Earth fault monitoring system for cable networks - uses fifth harmonic current detector with heavy fundamental suppression

摘要

It is well established that an earth fault in a cable network leads to a marked increase in fifth harmonic current. The detection of this current, through a current transformer (T), is made more sensitive by heavy damping of the fundamental frequency. The required damping is achieved by adding a series capacitor (C2) between the parallel tuned filter (L, C1) and a bridge rectifier (G) which passes a fifth harmonic dependant signal to a trigger circuit (MT). The addition of the series capacitor increases the damping of the fundamental of the power supply frequency. The inductive elements (L) of the parallel filter can be combined with the inductance of the transformer winding.