Comparison signal error detector for protective relay system

Abstract

A protective relay, for use in protecting a predetermined protected zone of a three phase alternating current electrical power transmission line, the relay being of the type which receives a comparison signal from a remote relay by way of a communication channel, includes circuitry for detecting abnormalities in the received comparison signal. Such abnormalities are caused by, for example, errors in the communication channel, non-fundamental frequency components in the secondary current caused by current transformer saturation, and high frequency transients in the protected zone. The abnormalities are detected by subtracting a filtered demodulated signal having a frequency which is substantially equal to a fundamental frequency of the comparison signal, from an unfiltered demodulated signal. The resulting difference is a signal which comprises components caused by the abnormalities. The resulting difference signal is utilized as a restraint signal to prevent the protective relay from incorrectly generating a circuit breaker trip signal. The filtered demodulated signal is utilized as an operate signal, in conjunction with a locally generated operate signal, to produce circuit breaker trip signals.