The outflow current during an internal fault on the bus will decrease the sensitivity of the relays for the overall differential element and even lead to operation refusal or trip time delay for the total protection. To solve the problem

improved criteria and a concrete implementing scheme are put forward. First

the influence of outflow current on conventional bus protection for double-bus and double-bus double-segment connection modes is analyzed. Then

the restraint current calculation method of the new ratio differential criterion is proposed and the characteristics of the new algorithm during internal and external faults are analyzed. Finally

the implementation scheme matching small range differential relay is put forward and the validity of scheme is verified by dynamic simulation test.