The primary or main protection is the first line of defense in a power system's protection scheme. The selection and applications of. The relays are in round glass cases. : 4 The first. When a fault occurs on any part of electric power system, it must be cleared quickly in order to avoid damage and/or interference with the rest of the system. Primary. Engineering use: Relays are used on feeders, transformers, buses, motors, generators, and transmission lines to protect equipment and improve system reliability. What controls it: Relay performance depends on the protected zone, CT/PT inputs, pickup settings, time delay, breaker clearing time, trip. State Key Laboratory of HVDC, Electric Power Research Institute, China Southern Power Grid, Guangzhou, China 2. National Energy Power Grid Technology R&D Centre, Guangzhou, China 3.
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