Power networks generate two kinds of power – real and reactive. While real power accomplishes the useful work of running appliances, reactive power supports the voltages that must be controlled for system reliability. Several recent power outages worldwide may have been a result of inadequate reactive power supply, which subsequently led to a voltage collapse. By supporting voltage control, reactive power prevents damage to an electric system (such as the overheating of generators and motors), reduces transmission losses and maintains system stability. It also improves the efficiency with which real power is delivered to customers.
If there is an excess of reactive power, it greatly reduces the system power factor and hence lowers the operational efficiency. Alternatively, when there is not enough reactive power, the voltage sags and it is not possible to deliver the required power through the transmission lines. Reactive power is difficult to transmit since the reactance of transmission lines is often four to ten times higher than the resistance of the lines. Hence, the amount of power generated and consumed should be as close as possible; otherwise it results in an unbalanced voltage profile.
Further, with the increase in renewable energy penetration, including solar and wind, the need for reactive power support is much more than the earlier scenario when most of the energy was supplied by conventional synchronous generators, which also readily supplied reactive power. Variations in renewable energy generation cause variations in reactive power exchange of the wind and solar generators with the grid, thereby disturbing the voltage balance in the grid. A number of static and dynamic devices and management options are deployed for the optimisation of reactive power and maintenance of grid stability.
Traditionally, rotating synchronous condensers and fixed or mechanically switched capacitors or inductors have been used for reactive power compensation. However, in recent years, flexible AC transmission systems (FACTS) have been developed based on the use of reliable high-speed power electronics, powerful analytical tools, and advanced control and microcomputer technologies, which represent a new concept for the operation of power transmission systems. They increase the power transfer capability of existing transmission systems, directly control active and reactive power flow, provide fast reactive power support and voltage control, and dampen power oscillations in the system. Based on how the controllers are connected in the system, FACTS devices are categorised as series controllers and shunt controllers. The key FACTS devices are static VAR compensators (SVCs) and static synchronous compensators (STATCOMs). Other high performance power system controllers include dynamic voltage restorers (DVRs) and unified power flow controllers (UPFCs). A look at the key reactive power equipment…
Series and shunt controllers
Series FACTS devices increase stability in the power system while shunt FACTS devices provide reactive power compensation. Series capacitors are inserted in series with the existing transmission line (typically more than 200 km) for improving system impedance. Connecting a capacitor in series reduces both the angular deviation and the voltage drop, thereby resulting in increased loadability and stability of the transmission system. Further, series compensation ensures proper load division amongst parallel feeders, which is difficult to achieve when a new transmission line is made parallel to a pre-existing one. About 40 fixed series compensators are under operation at 400 kV lines.
Shunt compensators are connected in parallel with the transmission line, and are located at the receiving end. Shunt-connected reactors are used to reduce the excess voltage in the line by consuming the reactive power, while shunt-connected capacitors are used to maintain the voltage level by compensating the reactive power to that transmission line. It automatically adjusts the reactive power compared to the reference voltage level.
A static VAR compensator is a shunt-connected static VAR generator or absorber whose output is adjusted to exchange capacitive or inductive current so as to maintain specific parameters of the electrical power system (typically the bus voltage). An SVC has two main components and their combinations: thyristor-controlled reactors (TCRs) and thyristor-switched reactors (TSRs); and thyristor-switched capacitors (TSCs). TCRs and TSRs are both composed of a shunt-connected reactor controlled by two parallel reverse-connected thyristors. While TCRs are controlled with a proper firing angle input to operate in a continuous manner, TSRs provide change in reactance in steps. TSCs share a similar composition and the same operational mode as TSRs, but the reactor is replaced by a capacitor. SVCs essentially 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 their features, they have 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.
STATCOMs comprise 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 independently 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. With respect to topology, STATCOMs are more complicated, with the switching device usually being a gate turn-off device paralleled by a reverse diode. The gate turn-off ability shortens the dynamic response time from several utility period cycles to a portion of a period cycle. STATCOMs are also much faster in improving the transient response time (typically around 3 ms to 5 ms), offering higher reliability and a larger operating range. Given its features, this device helps utilities to increase power quality by providing reactive power control, power oscillation damping and increased power transfer capacity. Further, it enables renewables to be connected to the grid in compliance with the grid code requirements, by providing fault ride-through voltage control and support.
DVRs and UPFCs
A DVR is a device connected in series with the power system and 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 in voltages. The key components of a DVR are switchgear, booster transformer, harmonic filter, integrated gate commutated thyristor-based voltage source converter, DC charging unit, control and protection system, and energy source (that is a storage capacitor bank). Further, the DVR can be integrated with static synchronous series compensators to get 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 consist of two switching converters operated from a common DC link provided by a storage capacitor. One of these is connected in series with the line and the other in parallel. This arrangement functions as an ideal power converter in which the real power can freely flow in either direction between the terminals of the two inverters and each inverter can independently generate (or absorb) reactive power at its own output terminal. The inverter connected in parallel supplies or absorbs the real power demanded by the inverter connected in series to the AC system and the corresponding reactive power exchanged is supplied or absorbed by the inverter connected in series. There is no continuous reactive power flow through a UPFC.
In India, SVCs and STATCOMs are being implemented by Power Grid Corporation of India Limited for dynamic control of reactive power in order to maintain the voltage and improve the stability of the grid. While two SVCs in Kanpur, Uttar Pradesh, have been in operation at the 400 kV grid since 1992, more of these have been recently commissioned, including those at New Wanpoh (Jammu & Kashmir), Kankroli (Rajasthan) and Ludhiana (Punjab). The recently commissioned STATCOMs include one in the southern region (NP Kunta), two in the western region (Satna and Aurangabad) and three in the eastern region (Rourkela, Jeypore and Ranchi). Eight STATCOMs, two with 300 MVAR capacity at Solapur and Lucknow, and six with 200 MVAR capacity each at Hyderabad, Trichy, Udumalpet, Gwalior, Kishanganj and Nallagarh, are at various stages of implementation. In 2019-20, Powergrid plans to complete the installation of STATCOMs in the southern region. In addition, six installations of fixed series compensators and thyristor-controlled series compensators are operational; two each on the 400 kV Muzaffarpur-Purnea I and II lines, the 400 kV Gorakhpur-Muzaffarpur I and II lines, and the 400 kV Raipur–Raigarh I and II lines.
The installation of this advanced reactive power equipment gives better grid control to utilities, reducing investment requirements in transmission lines. Going ahead, reactive compensators will be used on a much wider scale as grid performance and reliability become an even more important factor and the country moves towards the integration of vast amounts of renewable energy over long distances.