The Electricity Council

David R Aubrey

Stephen J Kearley

Ian Jameson

Excess currents due to faults in an electrical power system are generally larger than excess currents due to the magnetization of distribution transformers. Discrimination has hitherto relied on a threshold detector. Magnetizing currents are, however, asymmetrical, whereas fault currents are symmetrical. A detector detects symmetrical current waveforms on a conductor 1 and thus discriminates between a fault and a magnetizing excess current. A resistor 4, placed across the output terminals of a bridge rectifier 3 has a resistance selected so that the voltage across it exceeds the breakdown voltage of trigger diodes 7 and 8 when the current in conductor 1, in any half cycle, exceeds a desired maximum. Capacitors 13 and 14 will thus only both charge up in response to an excessive symmetrical current. A logic control 23 is connected via lines 21 and 22 to detect such a condition.

Symmetrical fault current detector

A method of isolating a spur line of an electrical power system protected by a multi-shot resetting circuit breaker wherein at least two successive current surges in the spur line which activate the circuit breakers are detected. In response to the detection the spur line is isolated during a subsequent current dead period. A sectionalizer is described for isolating a spur line of an electrical power system protected by a multi-shot resetting circuit breaker. The sectionalizer comprises a metal casing 22 providing a current path across a fuse mounting 10 while the sectionalizer is in a conducting position. An electrical circuit 23 inside the tube detects at least two successive current surges in the casing and during the following dead period actuates a chemical actuator 14 to trip the sectionalizer out of its conducting position.

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Chester, United Kingdom

David R Aubrey