Importance of Power Quality in Power Systems

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.

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:

Where V_n-rms is the RMS voltage of nth harmonic in the signal and V_fund-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.

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

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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  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.