Switching limits in static converters using soft commutation

The authors investigate the problem of switching limits called `commutation limits´ and show how it can be solved in the case of a PWM inverter in soft commutation operation. The authors discuss motions of `the commutation cell´ and then point out those cells which are susceptible to operating in soft commutation mode, particularly the one which, according to the real nature of the power switches, seems to be the most interesting. This one is the cell encountered in voltage inverters, using `thyristor dual´ type switches, i.e. with controlled turn-off and spontaneous turn-on. Next are shown the changes to be made in a PWM inverter equipped with its filter, in order for the switches to operate in that way, to take advantage of soft commutation. The authors then propose a theoretical approach to determine the commutation limits and show, in particular, that these limits introduce little restriction in the operation of this kind of inverter structure; they then deduce criteria for the filter design. This approach is validated by experimental results obtained with PWM inverters using MOSFETs or IGBTs