Early recognition of trouble in mains=connected inverters - comparing absolute DC and difference between DC and AC side currents with permissible limits

摘要

A subsynchronous inverter cascade is constituted by an induction motor (M) with starting resistance (R), power switches (Q), a bridge rectifier (LGR), inverter (WR), DC smoothing choke (Ld) and transformer (T). Instabilities associated with overcurrent cause the inverter (WR) to be inhibited forcibly by a thyristor (LT) fired from a control circuit (St) sensitive to outputs from a differential current comparator (DK) and overcurrent comparator (UK). The unstable inverter is thereby turned off at the precise instant when the cyclic current is at its min. value. ADVANTAGE - Undelayed tripping of suitable overcurrent protection arrangements safeguards inverter before excessive current can arise.