While there are several factors that degrade the quality of power, two of the most critical are reactive power and harmonic distortions. When there is not enough reactive power, the voltage sags and it is not possible to push the power demanded by loads through the lines. Several recent power outages worldwide may have been results of inadequate reactive power supply, which subsequently led to voltage collapse. Meanwhile, the issue of harmonic distortions is one of the most common power quality problems that power distribution systems need to tackle. The problem of harmonics arises due to the use of non-linear load or solid-state component. Traditional distribution equipment such as rotating machines and overloaded transformers produce harmonics. Today, most of the loads produce harmonics at varying magnitude, particularly, in view of the increasing application of electronics. These create additional stress on the networks, potentially leading to equipment malfunction and making installations run less efficiently.
Power Line looks at the causes of distortions, how these impact power systems and the solutions for mitigating them…
Causes and impact of harmonics and reactive power
Power is transported through and consumed in alternating current (AC) networks, whose elements produce or absorb real and reactive power. Whereas real power accomplishes useful work like running motors or lighting lamps, reactive power supports voltages in the system, controlling system reliability, voltage stability and operational acceptability. Both real and reactive powers are essential for the supply of power. Reactive power flows describe the unique local voltage profile of the background energy movement in AC systems. This movement is caused by the production of electric and magnetic fields, which store and change energy through each AC cycle. These variations must be controlled for the power system to operate within acceptable voltage limits.
Reactive power management is crucial for maintaining the quality of power supplied because reactive power maintains the voltage required in transmission lines for the delivery of active power. Thus, three objectives dominate reactive power management: first, adequate voltage must be maintained throughout the transmission system under normal and contingent conditions; second, the congestion of real power flows must be minimised; and third, real power losses must be minimised.
Meanwhile, a harmonic distortion is the deviation of current and voltage wave forms in distribution systems due to non-sinusoidal currents consumed by non-linear loads. Over time, there has been an increase in the use of electronic equipment that produce non-linear loads such as single-phase loads, switched-mode power supplies, fluorescent lighting ballasts, adjustable speed drives, data processing equipment and small uninterruptable power supply units. The existence of harmonic currents leads to the overloading of neutrals, overheating of transformers, tripping of circuit breakers and fuses, excessive stress on power factor correction capacitors and measurement errors in the metering system. Harmonic voltages lead to voltage distortions and zero-crossing noise, while harmonic deviations lead to disruptions in power quality.
The greatest detrimental effect of harmonic distortions is the overheating of the transformer core by increasing copper, stray flux and iron losses.
Many types of non-linear loads can cause harmonic currents. However, their presence does not necessitate the existence of harmful harmonic phenomena. In order to detect whether a system is suffering from harmonics problems, voltage and current must be measured using root-means-square digital meters. Spectrum analysers, oscilloscopes, power distribution monitors and harmonic analysers are able to provide graphic representations of the waveform and thus detect harmonic disruptions. Harmonic problems can be divided into two categories: current waveform distortion and voltage waveform distortion. Voltage distortion affects the delivery of electricity, and is, therefore, of greater concern to utilities. Both voltage distortion and waveform distortion are best measured at the power source, for example, the isolation transformer or main service entrance.
Reactive power management for reactive power flows, over and above those that occur normally, is provided by an appropriate combination of static and dynamic devices. Static devices, such as capacitors and reactors, respond to voltage changes slowly and in discrete steps. Though the devices themselves are inexpensive, the associated switches, controls, communication and maintenance can amount to over 30 per cent of the operations and maintenance budget of a distribution system. From the system point of view, static devices are typically used to provide normal or intact system voltage support, and adapt to slowly changing conditions such as daily load cycles and scheduled transactions.
There are also a number of flexible AC transmission system devices that provide dynamic technology solutions to address reactive power compensation. For example, reactive power compensators like synchronous condensers, synchronous generators and solid-state devices such as flexible alternating current transmission systems, static volt amp reactive compensators, static synchronous compensators, dynamic volt amp reactive system, and super volt amp reactive system are dynamic and can respond within cycles to the changing reactive power requirement. Dynamic reactive power sources are deployed to allow the system to respond to rapidly changing conditions such as the sudden loss of generators or transmission facilities. The right combination of both static and dynamic resources is used to ensure reliable operation of the systems at reasonable costs.
Harmonics present in the system can be mitigated by harmonic filters, which are of three varieties: passive, active and hybrid. Passive harmonic filters require constant loading conditions and their performance depends on the source of impedance, which is hard to determine and varies with system changes. They are, however, able to improve the power factor and reduce high-frequency harmonics of large size. Passive filters must be used in conjunction with tuning reactors to prevent instability due to parallel resonance with source inductance. Active harmonic filters are able to provide harmonic mitigation under any load condition. These filters also allow the control of output current and provide stable operations against AC source impedance variations. Active filters can respond quickly, irrespective of the order and magnitude of the harmonics. However, in comparison to passive filters, active filters tend to be more expensive as their initial and running costs are usually higher. Hybrid filters use the properties of both active and passive filters to mitigate harmonic distortions in the system.
Besides filters, there are a few other harmonic mitigation techniques for low order and uncharacteristic harmonics. Phase multiplication, for example, is effective in reducing low order harmonics so long as there is a balanced load. Harmonic injections can remedy uncharacteristic harmonics, but because system impedance is not part of the design criteria, these may give rise to low-order harmonics. Harmonic mitigation techniques with pulse width modification are capable of obtaining harmonic reductions in very minor frequency deviations. These can also be programmed to eliminate specific harmonics.
The way forward
The Central Electricity Authority and the Central Electricity Regulatory Commission at the central level and the state electricity regulatory commissions at the state levels have regulations for the quality of electricity supply that apply to generation, transmission and distribution companies. Consumers also face specific standards that must be met to maintain the grid. While there is a strong system of frequency regulation, the lack of enforcement of standards specified for reliability parameters and inconsistencies in regulatory approaches for dealing with issues like reactive power management or harmonics across the various states have been a key issue.
There are international standards for power quality such as the IEEE 519-1992 standard for harmonic control in electrical power systems, which emphasises voltage and current harmonics for consumers and utilities. The three standards specified for various users are: harmonics voltage limits for individual consumers, harmonics current distortion limits for distribution systems and voltage distortion limits for utilities. Following such a structured approach for maintaining the quality of power would help reduce the direct costs of downtime in India, which are to the tune of Rs 200 billion per annum. About 57 per cent of these financial losses are due to voltage sags and short interruptions, while about 35 per cent of the losses are due to transients and surges.