Static synchronous compensator
When gate turn-off thyristors (GTO) became more widely available in the 1990s[1] and had the ability to switch both on and off at higher power levels, the first STATCOMs began to be commercially available.
Modern STATCOMs now make use of insulated-gate bipolar transistors (IGBTs), which allow for faster switching at high-power levels.
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.
As operating a transmission line only at it surge impedance loading (SIL) was not feasible,[2] 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.
In particular, shunt capacitors switched by circuit breakers provided an effective means to managing varying reactive power requirements due to changing loads.
This required either a careful study of the exact size needed,[4] or accepting less than ideal effects on the voltage of a transmission line.
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.
Arc valves continued to dominate power electronics until the rise of solid-state semiconductors in the mid 20th century.
[6] As semiconductors replaced vacuum tubes, the thyristor created the first modern FACTs devices in the Static VAR Compensator (SVC).
[8] The thyristor dominated the FACTs and HVDC world until the late 20th century, when the IGBT began to match its power ratings.
A prototype 1 MVAr STATCOM was described in a report by Empire State Electric Energy Research Corporation in 1987.
[10] 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.
[11] 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.
a fixed size, reactive power flow is controlled by the difference in magnitude of the two AC voltages.
Since a STATCOM varies its voltage magnitude to control reactive power, the topology of how the VSC is designed and connected defines how effectively and quickly it can operate.
There are numerous different topologies available for VSCs and power electronic based converters, the most common ones are covered below.
Some harmonic reduction can be achieved by analytical techniques on different switching patterns; however, this is limited to controller complexity.
[17] Each level of the two-level converter also generally comprises multiple series IGBTs, to create the needed final voltage, so coordination and timing between individual devices is challenging.
Adding additional levels to a converter topology has the benefit of more closely mirroring a true voltage sine wave, which reduces harmonic generation and improves performance.
[19] A transformer with two secondaries, one Wye-Wye and the other Wye-Delta, can be connected to two separate three-phase, three-level converters to double the number of levels.
With enough levels, PWM is not necessary as the waveform created is close enough to a true voltage sine wave and generates very little harmonics.
If a DC bus is not needed, and there are benefits to connecting the three phases into a delta arrangement to eliminate zero sequence harmonics, four IGBTs can be used to surround the capacitor to bypass or switch it in at either polarity.
[24] This offers an advantage over SVCs, as a STATCOM's effectiveness is not dependent on the voltage drop caused by the fault.
A STATCOM may also have a transient rating, where it can provide above its maximum current for very short time, allowing it to help the system better for larger faults.
A simplified PID regulator is shown, however a separate closed loop is sometimes used to determine the reference voltage with respect to the slope and any other modes a STATCOM may have.
[28] A STATCOM may also have additional modes besides voltage regulation or VAR control, depending on specific needs of the system.
