In the paper, a new field ground fault protection with voltage injection is introduced. The injection voltage of protection is a low frequency square wave voltage, it is transformed from an AC voltage by transformer, rectifier and electronic switches, and its frequency is controllable. The principle, criterion and hardware configuration of protection are given, and several operation problems are analyzed. The results of static simulation test and dynamic simulation test is shown, the operational aspect of protection in three gorges left bank power plant is also introduced, these test and operation results indicate that the protection performance can meet the practical operation requirement, so the application prospect of this protection is good.