5 Must-Have Features in a power company power quality

Measurement of power meeting specifications

Electric power quality is the degree to which the voltage, frequency, and waveform of a power supply system conform to established specifications. Good power quality can be defined as a steady supply voltage that stays within the prescribed range, steady AC frequency close to the rated value, and smooth voltage curve waveform (which resembles a sine wave). In general, it is useful to consider power quality as the compatibility between what comes out of an electric outlet and the load that is plugged into it.[1] The term is used to describe electric power that drives an electrical load and the load's ability to function properly. Without the proper power, an electrical device (or load) may malfunction, fail prematurely or not operate at all. There are many ways in which electric power can be of poor quality, and many more causes of such poor quality power.

The electric power industry comprises electricity generation (AC power), electric power transmission and ultimately electric power distribution to an electricity meter located at the premises of the end user of the electric power. The electricity then moves through the wiring system of the end user until it reaches the load. The complexity of the system to move electric energy from the point of production to the point of consumption combined with variations in weather, generation, demand and other factors provide many opportunities for the quality of supply to be compromised.

While "power quality" is a convenient term for many, it is the quality of the voltage—rather than power or electric current—that is actually described by the term. Power is simply the flow of energy, and the current demanded by a load is largely uncontrollable.

Frequency stability of some large electrical grids

Introduction

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The quality of electrical power may be described as a set of values of parameters, such as:

• Continuity of service (whether the electrical power is subject to voltage drops or overages below or above a threshold level thereby causing blackouts or brownouts[2])
• Variation in voltage magnitude (see below)
• Transient voltages and currents
• Harmonic content in the waveforms for AC power

It is often useful to think of power quality as a compatibility problem: is the equipment connected to the grid compatible with the events on the grid, and is the power delivered by the grid, including the events, compatible with the equipment that is connected? Compatibility problems always have at least two solutions: in this case, either clean up the power, or make the equipment more resilient.

The tolerance of data-processing equipment to voltage variations is often characterized by the CBEMA curve, which give the duration and magnitude of voltage variations that can be tolerated.[3]

CBEMA curve

Ideally, AC voltage is supplied by a utility as sinusoidal having an amplitude and frequency given by national standards (in the case of mains) or system specifications (in the case of a power feed not directly attached to the mains) with an impedance of zero ohms at all frequencies.

Deviations

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No real-life power source is ideal and generally can deviate in at least the following ways:

Voltage

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• Variations in the peak or root mean square (RMS) voltage are both important to different types of equipment.
• When the RMS voltage exceeds the nominal voltage by 10 to 80% for 0.5 cycle to 1 minute, the event is called a "swell".
• A "dip" (in British English) or a "sag" (in American English the two terms are equivalent) is the opposite situation: the RMS voltage is below the nominal voltage by 10 to 90% for 0.5 cycle to 1 minute.
• Random or repetitive variations in the RMS voltage between 90 and 110% of nominal can produce a phenomenon known as "flicker" in lighting equipment. Flicker is rapid visible changes of light level. Definition of the characteristics of voltage fluctuations that produce objectionable light flicker has been the subject of ongoing research.
• Abrupt, very brief increases in voltage, called "spikes", "impulses", or "surges", generally caused by large inductive loads being turned ON, or more severely by lightning.
• "Undervoltage" occurs when the nominal voltage drops below 90% for more than 1 minute.[4] The term "brownout" is an apt description for voltage drops somewhere between full power (bright lights) and a blackout (no power – no light). It comes from the noticeable to significant dimming of regular incandescent lights, during system faults or overloading etc., when insufficient power is available to achieve full brightness in (usually) domestic lighting. This term is in common usage has no formal definition but is commonly used to describe a reduction in system voltage by the utility or system operator to decrease demand or to increase system operating margins.
• "Overvoltage" occurs when the nominal voltage rises above 110% for more than 1 minute.[4]

Frequency

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• Variations in the frequency.
• Nonzero low-frequency impedance (when a load draws more power, the voltage drops).
• Nonzero high-frequency impedance (when a load demands a large amount of current, then suddenly stops demanding it, there will be a dip or spike in the voltage due to the inductances in the power supply line).
• Variations in the wave shape – usually described as harmonics at lower frequencies (usually less than 3 kHz) and described as Common Mode Distortion or Interharmonics at higher frequencies.

