Which factors affect the power supply quality?

The harmonious and uniform standards should be specified by the State Regulatory Commissions to serve the best interest of the utilities and consumers connected to the national grid. - Ashok Upadhyay

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Which factors affect the power supply quality?

The objective of the power quality Standards is to ensure reliable and quality power to the electricity consumers. The Electricity Act 2003 has enshrined the basic need of consumers to be provided with continuous, reliable and quality supply by the distribution utilities. Meanwhile the accelerated growth of renewable energy along with meteoric rise of non-linear loads, are posing serious challenges for quality of conventional unidirectional power flow from generation to consumption points. India is the 4th largest consumer of electricity in the world but in spite of being one of the leaders both in electricity generation and consumption, it is facing major issues related to power quality. The issue of power quality remained largely ignored in the electricity supply industry of India. There are many reasons like huge gap between demand and supply just a decade back, lack of awareness and capacity to understand issues and challenges associated with quality of power, restricted availability of technology in detecting and overcoming such challenges.

Power quality is drawing increasing attention due to the heavy penetration of power electronics-based loads in every walk of our lives. Power quality parameters like frequency, voltage quality (interruptions, variations, unbalances, flicker, sags, and swells), harmonics and power factor are key matrices/indicators for defining a good power quality environment. Poor quality of power lead to premature failure or reduced/degraded performance of equipment. It also caused increased system losses. Discerning consumers are looking for clean and quality power to drive their sensitive equipment at all levels. In this context, issues pertaining to power quality need for greater regulatory intervention in ensuring quality of power supply.

At present, a few parameters related to power quality are covered under the Central Electricity Authority (CEA) and SERCs Regulations. The Grid Code, Supply Code and Standard of Performance laid by various SERCs do mandate the quality of power to be maintained. The State Regulations, when dealing with the aspect of power quality through Supply Code/Grid Code or Standards of performance are not harmonious across different States and does not cover all aspects of power quality. Even there are lot of variations in similar power quality parameters specified by different SERCs. Therefore, there is a strong need to introduce a harmonised regulation on power quality across all states. SERCs are also required to put emphasis on measurement and introducing incentive/dis-incentive mechanism to ensure compliance to power quality parameters within certain limits.

Business and the economy in digital era also depend upon reliable and quality power supply. So far, the focus of the sector was limited to providing uninterrupted power supply to consumers. This was understandable at the time of deficit when the limited supply of power was available to meet peak demand and the expectation of end consumers was availability of power supply. But now the India has become one of the purples power country, thus, the quality power supply becomes the priority.

What is Power Quality

The reliability and quality are two important aspects of any electrical power supply system. Power Reliability means availability of power supply 24×7 basis which constitutes adequacy of electrical system at all levels from generation, transmission to distribution. However, power quality refers to both the extent of deviation or distortion in pure supply waveform and the continuity of supply. Any significant deviation in the magnitude, frequency, waveform or symmetry of line voltages is a potential power quality problem. Ideally, a wave form should be smooth and free from disturbances. But even the best power systems are subject to fluctuations and all electrical equipment are susceptible to damages caused by these fluctuations. When quality of the power supplied is deficient, it results in performance degradation and reduced life expectancy of equipment. Therefore, we may understand poor power quality as any power problem manifested in voltage, current, or frequency deviations that result in failure, increased energy loss or malfunctioning of equipment, thus causing economic loss.

Poor power quality can also result in problems with electromagnetic compatibility and noise. It can affect sophisticated protection systems and/or malfunctioning/failure of vital control and signal systems. Typical electrical loads, such as lighting, heating, and motor, are less sensitive to variations in the supply voltage, and more sensitive to availability (free from interruptions) of supply. However, electronic/digital equipment are more sensitive to variations in supply voltages. Characteristics that affect power quality are voltage fluctuation, harmonic distortion, voltage unbalance, flicker, supply interruptions, voltage sags, voltage swells and transients etc.

Need of Quality Power

In the emerging surplus power scenario, the characteristics of loads and the requirements of electrical systems have changed significantly. The devices and equipment used presently in industrial, commercial and domestic facilities are more sensitive to supply variations than equipment used in the past. It is due to increased use of power electronics and microprocessor-based technologies in equipment and appliances. The increasing penetration of Renewable sources of energy, semiconductor based electronic equipment, non-linear loads, data centres, industries running on adjustable speed drives and arc furnaces, etc. distort voltage/current waveforms in non-conformity to their desired form. This brings challenges to maintain the quality of power to ideal one and ensuring efficacy.

