Excess loading protection circuit - uses derivative of saturated output of line transformer with additional voltage injection from auxiliary transformer

摘要

A transducer (T1, T2) gives the first derivative of the saturated output of the line current transformer (W1, W2) and employs it to control a square wave generator (S1, S2) feeding a phase comparing network. Under certain circumstances, however, the transducer output can be virtually zero. In such a case control of the square wave generator may be lose and the protection rendered ineffective. This occasional condition is overcome by including the primary winding of an auxiliary current transformer (T1', T2') in series with the transducer primary, and adding part of the secondary output to the transducer output. A voltage divider (R1, R1'; R2, R2') may be put across the current transformer terminals for this purpose. The resultant signal has precise zero cross-over points and always has a finite value above or below these points.