Relay protection reliability is mainly ensured by reasonably configured, high-quality and technically superior relay protection devices, as well as normal operation, maintenance, and management. Any power equipment (lines, busbars, transformers, etc.) is not allowed to operate without relay protection. All operating equipment in 220kV and above power grids must be protected by two sets of relay protection devices with mutually independent AC and DC input/output circuits, each controlling different circuit breakers. When any set of relay protection devices or any set of circuit breakers refuses to operate, the other set of relay protection devices can operate the other set of circuit breakers to clear the fault. In all cases, it is required that the DC power sources for these two sets of relay protection devices and circuit breakers be supplied through different fuses.