Waveform

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• The oscillation of voltage and current ideally follows the form of a sine or cosine function, however it can alter due to imperfections in the generators or loads.
• Typically, generators cause voltage distortions and loads cause current distortions. These distortions occur as oscillations more rapid than the nominal frequency, and are referred to as harmonics.
• The relative contribution of harmonics to the distortion of the ideal waveform is called total harmonic distortion (THD).
• Low harmonic content in a waveform is ideal because harmonics can cause vibrations, buzzing, equipment distortions, and losses and overheating in transformers.

Each of these power quality problems has a different cause. Some problems are a result of the shared infrastructure. For example, a fault on the network may cause a dip that will affect some customers; the higher the level of the fault, the greater the number affected. A problem on one customer’s site may cause a transient that affects all other customers on the same subsystem. Problems, such as harmonics, arise within the customer’s own installation and may propagate onto the network and affect other customers. Harmonic problems can be dealt with by a combination of good design practice and well proven reduction equipment.

Power conditioning

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Power conditioning is modifying the power to improve its quality.

An uninterruptible power supply (UPS) can be used to switch off of mains power if there is a transient (temporary) condition on the line. However, cheaper UPS units create poor-quality power themselves, akin to imposing a higher-frequency and lower-amplitude square wave atop the sine wave. High-quality UPS units utilize a double conversion topology which breaks down incoming AC power into DC, charges the batteries, then remanufactures an AC sine wave. This remanufactured sine wave is of higher quality than the original AC power feed.[5]

A dynamic voltage regulator (DVR) and static synchronous series compensator (SSSC) are utilized for series voltage-sag compensation.

A surge protector or simple capacitor or varistor can protect against most overvoltage conditions, while a lightning arrester protects against severe spikes.

Electronic filters can remove harmonics.

Smart grids and power quality

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Modern systems use sensors called phasor measurement units (PMU) distributed throughout their network to monitor power quality and in some cases respond automatically to them. Using such smart grids features of rapid sensing and automated self healing of anomalies in the network promises to bring higher quality power and less downtime while simultaneously supporting power from intermittent power sources and distributed generation, which would if unchecked degrade power quality.

Compression algorithm

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A power quality compression algorithm is an algorithm used in the analysis of power quality. To provide high quality electric power service, it is essential to monitor the quality of the electric signals also termed as power quality (PQ) at different locations along an electrical power network. Electrical utilities carefully monitor waveforms and currents at various network locations constantly, to understand what lead up to any unforeseen events such as a power outage and blackouts. This is particularly critical at sites where the environment and public safety are at risk (institutions such as hospitals, sewage treatment plants, mines, etc.).

Challenges

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Engineers use many kinds of meters,[6] that read and display electrical power waveforms and calculate parameters of the waveforms. They measure, for example:

• current and voltage RMS
• phase relationship between waveforms of a multi-phase signal
• power factor
• frequency
• total harmonic distortion (THD)
• active power (kW)
• reactive power (kVAr)
• apparent power (kVA)
• active energy (kWh)
• reactive energy (kVArh)
• apparent energy (kVAh)
• and many more

In order to sufficiently monitor unforeseen events, Ribeiro et al.[7] explains that it is not enough to display these parameters, but to also capture voltage waveform data at all times. This is impracticable due to the large amount of data involved, causing what is known the “bottle effect”. For instance, at a sampling rate of 32 samples per cycle, 1,920 samples are collected per second. For three-phase meters that measure both voltage and current waveforms, the data is 6–8 times as much. More practical solutions developed in recent years store data only when an event occurs (for example, when high levels of power system harmonics are detected) or alternatively to store the RMS value of the electrical signals.[8] This data, however, is not always sufficient to determine the exact nature of problems.

