In the new field winding ground fault protection with low frequency square-wave voltage injection， the injection voltage 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 introduced. The accuracy and sensitivity of new protection are also analyzed. The results of static simulation test and dynamic simulation test show that the accuracy and sensitivity of protection can meet the operation requirement. Furthermore， three problems about protection application are analyzed: taking the appropriate measures to ensure the protection accuracy， the application of two-point ground fault protection， and the detection of intermittent ground fault. The operational result of protection in Three Gorges Left Bank Power Plant indicates that the protection operates reliably， with stable performance， so the protection's engineering application prospect is good.