The distribution network becomes a multi-source network after distributed generators (DGs) are connected to it. The differential protection scheme is the most effective way to solve the problem about protections of the multi-source network. Limited by the level of distribution network communication automation

the double side synchronous sampling requirement is hard to meet

which means the traditional differential protection scheme cannot be applied in the distribution network with DGs. To solve the problem above

this paper presents a fault synchronous identification technique. Before and after the fault moment

the time interval between two adjacent zero crossing points of fundamental current with the same trend is measured. The time interval can be used to calculate the fundamental current phase difference and identify the fault

while effectively overcoming the difficulty of high precision synchronous sampling. Based on the fault synchronous identification technique

the compound criterion for sufficient differential protection is proposed. Only fundamental current amplitude and time information obtained by the fault synchronous identification technology are used to structure the compound criterion which consists of the improved scalar product restraining criterion and the sufficient current amplitude comparison criterion. The new criterion has solved the contradiction between improving the phase shift restraining capacity and sensitivity. Simulation has verified the accuracy of the fault synchronous identification technique and the action features of the compound criterion. This work is supported by National Natural Science Foundation of China (No. 51377022) and Graduate Research and Innovation Project of Jiangsu Province (No. KYLX_0122).