In electrical engineering , a static synchronous compensator (STATCOM) is a shunt-connected, reactive compensation device used on transmission networks. It uses power electronics to form a voltage-source converter that can act as either a source or sink of reactive AC power to an electricity network. It is a member of the flexible AC transmission system (FACTS) family of devices. STATCOMS are alternatives to other passive reactive power devices, such as capacitors and inductors (reactors). They have a variable reactive power output, can change their output in terms of milliseconds, and are able to supply and consume both capacitive and inductive vars. While they can be used for voltage support and power factor correction, their speed and capability are better suited for dynamic situations like supporting the grid under fault conditions or contingency events. The use of voltage-source based FACTs device had been desirable for some time, as it helps mitigate the limitations of current-source based devices whose reactive output decreases with system voltage. However, limitations in technology have historically prevented wide adoption of STATCOMs. When gate turn-off thyristors (GTO) became more widely available in the 1990s and had the ability to switch both on and off at higher power levels, the first STATCOMs began to be commercially available. These devices typically used 3-level topologies and pulse-width modulation (PWM) to simulate voltage waveforms. Modern STATCOMs now make use of insulated-gate bipolar transistors (IGBTs), which allow for faster switching at high-power levels. 3-level topologies have begun to give way to Multi-Modular Converter (MMC) Topologies, which allow for more levels in the voltage waveform, reducing harmonics and improving performance.
History When AC won the War of Currents in the late 19th century, and electric grids began expanding and connecting cities and states, the need for reactive compensation became apparent. While AC offered benefits with transformation and reduced current, the alternating nature of voltage and current lead to additional challenges with the natural capacitance and inductance of transmission lines. Heavily loaded lines consumed reactive power due to the line's inductance, and as transmission voltage increased throughout the 20th century, the higher voltage supplied capacitive reactive power. As operating a transmission line only at it surge impedance loading (SIL) was not feasible, other means to manage the reactive power was needed.
Synchronous Machines were commonly used at the time for generators, and could provide some reactive power support, however were limited due to the increase in losses it caused. They also became less effective as higher voltage transmissions lines moved loads further from sources. Fixed, shunt capacitor and reactor banks filled this need by being deployed where needed. In particular, shunt capacitors switched by circuit breakers provided an effective means to managing varying reactive power requirements due to changing loads. However, this was not without limitations. Shunt capacitors and reactors are fixed devices, only able to be switched on and off. This required either a careful study of the exact size needed, or accepting less than ideal effects on the voltage of a transmission line. The need for a more dynamic and flexible solution was realized with the mercury-arc valve in the early 20th century. Similar to a vacuum tube, the mercury-arc valve was a high-powered rectifier, capable of converting high AC voltages to DC. As the technology improved, inverting became possible as well and mercury valves found use in power systems and HVDC ties. When connected to a reactor, different switching pattern could be used to vary the effective inductance connected, allow for more dynamic control. Arc valves continued to dominate power electronics until the rise of solid-state semiconductors in the mid 20th century. As semiconductors replaced vacuum tubes, the thyristor created the first modern FACTs devices in the Static VAR Compensator (SVC). Effectively working as a circuit breaker that could switch on in milliseconds, it allowed for quickly switching capacitor banks. Connected to a reactor and switched sub-cycle allowed the effective inductance to be varied. The thyristor also greatly improved the control system, allowing an SVC to detect and react to faults to better support the system. The thyristor dominated the FACTs and HVDC world until the late 20th century, when the IGBT began to match its power ratings. With the IGBT, the first voltage-sourced converters and STATCOMs began to enter the FACTs world. A prototype 1 MVAr STATCOM was described in a report by Empire State Electric Energy Research Corporation in 1987. The first production 100 MVAr STATCOM made by Westinghouse Electric was installed at the Tennessee Valley Authority Sullivan substation in 1995 but was quickly retired due to obsolescence of its components.
Theory The basis of a STATCOM is a voltage source converter (VSC) connected in series with some type of reactance, either a fixed inductor or a power transformer. This allows a STATCOM to control power flow much like a transmission line, albeit without any active (real) power flow. Given an inductor connected between two AC voltages, the reactive power flow between the two points is given by:
Q = V S ⋅ ( Δ V ) X cos ( δ ) {\displaystyle Q={\frac {V_{S}\cdot (\Delta V)}{X}}\cos(\delta )}
where
Q {\displaystyle Q} : Reactive Power
V S {\displaystyle V_{S}} : Sending-End Voltage
Δ V {\displaystyle \Delta V} : Magnitude difference in V S {\displaystyle V_{S}} and receiving end voltage V R {\displaystyle V_{R}}
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