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.
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.