Raw data compression

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Nisenblat et al.[9] proposes the idea of power quality compression algorithm (similar to lossy compression methods) that enables meters to continuously store the waveform of one or more power signals, regardless whether or not an event of interest was identified. This algorithm referred to as PQZip empowers a processor with a memory that is sufficient to store the waveform, under normal power conditions, over a long period of time, of at least a month, two months or even a year. The compression is performed in real time, as the signals are acquired; it calculates a compression decision before all the compressed data is received. For instance should one parameter remain constant, and various others fluctuate, the compression decision retains only what is relevant from the constant data, and retains all the fluctuation data. It then decomposes the waveform of the power signal of numerous components, over various periods of the waveform. It concludes the process by compressing the values of at least some of these components over different periods, separately. This real time compression algorithm, performed independent of the sampling, prevents data gaps and has a typical 1000:1 compression ratio.

Aggregated data compression

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A typical function of a power analyzer is generation of data archive aggregated over given interval. Most typically 10 minute or 1 minute interval is used as specified by the IEC/IEEE PQ standards. A significant archive sizes are created during an operation of such instrument. As Kraus et al.[10] have demonstrated the compression ratio on such archives using Lempel–Ziv–Markov chain algorithm, bzip or other similar lossless compression algorithms can be significant. By using prediction and modeling on the stored time series in the actual power quality archive the efficiency of post processing compression is usually further improved. This combination of simplistic techniques implies savings in both data storage and data acquisition processes.

Standards

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The quality of electricity supplied is set forth in international standards and their local derivatives, adopted by different countries:

EN50160 is the European standard for power quality, setting the acceptable limits of distortion for the different parameters defining voltage in AC power.

IEEE-519 is the North American guideline for power systems. It is defined as "recommended practice"[11] and, unlike EN50160, this guideline refers to current distortion as well as voltage.

IEC 61000-4-30 is the standard defining methods for monitoring power quality. Edition 3 (2015) includes current measurements, unlike earlier editions which related to voltage measurement alone.

See also

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References

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Additional resources:
The Evolution of Digital DC Millivoltmeters: From Analog to Precision
What tool is used to check for electrical power?
Types of Fittings for Gas and Water Pipes
Benefits of High-Voltage Direct Current Transmission Systems
What is a non-contact voltage tester?

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Literature

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• Dugan, Roger C.; Mark McGranaghan; Surya Santoso; H. Wayne Beaty (2003). Electrical Power Systems Quality. McGraw-Hill Companies, Inc. ISBN 978-0-07-138622-7.

• Meier, Alexandra von (2006). Electric Power Systems: A Conceptual Introduction. John Wiley & Sons, Inc. ISBN 978-0471178590.

• Heydt, G.T. (1991). Electric Power Quality. Stars in a Circle Publications. Library Of Congress 621.3191. ISBN 978-9992203040.

• Bollen, Math H.J. (2000). Understanding Power Quality Problems: Voltage Sags and Interruptions. New York: IEEE Press. ISBN 0-7803-4713-7.

• Sankaran, C. (2002). Power Quality. CRC Press LLC. ISBN 978-0-8493-1040-9.

• Baggini, A. (2008). Handbook of Power Quality. Wiley. ISBN 978-0-470-06561-7.

• Kusko, Alex; Marc Thompson (2007). Power Quality in Electrical Systems. McGraw Hill. ISBN 978-0-07-147075-9.

• Chattopadhyay, Surajit; Mitra, Madhuchhanda; Sengupta, Samarjit (2011). Electric Power Quality. Springer Science+Business. ISBN 978-94-007-0634-7.

Electricity with a bad quality is dangerous and uneconomical at both utility and consumer end. There is a big need to focus on the quality of power being supplied to the loads. Read more as we cover causes of poor power quality, different measuring parameters, power quality standards and various techniques to improve the power quality.

Power quality is the ability of a power grid to supply power to the consumers efficiently and it also expresses the ability of an equipment to consume the power being supplied to it. In technical terms, power quality is the measure, study and enhancement of sinusoidal waveform at the rated voltage and frequency.

