Page 200 - Power Electronic Control in Electrical Systems

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188 Power electronic equipment

change in the inductive reactive power of the TCR. Thereafter the conduction angle
will vary continuously according to the system requirements, until the next capacitor
switching occurs.
The performance of this hybrid arrangement of a TCR and switched shunt capaci-
tors depends critically on the method of switching the capacitors, and the switching
strategy. The most common way to switch the capacitors is with conventional circuit
breakers. If the operating point is continually ranging up and down the voltage/
current characteristic, the rapid accumulation of switching operations may cause a
maintenance problem in the circuit breakers. Also, in transmission system applica-
tions there may be conflicting requirements as to whether the capacitors should be
switched in or out during severe system faults. Under these circumstances repeated
switching can place extreme duty on the capacitors and circuit breakers, and in most
cases this can only be avoided by inhibiting the compensator from switching the capa-
citors. Unfortunately this prevents the full potential of the capacitors from being used
during a period when they could be extremely beneficial to the stability of the system.
In some cases these problems have been met by using thyristor controllers
instead of circuit breakers to switch the capacitors, taking advantage of the virtually
unlimited switching life of the thyristors. The timing precision of the thyristor
switches can be exploited to reduce the severity of the switching duty, but even so,
during disturbances this duty can be extreme. The number of separately switched
capacitor groups in transmission system compensators is usually less than four.

6.2.4 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'.
6.2.4.1 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 switch-
ing transients, the current is sinusoidal, that is, it contains no harmonics.