Reactive Power Management

The increased injection of renewables causes voltage fluctuations in the gr­id, causing harmonics, flicker ph­e­nomena, unbalanced loads and power oscillation, thus impacting pow­er quality and power transfer capability. There­fore, reactive power management is crucial to ensure grid stabilisation, enable voltage management and maintain po­wer quality. Reactive power ma­na­geme­nt reduces voltage fluctuations and im­proves the stability of AC systems. It im­pro­ves power system stability, thus preventing load shedding and system collapse, and improving system security and reliability.

Various reactive power ma­na­gement solutions are now available, based on specific requirements. These include shunt capacitor banks and several kinds of flexible alternating current transmission system (FACTS) de­vices such as thyristor-controlled se­ri­es capacitors (TCSCs), shunt-connected static synchronous compensators (STATCOMs) unified power flow controllers (UPFCs), dynamic voltage restorers (DVRs) and VAR compensators (SVCs).

Reactive power management solutions

• Shunt capacitor banks: Shunt capacitors are deployed at the substation le­vel to reduce power losses and increa­se the utilisation of equipment. Com­pen­sating for a load’s lagging power fa­ctor with a bus-connected shunt ca­pacitor bank improves the power factor and reduces current flow through transmission lines, transformers, generators, etc. This, in turn, reduces po­w­er losses in the equipment, cables and transmission lines. Shunt compe­nsation using capacitor banks reduces the kVA load on lines, transformers and generators, which means that with compensation they can be used to deliver more power without overloading the equipment. Shunt compensation can be installed near the load, in a distribution substation and along the distribution feeder. It impro­v­es the power factor of the source current, reduces power losses, improves vo­l­­ta­ge regulation, increases utilisati­on of equipment and reduces capital investment per megawatt of the load.
• FACTS devices: FACTS devices are used to control power flow, decrease losses, and enhance voltage and transmission line loadability. FACTS devic­es increase the power transfer capability of transmission systems; hence, it sa­v­es costs when developing an enti­re­ly new transmission line. It directly controls active and reactive power flow, provides fast reactive po­wer support and voltage control, and dampens power oscillations in the system. There are many ways to connect tra­nsmission lines to FACTS de­vices. These include different combinations of series, shunt, or combinations of both. For series combinations, TCSC is used; for shunt connections, STATCOM and SVC are used; and for series and shunt combinati­ons, UPFC is used.
• SVCs: An SVC is a shunt-connected static VAR generator or absorber, who­se output is adjusted to exchange capacitive or inductive current so as to maintain the specific parameters of an electrical power system (typically the bus voltage). An SVC consists of two main components and their combinations: thyristor-controlled reactors (TCRs) and thyristor-switched reactors (TSRs); and thyristor-switched ca­pacitors (TSCs). SVCs adjust their reactive power output to maintain the desired voltage. Hence, with different combinations of TCRs/TSRs, TSCs and fixed capacitors, an SVC can meet various requirements to absorb/supply reactive power from/ to the transmission line. Due to this, they offer several benefits such as fast, accurate regularisation of voltage and transient-free capacitor bank switching. SVCs are al­so used to dampen power swings and redu­ce system losses through op­timi­s­ed reactive power control. The Power Grid Corporation of India (Power­grid) has successfully commission­ed SVRs at three locations.
• STATCOMs: A STATCOM comprises power convertors, a set of coupling re­­actors or a step-up transformer, and a controller. STATCOMs can ab­sorb or su­pply reactive power in single- or thr­ee-phase AC systems, and help prevent sudden fluctuations in the trans­mi­s­sion system. Unlike SVCs, STATCOMs con­trol the output current in­depen-dently of the AC system voltage, while the DC-side voltage is automatically ma­i­ntained to serve as a voltage sour­ce. Further, STATCOMs do not require large inductive and capacitive components to provide cap­aciti­ve reactive power to high voltage tra­n­s­mission systems, resulting in smaller land requirements for the device, and a reduction in equipment volume as well as footprint. STATCOMs help utilities increase po­wer quality by providing reactive power control, power os­cillation da­m­ping and increased pow­er tra­nsfer ca­pacity. They enable the integration of re­ne­wables into the grid in compliance with the grid code re­qui­re­ments, by providing fault ride-through voltage control and support. In June 2018, Po­wergrid in­s­talled one of the world’s lar­gest STATCOM systems at Rourkela in Odisha, in association with Siemens Limited. The 400 kV STATCOM has a swing range of 600 MVAr, and 250 MVAr of me­chan­i­cally switched com­po­nents. With the fast response time of STATCOM controls, power consumers will receive benefits su­ch as constant vol­tage and frequency leading to availability of uninterrupted quality power.
• DVRs: A DVR is a device, connected in series with the power system, that is used to keep the load voltage constant in­dependent of source voltage fluctuations. When voltage increases at the lo­ad terminals, the DVR re­s­ponds by in­jecting three AC voltages in series with the incoming three-phase network voltages, compensating for the difference. Furth­er, a DVR can be integrated with static synchronous series compensators to create a system ca­pable of controlling the power flow of a tra­nsmission line during steady state conditions, and providing dynamic voltage compensation and short circuit current limitation during system disturbances.
• UPFCs: Among the available FACTS devices, UPFCs are the most versatile. They can be used to enhance steady sta­te stability, dynamic stability and transient stability. A UPFC is capable of both supplying and absorbing real and reactive power. It consists of two AC/DC converters. One of these is connected in series with the transmission line through a series transformer, and the other in parallel through a sh­unt transformer. The DC side of the two converters is connected through a common capacitor, whi­ch provides DC voltage for converter operations. Ma­intaining power ba­lance bet­we­en the series and shunt converters is a pr­erequisite to maintain a constant voltage across the DC capacitor. As the se­ries bra­n­ch of the UPFC injects a voltage of variable ma­g­nitude and phase angle, it can exch­ange real power into the transmission line, thus improving the po­wer flow capability of the line as well as its transient stability limit.