Power quality can have a big impact on the performance and cost of a power system. So, it is essential to make sure that the power being consumed by the system is of right quality and the system is compatible to function with the power delivered to it. Nowadays consumers have become well aware of power quality, that’s why many governments have revised their policies to force electric utilities for making sure the power quality according to the designed standards. Also the modern equipment is more sensitive to any changes in power quality. Manufacturers, utilities and consumers all are concerned about power quality and this concern is increasing day by day.

 

Causes of Poor Power Quality

 

There is need to identify the factors which lead to poor power quality in a power system. These possible causes are uncertain events, utility, consumer and manufacturer.

Uncertain Events

Most of the problems in power quality are caused by random events like faults, resonance, lightning surges etc. Electric utility is associated with such kind of disturbances.

Utility

Utility is responsible for poor power quality at three ends:

Generation end: Power quality issues at the generating end arise due to expansion, maintenance, scheduling, outages and load shifting.

Transmission end: Power quality gets affected in transmission lines due to wind interrupting the power supply, voltage variations, lightning, improper functioning of voltage regulation devices etc.

Distribution end: Voltage dips, interruptions, transients, spikes, transformer energization etc. are the reasons of poor power quality in the distribution system.

Consumer

Consumers contribute to a big chunk of power quality issues. Non-linear loads used by consumers produce harmonics in the power system, thus leading to poor power quality. If a load’s impedance varies with the applied voltage then it is said to be non-linear. The changing impedance means non sinusoidal current drawn by the non-linear load even if there is sinusoidal voltage in the system. The non sinusoidal current contains harmonic current which interferes the system’s impedance and leads to voltage distortion that can affect power system and the loads connected to it.

Manufacturer

Power quality issues can be related to manufacturers in two ways:

Standards: Lack of standards for installation, testing, certification, purchase, sale or use of any product may result in poor power quality.

Equipment sensitivity: The sensitivity of an equipment may cause power quality issues if it is incompatible with the electrical environment due to high sensitivity.

Common Power Quality Issues and Parameters

Transients

Transients are the pulses occurring in a sinusoidal waveform for a short duration but of high intensity. Transients can come from either internal or external sources i.e. either from outside or inside the facility. External sources may include lightning, wind, transformer switching etc. While faults in the system, load switching or arcing are considered as internal sources. These distortions in the waveform are undesirable as equipment can be harmed by several means like dielectric breakdown, fracture, insulation flashover, overload etc. In this way, the transients result in poor quality. For detailed video explanation, click here.

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Voltage Variations

Voltage of a system may vary from its nominal value, and the phenomenon is termed as voltage variation. One of the factors causing voltage variation is interruption which may occur due to equipment failure, control malfunction, or fuse/circuit breaker operation. Sag or voltage dip i.e. reduction in RMS voltage is another factor which is caused by starting of large motors, single line to ground fault, load shifting or energizing heavy loads. Also, the under or over voltages lead to voltage variations. Under voltages are caused when system is overloaded and over voltages occur when system is equipped with lesser loads as compared to the utility’s voltage level. For detailed video explanation, click here.

Unbalanced Voltages

Unbalanced voltages mean that voltages of a 3 phase system are different in either magnitude or phase difference between each of two phases is not same i.e. other than 120 degrees. Blown fuse in any of the 3 phases, unequal distribution of loads in a 3 phase system and no transposition in overhead transmission lines are the major causes of voltage imbalance in the power systems. Such unbalanced voltages may harm or damage the electrical equipment, thus causing poor power quality. For detailed video explanation, click here.

Flickers

Continuous variations in voltage of the supplied power causes rapid fluctuations in the load currents leading to instability of visual sensation. There is rapid and visible change in brightness of a lamp which puts harmful effect on a human eye. Sudden load changes, motor drives, arc furnaces, welding machines etc. are the common causes of flickering effect. So, the flickers put a question mark on the power quality.

