The safe operation of power systems mainly relies onrelay protection devices for maintenance, so relay protection devices should meet the following requirements:
(1) Selectivity: When a fault occurs in the system, the protection device is required to only trip the faulty equipment, ensuring that non-faulty equipment continues to operate, thereby minimizing the scope of power outages and achieving selective action.
(2) Speed: After a fault occurs in the system, if the fault cannot be quickly cleared, it may expand. For example, during a short circuit, the voltage drops significantly, and the motors of users near the fault point are braked, slowing down or stopping, disrupting normal production. On the other hand, during a short circuit, the generator cannot deliver power, which may cause instability in the power system. Furthermore, during a short circuit, the faulty equipment itself will pass a large short-circuit current, and due to the effects of electrodynamic force and thermal effects, the equipment will also be severely damaged. The longer the short-circuit current flows, the more severe the damage to the equipment. Therefore, after a fault occurs in the power system, relay protection devices should act as quickly as possible to clear the fault.
(3) Sensitivity: The protection device should accurately respond to faults and abnormal operating conditions within its protection zone. That is, the protection device should not only operate sensitively during three-phase metallic short circuits under maximum operating conditions, but also have sufficient sensitivity and reliable operation during two-phase short circuits under minimum operating conditions and with larger transition resistances.
(4) Reliability: The reliability of a protection device is very important. That is, when a fault occurs within its protection zone, it should not refuse to operate due to its own defects, and it should not misoperate under any circumstances where it should not operate. Otherwise, an unreliable protection device, once put into service, can itself become a source of expanding accidents or directly causing accidents.
