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.
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.
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 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.
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.
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.
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 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 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.
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.
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 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 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.
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.
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:
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.
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:
|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.|
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.
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.
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.
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.
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.
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 (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.
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.
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.