Thyristor-switched capacitor

When the TSC is on, or ‘’deblocked’’, the current leads the voltage 90° (as with any capacitor). The rms current is given by:

${\displaystyle I_{tsc}={V_{svc} \over {X_{tsc}}}}$

Where:

${\displaystyle X_{tsc}={{1 \over {2\pi fC_{tsc}}}-2\pi fL_{tsc}}}$

V_svc is the rms value of the line-to-line busbar voltage to which the SVC is connected

C_tsc is the total TSC capacitance per phase

L_tsc is the total TSC inductance per phase

f is the frequency of the AC system

The TSC forms an inductor-capacitor (LC) resonant circuit with a characteristic frequency of :

${\displaystyle f_{tsc}={1 \over {2\pi {\sqrt {C_{tsc}L_{tsc}}}}}}$

The tuned frequency is usually chosen to be in the range 150-250 Hz on 60 Hz systems or 120-210 Hz on 50 Hz systems.[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’’).

The TSC is usually tuned to a non-integer harmonic of the mains frequency so as to avoid the risk of the TSC being overloaded by harmonic currents flowing into it from the AC system.

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. However, when the TSC turns off, it does so at zero current, corresponding to peak capacitor voltage. The capacitor only discharges very slowly, so the voltage experienced by the thyristor valve will reach a peak of more than twice the peak AC voltage, about half a cycle after blocking. The thyristor valve needs to contain enough thyristors in series to withstand this voltage safely.

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.

Sometimes, however, the TSC may turn on at an incorrect instant (as a result of a control or measurement fault), or the capacitor may become charged to a voltage above the normal value so that even at the minimum of valve voltage, a large transient current results. The current in the TSC will then consist of a fundamental-frequency component (50 Hz or 60 Hz) superimposed on a much larger current at the tuned frequency of the TSC. This transient current can take hundreds of milliseconds to die away, during which time the cumulative heating in the thyristors may be excessive.

The largest item of equipment in a TSC, the capacitor bank, is constructed from rack-mounted outdoor capacitor units, each unit typically having a rating in the range 500 – 1000 kilovars (kVAr).

The function of the TSC reactor is to limit the peak current and rate of rise of current (di/dt) when the TSC turns on at an incorrect time. The reactor is usually an air-cored reactor, similar to that of a TCR, but smaller. The size and cost of the TSC reactor is heavily influenced by the tuning frequency of the TSC, lower frequencies requiring larger reactors.

The TSC reactor is usually located outside, close to the main capacitor bank.

The thyristor valve typically consists of 10-30 inverse-parallel-connected pairs of thyristors connected in series. The inverse-parallel connection is needed because most commercially available thyristors can conduct current in only one direction. The series connection is needed because the maximum voltage rating of commercially available thyristors (up to approximately 8.5 kV) is insufficient for the voltage at which the TCR is connected. For some low-voltage applications, it may be possible to avoid the series-connection of thyristors; in such cases the thyristor valve is simply an inverse-parallel connection of two thyristors.

Typical TSC valve

In addition to the thyristors themselves, each inverse-parallel pair of thyristors has a resistor–capacitor ‘’snubber’ circuit connected across it, to force the voltage across the valve to divide uniformly amongst the thyristors and to damp the "commutation overshoot" which occurs when the valve turns off.

The thyristor valve for a TSC is very similar to that of a TCR, but (for a given AC voltage) generally has between 1.5 and 2 times as many thyristors connected in series because of the need to withstand both the AC voltage and the trapped capacitor voltage after blocking.

The thyristor valve is usually installed in a purpose-built, ventilated building, or a modified shipping container. Cooling for the thyristors and snubber resistors is usually provided by deionised water.