In India, various sectors are prone to both generation of higher power quality pollution as well as susceptible to power quality disturbances. The losses due to power quality issues are economic as well as technical. Both utilities as well as consumers are heavily impacted due to the techno-economic losses arising out of poor power quality. Poor power quality not only causes performance degradation and premature failure of electrical equipment but also results in increased system losses, financial loss etc. Therefore, apart from the reliability i.e. continuous supply, the preference of the electricity consumers is shifting towards quality power supply from the distribution licensees. Optimum power quality can enhance productivity and reduce losses.

Power Quality Parameters

The standards for voltage and other technical criteria are there which can be used to measure power quality. Parameters affecting power quality can be divided into two categories, i.e. Steady-state (or continuous) and Disturbances. Steady-state power quality parameters include Harmonics (waveform distortion), frequency deviation, voltage unbalance, voltage fluctuations and flicker. Disturbances include outages, momentary interruptions, momentary or transient overvoltage or surges, voltage dips and voltage swell. The important parameters are defined below with their probable causes and effects on the electrical equipment or supply system:

Frequency

Any variation of the power system fundamental frequency from its specified nominal value (e.g. 50 Hz in India) is defined as a frequency deviation. Frequency variations that go outside of the accepted limits for a normal steady-state operation of the power system can be caused by faults on the bulk power transmission system, a large block of load being disconnected, or a large source of generation going offline. Large frequency variations result in long-term damage to both generator and end use rotating electrical equipment whose rated output may suffer in low frequency regime. It may affect system stability and also leads to blackout of the grid. In interconnected power systems, significant frequency variations are rare.

Voltage

Voltage is a most important parameter in power system which affects the quality power in several ways like supply voltage interruptions, voltage fluctuations, voltage unbalance, voltage sag, voltage swell, voltage transients and voltage harmonics etc. as given below:

  • Supply Voltage Interruptions: It is a condition in which the voltage at the supply terminals is lower than 10 per cent of the nominal voltage. It may be long or sustained interruption if duration is longer than 1 min. and short interruption if duration is up to and including 1 min. Voltage interruptions longer than 1 min. are often permanent and require human intervention to repair the system for restoration. For poly-phase systems, an interruption occurs when the voltage falls below 10 per cent of the nominal voltage on all the phases otherwise, it is considered to be a voltage dip. Long power interruptions are a problem for all users, but many operations e.g. continuous process operations, multi-stage batch operations, digital data processing semiconductor fabrication etc. are very sensitive to even very short interruptions.
  • Voltage Fluctuations: It is defined as a cyclic variation of the voltage envelope or series of random voltage changes, the magnitude of which does not normally exceed the specified voltage ranges. They are relatively small (less than +5 or +10 percent) variations in the rms line-voltage. These variations can be caused by static frequency converters, cyclo-converters, arc furnaces, rolling mill drives, main winders and large motors during starting, etc. Voltage fluctuations may cause nuisance tripping due to mal-operation of relays and contactors and unwanted triggering of UPS units to switch to battery mode. It may stress electrical and electronic equipment toward detrimental effects that may disrupt production processes with considerable financial loss.
  • Voltage Unbalance: It is a condition in a poly-phase system in which the root mean square values of the line-to-line voltages, or the phase angles between consecutive line voltages, are not equal. The sources of unbalanced voltages are due to malfunctioning of equipment, mismatched transformer taps and impedances, blown capacitor fuses, open-delta regulators, or open-delta transformers. It can also be caused by uneven single-phase load distribution among the three phases. Unbalanced systems indicate the existence of a negative sequence component of supply voltage, which is harmful to all poly-phase loads, especially three-phase induction machines. It can cause an over-load on induction machines and malfunctioning of frequency converters. Voltage unbalance can create a current unbalance which can be 6 to 10 times the magnitude of voltage unbalance. In turn, current unbalance produces heat in the motor windings which degrades motor insulation causing progressive performance deterioration and permanent damage to the motor.
  • Voltage Sag (dip): It is a condition in which the voltage reduces at the supply terminals ranges for a duration of about half a cycle to several seconds. Common sources of sag are the starting of large induction motors and system faults. Sags can happen due to an overloaded circuit, malfunction of a transformers tap changer, breakers connecting a large inductive load to the grid or a disconnected capacitor bank. Also, arc furnaces initially take large amperes to produce high temperatures causing voltage sag. Voltage sag results in malfunction of equipment/ relays and contactors, under voltage tripping, loss of efficiency of motors and intermittent reduction of light illumination etc. In case voltage is too low, accelerated aging may take place in components and eventually causing faults in the network.
  • Voltage Swell (rise): It is a condition in which the voltage rises at the supply terminals for a duration of about half a cycle to several seconds. Over-voltage could be the result of connecting a capacitor bank or disconnecting a large inductive load. Other sources of voltage swells are line faults and incorrect transformer tap changer settings in the sub-stations. It also occurs due to transfer of loads from one source to another. Voltage swells results in malfunction of an equipment, insulation failure, intermittent increase in light illumination, tripping of relays and contactors etc. In case of very high voltage, damage to electrical appliances may occur.
  • Voltage Transients: Transients are momentary changes in voltage or current which occur over a short period of time usually for microseconds. It is divided into two categories. Impulse transient, which is a brief, unidirectional variation in voltage, current, or both on a power line and Oscillatory transient, which is a brief, bidirectional variation in voltage, current, or both on a power line. The most common causes of impulsive transients are lightning strikes, switching of inductive loads, opening and closing of energised lines and tap changing on transformers. Oscillatory transient can occur due to the switching of power factor correction capacitors, or transformer ferro-resonance. Poor or loose connections in the distribution system can also generate transients.

