Patent ID: 6922318

Claim:
A ground relay system in a multiplex direct grounding system comprising: a plurality of first current transformers sensing currents flowing in respective phases of a sending end of a distributed high-voltage distributed from a distribution substation; a plurality of over-current relays respectively connected to corresponding current transformers of the plurality of first current transformers; a plurality of distribution transformers connected to the respective phases of the distributed high-voltage; a plurality of second current transformers respectively sensing currents flowing in secondary sides of corresponding distribution transformers of the plurality of distribution transformers; a plurality of current converters respectively connected to corresponding second current transformers of the plurality of second current transformers; a current comparator that sums current vectors of the currents flowing in respective phases of the sending end and sensed by the plurality of first current transformers to produce a first zero-phase current, sums current vectors of the currents flowing in respective phases of a load end, sensed by the plurality of second current transformers through the plurality of current converters, to produce a second zero-phase current, compares the first and second zero-phase currents with each other, and operates a ground protection relay that opens a circuit breaker inserted at the sending end of the high-voltage distributed if the first zero-phase current is larger than the second zero-phase current; and a ground over-current relay connected to the plurality of over-current relays, wherein, in a trip circuit of the circuit breaker, a contact of the ground protection relay and a contact of the ground over-current relay are connected in series, and a malfunction of the existing ground over-current relay is locked by the ground protection relay.