Transformer saturation criterion for line differential protection

CT(Current Transformer) saturation at external fault of short line and other special con-ditions will cause misoperation of line current differential protection for EHV / HV transmission line. Some preventive measures such as increasing setting threshold and enhancing restraint factor will reduce the protection sensitivity and delay the protective action. A CT saturation detection method is introduced,which combines short data window with distinguishing wave distortion. It ensures both sensitivity for internal faults and reliability for external faults,which is successfully applied in digital fiber current differential protection with 32 - bit DSP.