Principle and implementation of substation-area backup protection for digital substation

To solve the problems related to configuration and principle of backup protection in conventional substations, a new scheme of Substation-area Differential Backup Protection (abbr. SDBP) oriented to intelligent substations is proposed by this paper. The new scheme takes the whole substation and all connected transmission lines as an object to be protected. Through defining four types of differential zone (i.e. boundary differential zone, inner-station differential zone, equipment differential zone and extended differential zone), the SDBP can accurately locate the faulted component according to whether the differential zone lies in restraining state or operating state. In the proposed scheme, an intelligent substation will be configured by bay oriented main protection and only one set of SDBP, which simplifies the configuration and setting of backup protection. The tripping strategy of SDBP is determined by action signals from both main protection and circuit breakers. The validity of SDBP scheme has been verified by PSCAD simulation. The prototype of SDBP has been developed and tested in laboratory.