The development of high-power thyristors and Insulated Gate Bipolar Transistors (IGBT) enabled the cost-effective provision of FACTS devices. The actual behaviour of these devices is beyond the scope of this article, but on a basic conceptual level, they are simply fast acting switches, controlled by some external means (a trigger). Triggers can be either electric (a voltage applied at the gate terminal) or photonic (light), the latter of which is useful to isolate the control system electrically.
Thyristors can be conceptualized (and indeed, are drawn on schematic diagrams) as diodes with a switch (the gate voltage or photoelectric stimulus). In FACTS correction systems, whereby a thyristor should act as essentially a fast-acting switch, power needs to be transferred in both directions. Thyristors are also used in high-power rectifier circuits as well, particularly for High-Voltage Direct Current (HVDC) transmission.
How much faster is thyristor-based switching compared to mechanically-switched circuit breakers? Because thyristors are semiconductor devices, they can switch on the order of milliseconds. Conventional circuit breakers, on the other hand, take much longer to switch. They can switch in one or two cycles (of the 50-60Hz mains frequency), though for power system protection purposes, this is considered a rather slow switching speed.
The graph below illustrates the difference:
Additionally, mechanically-switched capacitors do not have sufficient switching speed to support extremely rapid switching nor can they be realistically switched more than a few times per day.
We can see that advanced communications and control systems play an important role in flexible transmission and distribution systems.
This article was taken from the introduction of a report which was written by a partner and I, submitted to ECE3333: Power Systems I, taught by Professor Rajiv Varma at the University of Western Ontario.