Page 261 - Power Electronics Handbook

Parallel capacitor-inductor commutation 25 I
polarity shown, from a previous cycle, and that the main thyristor THI is
on and supplying load current. When switch S, is closed the current
through inductor L starts to increase, due to the reverse polarity on the
capacitor. When this current equals that of the load current all the load
power is supplied via commutation capacitor C. The reverse voltage of the
capacitor is now placed across the main thyristor and it turns off. This
thyristor will remain off so long as it is reverse biased for a period in excess
of its turn-off time. The commutation capacitor charges via the load to at
least the supply voltage. For heavy loads it would charge to a voltage
greater than that of the supply since the energy stored in L, whilst it was
passing the load current, would transfer to the commutation capacitor,
boosting its voltage. The peak voltage on the commutation capacitor, and
its rate of rise, must not exceed the rating of the main thyristor or it will
break over into conduction.
Assuming that the commutation capacitor is charged to the value of the
battery voltage V, prior to commutation, the value of the commutation
capacitor and inductor can be found from equations (1 1.5) and (1 1.6) where
I,,,!, is the peak load current at the time commutation commences (switch
S, is closed) and tow is the turn-off time of the thyristor.

(11.5)

(11.6)

From these equations it can be seen that parallel capacitor-inductor
commutation is unsuitable for use in systems controlling large currents and
operating from low supply voltages since it would require a large value of
commutation capacitor and an impractical small value of series inductance.
For example, to turn off 250A from a 4OV supply, using thyristors with
turn-off times of 50p, would require a commutation capacitor of 28OpF
and an inductor of 2.5pJ-I. In all probability, the inductance of the
connecting leads would exceed this value.
Figure 11.8 shows the most basic commutation circuit, in the parallel
capacitor-inductor group. When the power is first applied capacitor C
charges to the supply voltage with plate b positive and the main thyristor TH,
cannot be fired until this has been completed. When "HI is turned on the load
cycle commences and simultaneously the capacitor voltage is placed across
inductor L1, causing it to resonate and recharge with plate a positive. Once
this has been completed, the circuit conditions are as in Figure 11.2(b) with
the switch closed. The capacitor commences to discharge through the load
and as current builds up through the inductor, to reach the value of the load
current, thyristor THI turns off. The capacitor now continues to charge
towards the value of the supply voltage, as before.
This circuit can be analysed using the six comparative factors introduced
earlier:
(i) It can only operate in a variable-frequency mode, since there is
control over the firing time of thyristor TH1 but no control over when