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.
In a protective relay for use in protecting a predetermined protected zone of a three phase alternating current power transmission line, said relay being of the type which receives a comparison signal, having at least a fundamental frequency, from a remote relay and generates a first restraint signal which is related to the difference between the receive comparison signal and a filtered comparison signal having a frequency which substantially comrpises said fundamental frequency only, an improvement comprising means for preventing generation of a relay trip signal upon occurrence of a high level noise signal component in said comparison signal, said means comprising: (a) means, adapted to receive said restraint signal, for producing an output signal when the magnitude of said restraint signal exceeds a pre-set level; (b) means for amplifying said output signal and producing an amplified output signal; (c) means, adapted to receive said amplified output signal, for producing a second rstraint signal which remains for a predetermined period of time after discontinuance of said amplified output signal; and (d) means for producing an output restraint signal which is substantially equal to the sum of the magnitudes of the first restraint signal and the second restraint signal.
Claim 2
Dependent Claim
198 words
In a protective relay for use in protecting a predetermined protected zone of a three phase alternating current power transmission line, said relay being of the type which receives a comparison signal, having at least a fundamental frequency, from a remote relay and generates a first restraint signal which is related to the difference between the receive comparison signal and a filtered comparison signal having a frequency which substantially comrpises said fundamental frequency only, an improvement comprising means for preventing generation of a relay trip signal upon occurrence of a high level noise signal component in said comparison signal, said means comprising: (a) means, adapted to receive said restraint signal, for producing an output signal when the magnitude of said restraint signal exceeds a pre-set level; (b) means for amplifying said output signal and producing an amplified output signal; (c) means, adapted to receive said amplified output signal, for producing a second rstraint signal which remains for a predetermined period of time after discontinuance of said amplified output signal; and (d) means for producing an output restraint signal which is substantially equal to the sum of the magnitudes of the first restraint signal and the second restraint signal.
Claim 3
Dependent Claim
31 words
The improvement in accordance with claim 1 wherein said means for producing an output signal when the magnitude of said restraint signal exceeds a pre-set level comprises a zero suppression network.
Claim 4
Dependent Claim
63 words
The improvement in accordance with claim 2 wherein said means for producing an output restraint signal comprises an operational amplifier having a first input to which is applied said first restraint signal, and a second input to which is applied a second restraint signal, and an output which is equal to the sum of the magnitudes of the signals applied to said inputs.