Distorted Waveforms

Deviation of a waveform from the steady state sinusoidal waveform is known as distortion in the waveform. These distortions can be of different types like DC offset, harmonics and electric noise. Presence of a DC current or voltage component in an AC system is known as DC offset which is mainly caused by switching devices, leakage inductance of inductor loads etc. DC offsets can harm the power system as it may lead to overheating of an equipment thereby reducing its lifetime. Sinusoidal waveforms having frequencies as integral multiple of the fundamental frequency are known as harmonics. Non-linear loads, switching devices etc. are the main causes of harmonics in the power system which lead to malfunction of controlling devices, losses in an equipment, additional noise etc. Another type of waveform distortion is electric noise which is defined as undesirable electric signals overlaid on power system voltage or current waveform. Common causes of electric noise are improper connections in power system, electronic devices, corona effect etc. All of these distortions have bad impact on the power quality so they must be mitigated.

Total Harmonic Distortion (THD) is defined as the measurement of the harmonic distortion present in a waveform. Power quality of a power system is inversely proportional to THD. More harmonic distortion in the system, lower will be the power quality and vice versa. THD is equal to the ratio of the RMS harmonic content to the fundamental:

√

 ∑

∞n=2

V2n-rms 

   Vfund-rms     

THD =

Where Vn-rms is the RMS voltage of nth harmonic in the signal and Vfund-rms is the RMS voltage of the fundamental frequency.

 

Power Factor

Power factor is directly linked with power quality. Power factor’s value closer to 1 indicates high power quality. As much the value of power factor is less than one, the more poor will be the power quality and higher will be the costs.

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Varying Frequency

Fluctuations in the magnitude of frequency from its nominal value (50 or 60 Hz) are defined as frequency variations. Frequency of power system deviates from the fundamental value if there is an imbalance between generation and demand. Faults in the transmission system are also one of the causes of frequency variations. These fluctuations result in poor power quality as all of the electrical devices are designed according to the rated frequency and any variations in this value may put harmful impact on them.

Temporary interruptions

Electric supply to consumers can also be interrupted or cut off i.e., total loss of power occurs, and voltage becomes zero, for short or long periods of time in a localized area. This interruption might be planned by your utility or can also be a result of the following causes:

• Short term power interruptions usually occur because of insulation failure, lightning and insulator flashovers etc.
• Long term interruptions can be caused by equipment failure in the power system network, human errors, unfavorable weather conditions, poor coordination of protection devices etc.

Voltage notching

Notching is a recurring power quality disturbance caused by the normal operation of power electronic devices (rectifiers, SCRs etc.) when current is commutated from one phase to another. The abrupt change in voltage due to notch excites natural frequencies of the electrical system. This creates additional non-characteristics harmonics to appear in the system voltage. Due to these high frequency harmonics created by notching, sensitive logic and communication electronic circuits in the facility can be damaged, radio interference might occur.

Effects of Poor Power Quality on Power System

There are harmful impacts of poor power quality on both the utility and consumer end. Some of the main effects of poor power quality in the power system are as following:

• Harmonics add up to the waveform and equipment may receive high peak of waveforms thereby damaging the equipment. High voltages may also cause the equipment to operate in saturation region producing additional disturbances.
• Due to overheating, noise etc. lifetime of an equipment is reduced.
• System’s efficiency or performance is highly decreased due to poor power quality.
• Due to power failure or interruption, important data can be lost or corrupted which may lead to a great loss.
• Costs of a power system are highly increased if there is poor power quality.
• When there is a power failure, consumers can face many problems due to unavailability of power and it affects the utility costs as well.
• Consumer loads are badly affected or even get damaged due to power quality issues.
• Sometimes there is need to oversize the power system due to additional stress imposed by poor power quality. This expansion results in high installation costs.

Power Quality Standards

Organization

Standard

Title

Description

IEEE 141-1993 Electric power distribution for industrial plants Guidance for equipment and life safety, reliability, voltage regulations and flexibility of expansion etc. IEEE 242-1986 Protect & coordination of industrial and commercial application.

Standard of proper selection, application and coordination of component for protection.

IEEE 519-1992 Harmonic control in electric power system Recommended for harmonic control and reactive compensation.  IEEE 1159-1995 Monitoring electric power quality.

Guidance for monitoring objectives, measuring instrument, monitoring application techniques.

IEEE 1250-2018 Voltage quality in power system.

Standard of ways to identify and improve the quality of power in electrical system.

SEMI F-47-1999 Equipment reliability, availability and maintainability.