Due to transients, electronic devices may operate erratically. Motor winding insulation is degraded and resulting in eventual failure. The electrical distribution system is also affected by transient activity. Voltage Transients degrade the contacting surfaces of switches, isolators, and circuit breakers. Intense transient activity can produce nuisance tripping of breakers. Transformers may get saturated if exposed to high voltage transients. In such cases hysteresis losses will increase thus causing transformers to run hotter than normal.

  • Voltage Harmonics: It is a sinusoidal component of a periodic voltage waveform having a frequency that is an integral multiple of the fundamental frequency. It is the deviation from the original or pure voltage sine waveform. Generally, at the source point, the Voltage harmonics is absent. As the power flow progresses towards load end, voltage harmonic creeps in due to the effect of current characteristics of non-linear loads reflecting on network impedances. Voltage harmonics is generally expected to be managed by the utility service provider.

Flicker

It is the impression of uncomfortable visual sensation induced by a light stimulus whose luminance or spectral distribution fluctuates rapidly with time. It is caused under certain conditions by voltage fluctuations resulting in change of the luminance of lamps. Quantitatively, it may be expressed as the change in voltage over nominal voltage expressed as a percent. The main cause of these effects is fast switching operations of industrial processes and electrical appliances connected to the supply system. Flicker is considered the most significant effect of rapid voltage fluctuations because it can affect the production environment by causing personnel fatigue and lower work concentration levels.

Current Harmonics

It is sinusoidal component of a periodic current waveform having a frequency that is an integral multiple of the fundamental frequency. It is the deviation from the original or pure current sine wave. Voltage and Current Harmonic pollution can be quantified by Total Harmonic Distortion. Current harmonics in the system are produced by non-linear loads and causes power pollution akin to air pollution caused by automobile emission. Examples of such non-linear loads are power electronic equipment including variable speed drives, fan regulators, CFLs, LEDs, Televisions, Switched Mode Power Supplies, Data Processing equipment, high efficiency lighting, electrical machines working under magnetic saturation, arc furnaces, welding machines, rectifiers, DC brush motors, etc. These harmonics have serious effects on various electrical equipment such as overheating of cables and equipment. Further Harmonic causes increased system losses, interference with communication lines, errors while indicating electrical parameters, probability to produce resonant conditions, etc.

Power Factor

Power factor is a key indicator for an efficient energy delivery in AC electrical system. It is a measure of how effectively a specific load consumes electricity to produce work. Power factor may be categorised into displacement power factor and true power factor. Displacement power factor is the cosine of the angle between the fundamental voltage and current waveforms. However, presence of harmonics introduces additional phase shift between voltage and the current. True power factor is calculated as the ratio between the total active power used in a circuit (including harmonics) and the total apparent power (including harmonics) supplied from the source. True power factor is always less than displacement power factor if harmonics are present in the system.

