According to the role it plays, protection can be classified into main protection, backup protection, auxiliary protection, etc. ①Main protection: Protection that meets the requirements of system stability and equipment safety, and can quickly and selectively remove faults in protected equipment and lines. ②Backup protection: Protection used to remove faults when the main protection or circuit breaker fails to operate, further divided into remote backup protection and local backup protection. a. Remote backup protection: Backup protection implemented by the protection of adjacent power equipment or lines. b. Local backup protection: When the main protection fails to operate, backup protection is implemented by another set of protection on the same power equipment or line; when the circuit breaker fails to operate, backup protection is implemented by the circuit breaker failure protection.