Standard of definition and measurement of equipment reliability, availability and its maintenance for better power quality.

ANSI C84.1-1995 Electric power system and equipment -voltage rating.

Recommendation for voltage ratings of equipment and power system to attain compatibility. 

NEMA MG 1-1998 Motors and Generators.

Standard for technical specification used by manufacturers to achieve power quality.

NEMA LSI-1992 Low voltage surge protective devices.

Guidance for quality construction of the devices.

IEC 816-1984 Transient on low- voltage power and signal line.

Recommended practice for methods of measurement of short duration transient on low- voltage power and signal line. 

IEC 868-0-1991 Flicker meter Evaluation of severity of voltage fluctuations on light flickers. 

Power Quality Improvement Techniques

There are many techniques proposed and implemented to mitigate the effect of poor power quality on the power system. Many devices have been introduced to reduce or suppress the bad impact of low power quality. Other than that, deep analysis and monitoring of the power quality is carried out to enhance or maintain the power quality as per requirements.

1. Power System Studies

A power system study is defined as different engineering investigations to ensure a secure, proficient and reliable electrical system of a facility in both normal and abnormal conditions. Engineers with great knowledge and understanding of power systems are needed to conduct power system studies. There are various types of power system studies, each having its own significance and methodology. The studies include short circuit, coordination, arc-flash, load flow , harmonic analysis and stability study . Comprehensive and exact data is required for a power system in order to conduct power system studies. Such studies are carried out using various software tools. Completion of power system studies reduces the risk level of ongoing operations and enhances the system’s efficiency.

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2. Power Conditioning Devices

Power conditioning devices are used to improve the power quality that is supplied to the equipment in power system. There are number of devices that act as power conditioners in different ways.

• Surge Protectors

  A device which protects an electrical equipment from voltage spikes are known as surge protectors or surge suppressors. Whenever there is a rapid increase in voltage, surge protectors detect such increment in magnitude and limit the voltage level up to the value which can be tolerated by the system by directing the excessive current flow to the ground.

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• Filters

  Filters are the devices used to remove the harmonics generated by non-linear loads in the system. Filters are placed near non-linear loads, they either bypass harmonic currents or block the harmonics to enter the power system.

• Voltage Regulators

  A device that automatically maintains a constant voltage level is known as voltage regulator. It produces a fixed output voltage irrespective of the input provided or load connected.

• UPS

  UPS (Uninterrupted Power Supply) is an electrical device that works as a backup and provide supply to the system when there is an emergency or main power failure.

3. Power Quality Monitoring

Power quality monitoring (PQM) is to collect, analyze and use the electrical data to improve the power quality and system’s performance. It ensures energy management, quality control, preventive maintenance and overall cost deductions. Nowadays, consumers are well aware of power quality and expect efficient electrical service. For this reason, electrical facilities are concerned about power quality monitoring and use digital fault recorders, smart relays or other special purpose power quality equipment. Modern power plants regularly monitors the quality of voltage and currents supplied to the consumer to optimize the power quality. Every power system should improve its performance, efficiency and elongate the lifetime of the equipment.

Conclusively, power quality problems are often interconnected. It is compulsory to analyze power quality issues from aspect of an entire plant along with complete focus on how they affect individual loads. Sometimes resolving a power quality issue can make another problem worse. By having a look at big picture, power quality analysis enables you to identify and mitigate the reasons of power quality issues. Increasing power quality problems are also giving rise to the awareness of power quality among the utilities and consumers as well. A deep analysis and understanding is required in every power system to maximize the proficiency of the electrical systems everywhere.

Does your facility really needs a high level Power Quality Study?

At AllumiaX Engineering, we aim to provide high quality power system study in compliance with the set international standards of IEEE, NEC and NFPA. We ensure that power systems are safe, reliable, operable, maintained, protected and well designed. Contact us for a Power Quality Study of your facility.

• About The Author

  Abdur Rehman is a professional electrical engineer with more than eight years of experience working with equipment from 208V to 115kV in both the Utility and Industrial & Commercial space. He has a particular focus on Power Systems Protection & Engineering Studies.