Fault current protective circuit - has summator transformer and has auxiliary windings feeding biasing generator and RC control

摘要

A fault current protective circuit consists of a summing current transformer whose primary windings are formed by the main conductors of an AC single-phase or three-phase supply. The secondary windings of the transformer are connected to a trigger. There are auxiliary windings connected to a generator to produce an AC biasing magnetisation of the current transformer. The secondary windings are connected to a rectifier circuit. The frequency of the biasing magnetisation is large compared to the main frequency. A RC network is connected in parallel to the rectifier circuit. The time constant of the RC network is approximately equal to the period of the biasing oscillations. There is a discriminator circuit parallel to the RC network. When the voltage of the RC network drops below a limiting value it will produce a trigger pulse.