Protective relay circuit for interphase faults

A protective relay circuit for controlling tripping of three poles of a circuit breaker connected in a three phase alternating current power line upon the occurrence of a fault condition involving more than one of the three conductors of the power line, comprising:

• (a) OR circuit means provided with a first, a second and a third input and an output, said OR circuit means providing a signal output in response to a signal being applied to at least one of said inputs;
• (b) means for initiating tripping of said three poles of the circuit breaker in response to a signal output being developed on said output of said OR circuit means;
• (c) first circuit means for providing a three pole trip signal to said first input of said OR circuit means, said first circuit means generating said trip signal in response to a restraining signal falling below a predetermined value, said restraining signal being representative of the sum of first and second absolute value quantities, said first absolute value quantity being a difference signal representative of a first signal less a second signal, said first signal being representative of the positive sequence symmetrical component voltage of the line at the relay location, and said second signal being representative of the product of the positive sequence positive sequence component of line current and a replica impedance representative of the positive sequence impedance of said power line, said second absolute value quantity being the absolute value of a third signal which is representative of the product of the zero sequence symmetrical component current and the replica impedance of said power line;
• (d) second circuit means for producing a second three pole trip signal on said second input of said OR circuit means, said second circuit means including:
• a positive sequence distance relay, a zero sequence overcurrent relay provided with positive sequence current restraint, and logic means for restraining the output of said distance relay while said overcurrent relay is producing an output but producing said second trip signal when said distance relay is unrestrained by said overcurrent relay, and
• (e) third circuit means for provding a third three pole trip signal to said third input of said OR circuit means, said third circuit means including a positive sequence voltage level detector, said positive sequence voltage level detector producing said third trip signal when the positive sequence symmetrical component voltage of said power line falls below a predetermined level.