The thyristor-switched capacitor (TSC)

Thyristor switched capacitor is defined as 'a shunt-connected, thyristor-switched capacitor whose effective reactance is varied in a stepwise manner by full- or zero- conduction operation of the thyristor valve'.

Principles of operation

The principle of the TSC is shown in Figures 6.12 and 6.13. The susceptance is adjusted by controlling the number of parallel capacitors in conduction. Each capacitor always conducts for an integral number of half-cycles. With k capacitors in parallel, each controlled by a switch as in Figure 6.13, the total susceptance can be equal to that of any combination of the k individual susceptances taken 0, 1, 2 . . . . or k at a time. The total susceptance thus varies in a stepwise manner. In principle the steps can be made as small and as numerous as desired, by having a sufficient number of individually switched capacitors. For a given number k the maximum number of steps will be obtained when no two combinations are equal, which requires at least that all the individual susceptances be different. This degree of flexibility is not usually sought in power-system compensators because of the consequent complexity of the controls, and because it is generally more economic to make most of the susceptances equal. One compromise is the so-called binary system in which there are (k - 1) equal susceptances B and one susceptance B/2. The half-susceptance increases the number of combinations from k to 2k.

The relation between the compensator current and the number of capacitors conducting is shown in Figure 6.14 (for constant terminal voltage). Ignoring switching transients, the current is sinusoidal, that is, it contains no harmonics.

Fig. 6.12 Alternative arangements of three-phase thyristor-switchet capacitor. (a) delta-connected secondary, Delta-connected TSC; ant (b) wye-connected secondary, wye-connected TSC (four-wire system).

Fig. 6.12 Principles of operation of TSC. Each phase of Figure 6.12 comprises of parallel combinations of switched capacitors of this type.

Fig. 6.14 Relationship between current and number of capacitors conducting in the TSC.

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