The Electricity Act, 2003 enshrines the basic need to provide consumers with a continuous, reliable and quality power supply. Power quality is the ability of electrical equipment to consume the energy being supplied in an efficient and effective manner. A number of power quality issues, including electrical harmonics, poor power factor, voltage instability and imbalance, impact the efficiency of an electrical equipment. This leads to greater energy usage, and higher maintenance and repair, which increase the supply cost, besides equipment instability and failure, ultimately resulting in business disruption. For good power quality, it is therefore important to ensure supply network availability and reliability, energy efficiency, industrial productivity, cost efficiency and safety.
The problem of poor power quality exists in every part of the power supply chain and, therefore, solutions are also deployed throughout the chain. There are also a large number of technological solutions, that can be deployed in different segments of transmission, distribution and end-use equipment to mitigate poor power quality issues. A look at some of the key technological solutions that can be deployed to enhance power supply quality to prevent disruptions occurring from transients, harmonics, reactive power, network unbalance, voltage variations and flicker…
Harmonic filters
Harmonic filters help eliminate unwanted harmonic waveform distortions in electrical systems created by non-linear loads. They minimise thermal and electrical stress on electrical infrastructure by eliminating the risk of harmonics-related reliability issues and allow for long-term energy efficiency and cost savings. Harmonic filters are classified as: active, passive and hybrid. Active harmonic filters monitor non-linear load and dynamically provide controlled current injection, which cancels out harmonic currents in the electrical system. Passive harmonic filters provide low impedance path to harmonic frequencies to be attenuated using passive components. A hybrid harmonic filter is a combination of both passive and active harmonic filters. It improves phase current and voltage balance and reduces losses caused by harmonics on lines and transformers.
Shunt capacitors
Shunt capacitors, also called power factor correction (PFC) capacitors aim to improve power factor and hence, power quality, switching capacitors to offset usually inductive loads such as electric motors. A PFC system increases power supply efficiency and provides immediate cost savings on electricity. These capacitors are used either at the customer location or in the distribution system for voltage control. Utilities use shunt capacitors at distribution and utilisation levels to provide reactive power near inductive loads that require it. At the transmission and sub-transmission levels (66 kV and above), shunt capacitors increase the power transfer capability of a transmission system without requiring new lines or larger conductors.
Isolation transformer
An isolation transformer is a special type of transformer, wherein primary and secondary windings are physically separated through double insulation. Its leakage inductance electrically isolates people and equipment from the hazards of power quality problems. Isolation transformers can prevent the transfer of direct current (DC) signals from one circuit to the other, as well as block interference due to ground loops. However, they permit alternating current (AC) power or signals to pass. Capacitor switching and lightning transients can be attenuated by it, thus preventing tripping of adjustable-speed drives and other equipment. It reduces high frequency noise currents and provides complete isolation from the input AC line. Combined with surge protection devices, it offers continuous filtering of power line noise.
Constant voltage transformers
Constant voltage transformers (CVTs) provide a barrier to spikes and electrical noise disturbances and also prevent disturbances in the main load at the grid. They are capable of correcting main voltage sags and surges by keeping the iron core of the transformer’s secondary section saturated, thereby generating a constant voltage output. These transformers have a unique capability to store energy for about half of the cycle due to their specific design, which, when combined with an inverter and a static transfer switch, can provide uninterrupted power transfer to an alternative source. In case of a fault or overload, this feature of a CVT enables it to maintain power supply to the grid, thus preventing total loss.
Variable speed drives
Variable speed drives (VSDs) are used to control the rotation speed of electronic motors in many industrial applications. A VSD supply transformer acts as a galvanic isolation between the frequency converter and the feeding network. It can reduce voltage from the feeding medium voltage or high voltage network to the rectifier input level. It suppresses harmonics generated by the frequency converter, thus protecting the feeding network from harmonic contamination. Protection of ambient and feeding network against radio frequency interference produced by the rapidly commutating semiconductors and providing a suitable reactance to ensure the correct commutation of semiconductors are some of the other tasks performed by a VSD transformer.
