Page 165 - Power Electronics Handbook

Chapter 8
A.C. line control

8.1 Introduction

The power flowing in an a.c. line can conveniently be controlled by series
resistors, although this method is inefficient, so that for all but the lowest
power levels transformers or variacs are used.
The thyristor a.c. line regulator was introduced in Chapter 6. This is
relatively efficient since it works on the principle of blocking unwanted
power, rather than dissipating it across the control device, and it also gives
a system which is physically smaller and lighter than conventional methods.
This is especially true when the power being handled is large, as is evident
when comparing a 1 kV 1 kA thyristor a.c. regulator with an equivalent
auto-transformer.
Static switching, described in Chapter 7, is a method of a.c. line control,
although in this case the control is either on or off, there being no facility
for continuously varying the amount of power flowing from the supply to
the load. These variable systems are described in the present chapter.
Phase-control techniques give the simplest a.c. regulating system and are
best known. However, there are three other methods by which thyristor
control of a.c. lines is possible, namely a.c. chopper control, integral
half-cycle regulation and synchronous tap changing. These are described in
the sections which follow.

8.2 Phase control

8.2.1 Singic-pBose circuits
Figure 8.1 shows three typical single-phase control systems with their
circuit waveforms. The two-thyristor circuit of Figure 8.l(a) is the simplest,
and if a triac is used in place of the two thyristors shown only a single power
component is required. The circuit of Figure 8.l(b) uses two extra diodes
compared to the basic two-thyristor system. This increases its cost and
reduces the efficiency since for any conduction path there is now a series
thyristor and diode. It has the advantage that the cathodes of the thyristors
are commoned, so the gate drive circuit is simplified.
Referring to the circuit of Figure 8.l(b) and the waveforms given in
Figure 8.l(d), at time to input line A goes positive to B, and since thyristor
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