The ideas of differential charge and restraining charge are proposed along with an analysis of their relationship. Based on the ideas， a ratio-restraining differential protection criterion for lines and its setting rules are presented. The protection criterion is able to identify internal and external faults based on full wave current. The idea of avoiding the influence of anti-TA saturation and capacitive current is enumerated. Finally

simulation results show that the proposed protection criterion is capable of quick and accurate identification of an internal fault within 5~10 ms and reliable lockout protection during an external fault.