Page 253 - Power Electronics Handbook

A classification system for forced commutation 243

involved in forced commutating a conducting thyristor THI which is
operating from a d.c. supply. When switch S, is closed the load current,
flowing via the thyristor, is diverted through the bypass circuit, which also
applies a reverse voltage across the device, turning it off. Often this bypass
system consists of a capacitor, which has been charged during a previous
cycle to the polarity shown.
For successful commutation several conditions must be satisfied:
(i) The time for which the thyristor is reverse biased must exceed its
turn-off time.
(ii) The rate at which the forward voltage is re-applied across the device
must be less than its dv/dt rating.
(iii) The switch S, will have to carry a high rate of current increase (dildt)
and this must not exceed its rated value.
(iv) It is probable that the bypass circuit will need to be reset again, so as
to be able to apply the required reverse voltage across the thyristor, if
it is refired and needs to be turned off.
This chapter examines the different forced commutation techniques used
and their application to choppers and inverters are described in Chapters
12 and 13, respectively.

11.2 A classification system for forced commutation

A large number of different circuits are used for forced commutation of
power semiconductors, and in order to study them a classification system is
required, one system being described here. This is independent of the type
of application, e.g. chopper or inverter, so that it will be used again in
subsequent chapters when these circuits are described. Four divisions are
used in the forced commutation classification, as illustrated in Figure 11.2.

(0 Parallel-capacitor commutation. In this a charged capacitor C is
placed directly across the conducting thyristor, as in Figure 11.2(a),
turning it off. The circuit which is used to prime the capacitor at the
start of every cycle, with the polarity shown, is not illustrated, and the
semiconductor switch S, is also part of the commutation circuit. The
capacitor performs a dual role, that of applying a reverse bias across
the thyristor TH1 and of diverting the load current away from this
thyristor during the turn-off period. For inductive loads commutation
is more difficult and a larger value of capacitor must be used, or a
free-wheeling diode placed across the load, as in Figure 11.2.
(ii) Parallel capacitor-inductor commutation. In this method, illustrated
in Figure 11.2(b), an inductor is placed in series with the capacitor
which is connected across the thyristor being forced commutated.
Once again the capacitor carries the load current during commutation
and provides the reverse bias across the thyristor, the inductor having
auxiliary functions, as described later.
(iii) Series capacitor commutation. In this technique the capacitor is
connected in series with the power thyristor being turned off, as in
Figure 11.2(c), so that it is almost invariably in series with the load.