Locked current protection system and locked current protection method

The invention discloses a locked current protection method. The method is characterized in that a braking area of busbar differential protection is utilized to realize current back-up protection of an access branch and also can be used as back-up protection of busbar faults and feeder line faults of subordinate substations and failure protection of circuit breakers of the subordinate substations. With the adoption of the method, existing sampling data of busbar protection is utilized sufficiently, so that current back-up protection selectivity of the whole power supply system can be realized by transferring locked signals among protection devices between the substations merely. A preferable solution, namely, the method is provided for the problems of short power supply circuits, multiple levels, slight current protection level difference and difficulty to ensure protection selectivity in low-medium voltage power supply systems such as power supply systems of enterprises, medium voltage power supply systems of subways and the like. The method is used to ensure the current back-up protection selectivity at any circumstance and has the advantages of simple logic, strong adaptability, wide application, high protective redundancy and the like.