Thyristor-switched capacitor
It consists of a power capacitor connected in series with a bidirectional thyristor valve and, usually, a current limiting reactor (inductor).
Static VAR compensators are a member of the Flexible AC transmission system (FACTS) family.
An attempt to operate a TSC in ‘’phase control’’ would result in the generation of very large amplitude resonant currents, leading to overheating of the capacitor bank and thyristor valve, and harmonic distortion in the AC system to which the SVC is connected.
[4] It is an economic choice between the size of the TSC reactor (which increases with decreasing frequency) and the need to protect the thyristor valve from excessive oscillatory currents when the TSC is turned on at an incorrect point of wave (‘’misfiring’’).
When the TSC is switched off, or ‘’blocked’’, no current flows and the voltage is supported by the thyristor valve.
After the TSC has been switched off for a long time (hours) the capacitor will be fully discharged, and the thyristor valve will experience only the AC voltage of the SVC busbar.
When the TSC is turned on ("deblocked") again, care must be taken to choose the correct instant in order to avoid creating very large oscillatory currents.
Since the TSC is a resonant circuit, any sudden shock excitation will produce a high-frequency ringing effect which could damage the thyristor valve.
The optimum time to turn on a TSC is when the capacitor is still charged to its normal peak value and the turn-on command is sent at the minimum of valve voltage.
If the TSC is deblocked at this point, the transition back into the conducting state will be smooth.
This transient current can take hundreds of milliseconds to die away, during which time the cumulative heating in the thyristors may be excessive.
The thyristor valve is usually installed in a purpose-built, ventilated building, or a modified shipping container.
Some TSCs have been built with the capacitor and inductor arranged not as a simple tuned LC circuit but rather as a damped filter.
In several “Relocatable SVCs” built for National Grid (Great Britain),[3] three TSCs of unequal size were provided, in each case with the capacitor and inductor arranged as a “C-type” damped filter.
