Use of capacitors for reduction of commutation angle in static high-power convertors

With the usual methods of connecting static capacitors as a reactive-power source on the primary or tertiary of a convertor transformer, their latent ability to produce a rapid discharge current, which will directly assist the commutation process and so reduce commutation angle, is not utilised. By connecting the capacitors to the secondary winding (valve winding) of the convertor transformer, appreciable reduction of commutation angle can be obtained, resulting in a saving in reactive-power consumption, which is particularly important in the case of invertor operation. Investigation shows that the connection of capacitors alone on the valve winding produces transient overvoltages, and that these can be eliminated by incorporating the capacitors in a suitable filter configuration. This also results in improved filtering of current harmonics compared with primary- or tertiary-connected filters. Other advantages and disadvantages have been assessed, and theoretical calculations have been experimentally confirmed. Provision of capacitive filter circuits on the transformer secondary is particularly commendable for convertors using semiconductor rectifiers.