Surge arresters
Surge arresters provide primary protection against atmospheric and switching voltages. Arresters for special applications such as railways and DC voltage limiting devices are also available. Their design complies with IEC 60099-4 and ANSI/IEEC62.11 standards and finds application in the protection of AIS and GIS, of HVDC, series capacitor banks and transmission lines.
Static VAR compensator
A static VAR compensator (SVC) provides fast reactive power compensation in a power system using a combination of capacitors and reactors to regulate the voltage. These are primarily used to mitigate voltage fluctuations, as well as the resulting flicker. SVCs can also mitigate active power oscillations through voltage amplitude modulation and has the added benefit of bringing the system power factor close to unity. An SVC is usually installed near high and rapidly varying loads, such as electric arc furnaces, welding plants and other industries prone to voltage fluctuations and flicker. SVCs are basically of two types: thyristor-controlled reactors with fixed capacitors and thyristor switched capacitors.
Static synchronous compensator
A static synchronous compensator (STATCOM) is a shunt device, which uses force commutated power electronics to control power flow and improve transient stability on electrical power networks. It is also a member of the so-called flexible AC transmission system (FACTS) devices. The STATCOM basically performs the same function as Static VAR compensators but with some advantages. STATCOMs are typically applied in long distance transmission systems, power substations and heavy industries where voltage stability is the primary concern. In addition, STATCOMs are installed for voltage support and control, voltage fluctuation and flicker mitigation, unsymmetrical load balancing, power factor correction, active harmonics cancellation and to improve transient stability of the power system.
Thyristor switched reactive power compensators
They help in ultra-rapid transient-free stepped power factor compensation and voltage fluctuation mitigation. It avoids switching transients and can be connected up to 690 V. It is modular and compact, standardised in design, and easy to install and extend with advanced communication features. Typical applications include fast and/or large varying loads including cranes, welders, offshore platforms and railway applications.
Line voltage regulator
A line voltage regulator controls voltage rise or drop by allowing in-feed of additional wind power into existing grids. It is available for single-turbine or wind farm installations and is often a better economic solution than grid extension. It does not influence the protection system and can be realised fast. It makes existing grids flexible for changing in-feed of wind and other decentralised generation.
Dynamic voltage restorer
To mitigate the voltage-based power quality problems in the distribution system, solid state series compensators and dynamic voltage restorer (DVRs) are commonly used. The basic principle of the DVR is to inject a voltage of the required magnitude and frequency so that it can restore the load-side voltage to the desired amplitude and waveform even when the source voltage is unbalanced or distorted. A DVR can generate or absorb independently controllable real and reactive power at the load side.
Transient voltage surge suppressors
Transient voltage surge suppressor (TVSS) devices are used to protect other equipment from the potential dangers of harmful voltage surges caused by lightning and switching of inductive or capacitive devices. They are used as an interface between the power source and sensitive loads, so that the transient voltage is clamped. A TVSS contains a component with a non-linear resistance that limits excessive line voltage and conducts any excess impulse energy to the ground.
Energy storage systems
Energy storage systems (ESSs) improve power quality and protect downstream loads against short-duration disturbances in the grid by offering accurate and rapid response. ESSs allow the grid to draw power from the storage system in case of high demand and store power during low-demand periods, thus bridging the gap by providing power as per the load requirement. These solutions enable quick response to varying grid requirements, thus maintaining grid stability and high quality power supply.
One of the key hurdles in the large-scale adoption of ESS technologies has been the high cost associated with it. However, given the current deployment levels, industry research and development efforts under way globally, most experts estimate that energy storage costs will reduce at a faster pace than expected. The battery ESS stores the excess solar photo voltaic energy generated and delivers it during peak hours to enhance power supply reliability.
Conclusion
Besides these, a number of other devices and technologies can also be deployed by utilities to improve power quality such as power conditioning devices, uninterruptible power supply, lightning surge arresters, low voltage capacitor banks and transient voltage suppressors. In areas that suffer from frequent power outages, distributed generation systems can also be used to improve power reliability. Another way to deal with power quality problems is to have equipment and devices, that are less sensitive to power quality issues and provide stable performance.