High voltage asynchronous motor comprehensive protection circuit

The high-voltage asynchronous motor operating system and comprehensive protection circuit are shown in Figure 4-58. This circuit uses an electromagnetic operating mechanism
(DC contactor KM turns on the closing coil YC of the circuit breaker QF).
    The line is equipped with the following protections:
    ① Current quick-break and over-current protection (1KA, 2KA), which act on tripping;
    ② Low voltage protection [KV1, KVz, KV3 (not shown in the figure), 2KC], which act on tripping;
    ③ Low cycle rate protection (via intermediate relay contact KC), which acts on tripping;
    ④ Excitation fault protection, which acts on tripping;
    ⑤ Excitation fan fault protection (via time delay relay KT), which acts on tripping;
    ⑥ Oil system fault, which acts on tripping Trip.