A regional backup protection algorithm is proposed based on the adaptive current protection co-factor. With adaptive current protection as the basic principle

the algorithm will have more reliable information source

and can be adapted to the system operation mode and fault type changes. The effectiveness degree of adaptive current action for fault line recognition is used as the weight of coordinated protection function and expected coordinated protection function. The fault line can be identified using the adaptive current protection co-factor and the ratio of coordinated protection function to expected coordinated protection function. Meanwhile

in the circumstances of lack of or error in large figure protection action information

the algorithm can still estimate the fault line. Finally

the simulation results obtained from the real-time digital simulator (RTDS) of an IEEE 33-bus system and a distribution network in Tianjing have proved the reliability and effectiveness of the algorithm. This work is supported by National Basic Research Program of China (973 Program) (No. 2012CB215200)

National Natural Science Foundation of China (No. 51277193)

Fundamental Research Funds for the Central Universities (No. 2014ZZD02)

and Hebei Provincial Natural Science Foundation of China (No. E2012502034).