The main function of the relay protection device is to automatically cut off the power supply when the electrical equipment and electrical lines fail, to avoid the expansion of accidents, and to ensure the safety of the power grid and people. Therefore, the function of the relay protection device must be selective, rapid, sensitive and reliable.

1. Selectivity. When the system fails, the protection device should select the part to remove the fault. The non-fault part continues to work.

2, when the short circuit short circuit, rapid troubleshooting can reduce the fault range. Reduce the damage caused by short circuit current, reduce the impact on users, improve the stability of the system.

3. Sensitivity refers to the sensitivity of the relay protection device to the possible failure and normal operation of the protection device. The sensitivity of the protection device is expressed by the sensitivity coefficient KL.

4, reliability, relay protection device at any time to maintain a complete and flexible working state, in the event of failure, the protection device should be timely and reliable action, should not have their own defects, to prevent failure, therefore, the requirements of urgent construction, simple, accurate, good construction quality, correct debugging, proper operation and maintenance.