voltage instability has attracted attention of power system planning and operating engineers as well as researchers. This is due to the frequent voltage collapses occurring in different parts of the world. Therefore Voltage Stability Analysis is important for researchers and power system planners to prevent such incidents from occurring.

Voltage Stability Analysis

The different methods used are:

P-V curve method.
V-Q curve method and reactive power reserve.
Methods based on peculiarity of power flow Jacobean matrix at the point of voltage collapse.
Continuation power flow method.
Optimization Method

Load Flow Analysis

To begin the Voltage Stability Analysis of a power system, computation of the complex voltages at all the buses is essential. After this, power flows from a bus and the power flowing in all the transmission lines are to calculate. A computational tool for this purpose is Load Flow Analysis. This analysis helps compute the steady state voltage magnitudes at all the buses, for a particular load condition. Load flow is mainly used in planning studies, for designing a new network or expansion of an existing one. The next step would be to compare the calculated values of power flows and voltage with the steady state device limits, to estimate the health of the network

Load Flow Study

Power flow analysis is very important in planning and designing the future expansion of power systems or addition to existing ones like adding new generator sites, meeting increase load demand and locating new transmission sites.

The load flow solution yields the nodal voltages and the phase angles, the power injection, power flows and the line losses in a network.

The best place, as well as the optimal capacity of a generating station, substation and new lines can regulate by load flow study.

Minimization of System transmission losses and prevention of line overloads. The operating voltages of the buses being determined, it aids in voltage stability analysis and voltage levels at certain buses can keep within the closed tolerances. The power flow problem formulated assuming the power system network to linear, bilateral and balanced. However, the power and voltage constraints impose non-linearity in the power flow formulation and iterative techniques are essential for the solution. The different conventional techniques for solving the power flow problem are:

Gauss-Seidel (GS) Method
Newton Raphson (NR) Method
Fast Decoupled Load Flow (FDLF)

Voltage Stability and Line Compensation

One of the prime causes leading to voltage instability is reactive power imbalance in the power system network. This occurs when there is a sudden and unpredicted increase or decrease in reactive power demand in the system. Occurrence of voltage collapse can only be prevented by either reducing the reactive power load or by providing further supply of reactive power before the system reaches the point of voltage collapse. During situations of outage in some critical lines, the generators are capable of supplying limited reactive power. But in the process, the real powers of the generators compromised while supplying this reactive power. In long transmission lines, the line length and the degree of shunt compensation are the most important factors affecting the power frequency voltages under normal and fault conditions. An open-end or unloaded line experiences a rise in the receiving end voltage related to sinusoidal input voltage, known as Ferranti effect. On the other hand, an overloaded line experiences a sequential reduction in voltage leading to voltage collapse at the weakest bus. To stabilize the line voltage, reactive power (VAR) compensation required, which is control of reactive power to enhance power system network performance. The two important features of reactive power compensation are:

Load Compensation and
Voltage Support.

The aim of voltage support is to decrease the voltage variations at a given terminal of a transmission line.

Line inductance compensation done by sequence capacitors and the line capacitance to earth by shunt reactors. Optimal placement of sequence capacitors are at different places along the line, when that of the shunt reactors is in the stations at the end of the line. In this way, the voltage drop/rise between the ends of the line can decrease both in amplitude and phase angle.

Source: https://carelabz.com/what-why-load-flow-analysis-power-flow-analysis-done/

What are non-linear loads and why are they a concern today?

Published by Mirus International Inc., [2010-01-08] MIRUS-FAQ001-B2, FAQ’s Harmonic Mitigating Transformers, 31 Sun Pac Blvd., Brampton, Ontario, Canada. L6S 5P6.

A load is considered non-linear if its impedance changes with the applied voltage. The changing impedance means that the current drawn by the non-linear load will not be sinusoidal even when it is connected to a sinusoidal voltage. These non-sinusoidal currents contain harmonic currents that interact with the impedance of the power distribution system to create voltage distortion that can affect both the distribution system equipment and the loads connected to it.

In the past, non-linear loads were primarily found in heavy industrial applications such as arc furnaces, large variable frequency drives (VFD), heavy rectifiers for electrolytic refining, etc. The harmonics they generated were typically localized and often addressed by knowledgeable experts.

Times have changed. Harmonic problems are now common in not only industrial applications but in commercial buildings as well. This is due primarily to new power conversion technologies, such as the Switch-mode Power Supply (SMPS), which can be found in virtually every power electronic device (computers, servers, monitors, printers, photocopiers, telecom systems, broadcasting equipment, banking machines, etc.). The SMPS is an excellent power supply, but it is also a highly non-linear load. Their proliferation has made them a substantial portion of the total load in most commercial buildings.

Examples of the current drawn by various types of equipment are shown in Figure 1-1. The most common form of distorted current is a pulse waveform with a high crest factor. The SMPS is one such load since it consists of a 2-pulse rectifier bridge (to convert AC to DC) and a large filter capacitor on its DC bus. The SMPS draws current in short, high-amplitude pulses that occur right at the positive and negative peaks of the voltage. Typically these high current pulses will cause clipping or flat-topping of the 120VAC supply voltage. The “double-hump” current waveform of the 6-pulse rectifier in a UPS or a VFD also will cause clipping or flat-topping of the 480V or 600V distribution system.

**Figure 1-1:** Typical linear and non-linear current waveforms

Harmonics and Harmonic Mitigating Transformers (HMT’s) Questions and Answers