As the load varies in a distribution system, a variable voltage drop will occur in the system impedance, which is mainly reactive. Assuming the generator voltage remains constant, the voltage at the load bus will vary.

The voltage drop is a function of the reactive component of the load current, and system and transformer reactance. When the loads change very rapidly, or fluctuate frequently, it may cause ‘‘voltage flicker’’ at the customers’ loads.

Voltage flicker can be annoying and irritating to customers because of the ‘‘lamp flicker’’ it causes. Some loads can also be sensitive to these rapid voltage fluctuations.

An SVC can compensate voltage drop for load variations and maintain constant voltage by controlling the duration of current flow in each cycle through the reactor. Current flow in the reactor can be controlled by controlling the gating of thyristors that control the conduction period of the thyristor in each cycle, from zero conduction (gate signal off) to full-cycle conduction.

In Fig. 18.2a, for example, assume the MVA of the fixed capacitor bank is equal to the MVA of the reactor when the reactor branch is conducting for full cycle. Hence, when the reactor branch is conducting full cycle, the net reactive power drawn by the SVC (combination of capacitor bank and thyristor controlled reactor) will be zero.

When the load reactive power (which is usually inductive) varies, the SVC reactive power will be varied to match the load reactive power by controlling the duration of the conduction of current in the thyristor controlled reactive power branch.

Figure 18.3 shows current waveforms for three conduction levels, 60, 120 and 1808. Figure 18.3a shows waveforms for thyristor gating angle (a) of 908, which gives a conduction angle (s) of 1808 for each thyristor. This is the case for full-cycle conduction, since the two back-to-back thyristors conduct in each half-cycle.
This case is equivalent to shorting the thyristors. Figure 18.3b is the case when the gating signal is delayed for 308 after the voltage peak, and results in a conduction angle of 1208. Figure 18.3c is the case for a¼1508 and s¼608

With a fixed capacitor bank as shown in Fig. 18.2a, it is possible to vary the net reactive power of the SVC from 0 to the full capacitive VAR only. This is sufficient for most applications of voltage regulation, as in most cases only capacitive VARs are required to compensate the inductive VARs of the load.

If the capacitor can be switched on and off, the MVAR can be varied from full inductive to full capacitive, up to the rating of the inductive and capacitive branches. The capacitor bank can be switched by mechanical breakers (see Fig. 18.2b) if time delay (usually five to ten cycles) is not a consideration, or they can be switched fast (less than one cycle) by thyristor switches (see Fig. 18.2c). Reactive power variation with switched capacitor banks for an SVC is shown in Fig. 18.4.

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