The increase in transmission over long distances, generation of grid-connected variable renewable energy, as well as the synchronisation of an all-India network pose additional voltage and stability problems. Hence, the need to transmit power efficiently has become a key concern for transmission system operators. To achieve this, it is crucial to stabilise fluctuations in the voltage levels to ensure reliable operations. While the purpose of the transmission system is to supply real (or active) power through the network, the transfer of energy in the transmission network is made possible only through reactive (or imaginary) power. Reactive power occurs in alternating current (AC) circuits and is needed for the acceptable functioning of various electrical systems, such as transmission lines, motors, transformers, etc. It essentially facilitates the flow of energy in an electric system, and is generated or absorbed by many equipment connected to the power system network. Reactive power plays a key role in the security of power systems as it affects and regulates the voltage throughout the system.
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 the system’s ability to withstand and prevent voltage collapse. 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 to load through the transmission lines. To avoid such a scenario, the amount of power generated and consumed should be as close as possible, otherwise it results in an unbalanced voltage profile. To overcome these limitations, a number of static and dynamic devices and management options are deployed for the optimisation of reactive power and maintenance of grid stability. In view of the growing influx of renewables and increasing power fluctuations, these solutions allow transmission operators to maintain power quality and power transfer capability through active network management.
Technologies and applications
Various new and mature technology solutions are now available that can address these operations-related challenges prevalent in the electricity system. Reactive power compensation can be provided through Flexible AC Transmission System (FACTS) devices, which provide a way of balancing the active and reactive power in AC networks. FACTS devices allow for the flexible and dynamic control of power systems, and help balance active and reactive power to maintain system parameters and enhance the capacity of the transmission system.
FACTS devices enable utilities to reduce transmission congestion without compromising the system’s reliability and security. These static devices 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. In addition, these devices reduce financial costs and environmental impact by the possible deferral of transmission lines.
The application of this technology can be divided into two categories; steady state and dynamic applications. Steady state applications include voltage control, increased thermal loading, post-contingency voltage control, loop flow control, reduction in short circuit levels, and power flow control. Dynamic applications of FACTS controllers include transient stability improvement, oscillation damping (dynamic stability), dynamic control of voltage during system contingency and reducing the impact of primary disturbance, voltage stability enhancement, and subsynchronous resonance (SSR) elimination.
FACTS devices
Based on how the controllers are connected in the system, FACTS devices can be subdivided into four basic groups: series controllers, shunt controllers, series-shunt controllers and series-series controllers. Series FACTS devices increase stability and shunt FACTS devices provide reactive power compensation.
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 levels by compensating the reactive power to that transmission line; it automatically adjusts the reactive power compared to the reference voltage level.
Series compensation technology has been a preferred solution for improving grid stability as well as for the optimal utilisation of high voltage transmission lines. In this method, a capacitor is inserted in series with the existing transmission line (typically more than 200 km) for improving the 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 line.
The key components of a series capacitor include a capacitor bank, metal oxide varistors (for insulation), a damping circuit and spark gap. Moreover, since these capacitors are installed in series on a transmission line, the equipment must be housed on a platform that is fully insulated for the system voltage. However, series compensation can lead to malfunction of the remote distance relays of the line protection, if the degree of series compensation capacitor location is not proper.
SVCs and STATCOMs
FACTS devices can be further classified into two generations. First-generation devices, which have been in commercial use since the 1970s, employ conventional thyristor-switched capacitors and reactors with only current turn-on features. These devices employ capacitor and reactor banks with fast solid-state switches in traditional shunt or series circuit arrangements. Key first-generation controllers are the static var compensator (SVC), thyristor controlled series capacitor, and thyristor controlled phase shifting transformer.
SVCs are one of the most important FACTS devices that have been used for a number of years to improve transmission line economics by resolving dynamic voltage problems. SVCs combine conventional capacitors and inductors with fast switching capability. When there is excess reactive load, reactors (usually in the form of thyristor-controlled reactors) are used by the SVC to lower the system voltage and under-lagging conditions, and the capacitor banks are automatically switched on, thus providing a higher system voltage. Hence, SVCs essentially adjust their reactive power output to maintain the desired voltage. 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.
The second-generation devices, which have been in use since the mid-1980s, employ volatage-source conductors (VSCs), with features such as gate turn-offs (GTOs) and insulated-gate bipolar transistors (IGBTs) which have both current turn-on and turn-off capabilities. VSC-based FACTS controllers employ self-commutated DC to AC converters (using GTOs and IGBTs), which can internally generate capacitive and inductive reactive power for transmission line compensation, without the use of capacitor or reactor banks. In second- generation devices, the capacitors and reactors are replaced with intelligent switching semiconductors.
Important VSC-based FACTS controllers are static synchronous compensators (STATCOM). 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 to prevent sudden fluctuations in the transmission systems, such as voltage sag, voltage dip, transients, etc. Unlike SVCs, 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. 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. The response time of a STATCOM device is typically around 3 ms to 5 ms, which makes it faster and more flexible in adapting to the system needs.
Conclusion
In India, SVCs have so far been commissioned at three locations in the northern region and STATCOMs have been commissioned at one location in the southern region by the country’s premier transmission utility, Power Grid Corporation of India Limited (Powergrid). Overall, STATCOMs at 13 locations across the northern, eastern, western and southern region are planned. As per the Central Electricity Authority, the total investment in compensation devices in the Thirteenth Plan period is estimated at over Rs 28 billion.
Net reactive power compensation is critical for strengthening transmission networks and is the most economical means to increase their power transfer capability within the power quality constraints. As the country move towards the integration of vast amounts of renewable energy over long distances, reactive power compensation devices such as FACTs will continue to play a key role in stabilising voltage, increasing power flow capacity, preventing blackouts and reducing losses.