Conclusion

Reactive power management is being ta­k­en up in a big way by transmission utilities in the country to manage the grid and ensure uninterrupted power supply. Powergrid operates 14 STATCOMs, three SVCs and four series reactors, along with a number of TCSCs/fixed series compensators. It has four STATCOMs each in the western, eastern and southern regio­ns of the country, and two in the northern re­gion. Moving ahead, Powergrid is im­ple­menting one 500 MVAr TCSC in Ku­rukshetra to improve the static as well as dynamic voltage profile of the Kuru­k­shetra high voltage direct current station.

At the state utility level, Rajasthan Rajya Vidyut Prasaran Nigam Limited is planning to add reactive power compensation systems to its network, in order to sta­bilise the grid in the wake of inc­rea­sed renewable energy integration. The utility is planning to install bus reactors at several grid substations at Anta, Hee­ra­pura, Ajmer, Bhilwara and Babai, with financing from REC Limited. It is also planning to install TCSCs (dynamic co­mpen­sa­tion) at substations in Jaisal­mer, Bhadla and Bikaner, with proposed fun­d­ing support from the Asian Develop­ment Bank. Meanwhile, one of the key ongoing projects of the West Bengal State Electricity Transmission Company is reactive power management of the st­a­te grid. It plans to undertake the ins­talla­tion of 33 kV capacitor banks at 36 extra high voltage substations to impro­ve the power factor and the system voltage profile, and re­duce transmission losses. Fur­th­er, to control and overcome high voltage problems in the eastern re­gional grid, the installation of a 125 MVAr switchable bus reactor has been planned at the Arambag 400 kV substation.

With the increasing power demand, the capacity and quality of power need to be maintained. Poor power quality causes issues such as voltage sag, swelling, in­ter­ruptions, harmonics and transients. Thus, it is necessary to fix these issues for the system to operate without loss, and efficiently. Reactive power management has proved to be a viable solution to the­se kinds of problems. Old transmission networks should also be updated with new devices. That said, the transmission network should be strengthened by ins­talling the most suitable FACTS devices so that the overall performance of the sy­stem during various kinds of operations can be improved.