Page 267 - Power Electronics Handbook

Series capacitor commutation 257

Analysis of the circuit given in Figure ll.ll(b) shows that it is similar to
that in Figure ll.ll(a) regarding all the six comparative points, as follows:
(i) Only variable-frequency operation is possible.
(ii) The minimum on time is determined by the resonance through L1, the
load inductance not playing any part. The off time is determined by
the load and it should be resistive to prevent resonance during the
discharge cycle, which would reverse the voltage across it.
(iii) The commutation capacitor voltage is increased by the load current
flowing in inductor L1.
(iv) Unsuccessful commutation is more likely, with the thyristor
remaining permanently on through the relatively low impedance
load. When this occurs commutation is not re-attempted.
(v) The current rating of the thyristor is not increased by the
commutation capacitor reset action.
(vi) A low-impedance fault current path does not exist across the supply.
A more effective series capacitor commutated circuit is shown in Figure
ll.ll(c), where a separate resonant circuit is used to reset the
commutation capacitor. As before, the main thyristor TH1 is fired to
commence the load cycle and, assuming an underdamped circuit, this
thyristor will be turned off at the end of a half cycle of resonance, when C is
charged to a voltage in excess of VB with plate b positive. After a time
equal to the turn-off time of THI, auxiliary thyristor TH2 is fired, which
enables C to resonate through inductor b and recharge with plate a
positive, ready for the next half cycle of load current. This circuit has the
following characteristics:
(i) Only variable-frequency operation is possible, since the thyristor is
only capable of delivering a half cycle of power during its on period.
(ii) The on time is given by equation (1 1.10). The minimum off time is also
given by this equation where the effective inductance Le is now equal
to L. Since this inductance, L2, can be made much smaller than Le the
minimum off time is much less than the on period, so that the maximum
output voltage from the chopper circuit can be made to be close to that
of the supply.
(iii) The voltage on the commutation capacitor is boosted by the load
current flowing through L1 and the load. Furthermore, since the
voltage on C is built up from two sets of resonant circuits, it will reach
a value considerably in excess of the supply voltage VB, which will
result in faster commutation of the main thyristor.
(iv) A commutation failure path exists via L1, b and the load, and when
this occurs both thyristors THI and TH2 will remain permanently on,
supplying the load current, and commutation will not be re-
attempted. Since TH2 needs to carry the resonant reset current during
normal operation it is usually a low-current device, so it may be
destroyed when called upon to carry full-load current unless
protection circuitry is used.
(v) The rating of the main thyristor TH1 is not increased by the
commutation capacitor reset current, which occurs through TH2.
(vi) There is no low-impedance short-circuit current path across the
supply if commutation fails.