Polyphase hybrid rectifying bridge with commutation fault protection

Abstract

In association with a polyphase hybrid rectifying bridge having controlled rectifiers, there is employed, in addition to the normal means for controlling the firing time of the controlled rectifiers, additional circuitry for preventing or at least reducing the probability of the occurrence of a commutation fault. This is achieved by first producing a timing signal which is representative of the time intervals during which a voltage exists across a load being supplied by the bridge and by then combining the timing signal with a basic reference signal representing a minimum time period during which this voltage should exist to guard against a commutation failure of the controlled rectifiers. When a prescribed relationship exists between the timing signal and the basic reference signal, the normal control means is overridden and the firing angle of the controlled rectifiers is advanced to a point which will guarantee commutation. In addition, there is further provided an optional feature which facilitates recovery of the bridge in the event a commutation failure does occur. This feature takes the form of a recovery circuit which, in response to a sensing of an unbalance in the alternating voltage supplying the bridge, outputs a signal which also causes an advance in the firing angle of the controlled rectifiers.