Poor power factor results into requirement of higher apparent power and thus higher current flow at nominal voltage to do the same work against a higher power factor. To cope with these higher currents due to a poor power factor one has to increase conductor sizes or capacities of electrical equipment like generators or transformers thus resulting in blocked capital expenditure (capex) and increased operating cost of the system. The large current at low lagging power factor causes greater voltage drops in alternators, motors, transformers and transmission cum distribution lines. This leads to decrease in voltage at the driving end and forces the use of extra equipment like voltage stabilisers to counteract the voltage drop, or FACTS devices. Improving the power factor can maximise current-carrying capacity, improve voltage to equipment, reduce power losses, and lower electric bills. The simplest way to improve power factor is to add power factor correction capacitors preferably at load ends of the electrical system but ensuring network resonance due to harmonics is not magnified.

Recommendations 

The prevailing legal and policy framework with respect to power quality, provides that State Regulators are entrusted with the responsibility to specify or enforce standards with respect to quality, continuity and reliability of services by licensees to the consumers through Regulations. It was observed that main focus of the State Regulators is on management of power factor, frequency and reliability indices of power supply to the consumers. The other important power quality parameters such as voltage sags/swells, voltage fluctuations, voltage unbalance, harmonic distortion and voltage transients etc. are not covered comprehensively in the Regulations. These power quality parameters are not considered for assessing the health of DISCOMs and their obligation to provide quality supply as of now. Further, it is observed that power quality problems in distribution system are not yet studied extensively by the utilities. Some of the recommendations for quality power are as follows:

  • Regulations on power quality are need to be issued which define the power quality indices, roles and responsibilities of various entities, Standards/limits to be followed, incentive/disincentive mechanism to be deployed and procedure for monitoring, management and control of all aspects of power quality.
  • Since reliability and quality go hand in hand, the reliability indices should be included in the regulations.
  • For power quality parameters at transmission and sub-transmission system level, regulators should introduce appropriate reporting and incentive/dis-incentive mechanism in their grid/supply code or in standards of performance regulations for regular monitoring and implementation of the specified limits. incentive/dis-incentive mechanism may be structured and implemented in a phased manner.
  • Limits for some of power quality parameters like harmonic distortion, voltage variation and flicker, voltage unbalance, voltage sags/swells and supply interruptions have been specified in the regulations and code on power quality keeping in view the international or national standards. The limits for other power quality parameters may also be included in power quality regulations by the SERCs based on their experience and specific system requirements.
  • There is need of continuous monitoring and reporting of power quality parameters at identified locations by the distribution licensees.
  • Power quality measurements may be integrated with the smart grid applications for a reliable smart grid.
  • SERCs may prescribe power quality reporting format and fix the responsibility to maintain the power quality database by the distribution licensees or bulk consumers, as the case may be, for a sufficiently long period.
  • Regulatory framework may specify the training requirements for effective implementations of the power quality standards.
  • Regulatory framework should introduce the compliance audit of power quality parameters by Independent agencies.
  • Power quality may also be integrated with the smart grid application for a more reliable smart grid and promote adoption of technologies such as advanced power quality meters, wide-area power quality measurement, power quality enhancement devices for system component and sensitive loads that can provide fast diagnosis and correction of power quality disturbances.
  • State regulators have specified reliability indices such as SAIFI, SAIDI, CAIDI and MAIFI etc. in Grid/Supply Code or in Standards of Performance Regulations for reporting. However, there is a need that these reliability indices be also strictly monitored and implemented.

Conclusion

The existing regulations cover the power factor, frequency, reliability of supply and voltage regulations as power quality parameters. While there is a strong system of frequency regulation, enforcement of the standards specified for reliability parameters are required to be strictly monitored and implemented. Issues of voltage regulations, transients, and harmonics are not given the attention they deserve.

With increasing penetration of renewable energy, electronic equipment, non-linear loads, data centres and industries running on adjustable speed drives etc., there is a need of emphasising separate regulations covering exhaustively all parameters of power quality with a clear incentive/disincentive mechanism to ensure compliance of specified parameters.

The Act and the Tariff Policy emphasises the need for supply of reliable and quality power of specified standards at reasonable rates. It is desired that the harmonious and uniform standards should be specified by the State Regulatory Commissions to serve the best interest of the utilities and consumers connected to the national grid.