Misoperation of high impedance differential protection for main transformer

The misoperation of the high impedance differential protection for main transformer No.3 in a substation is analyzed based on its principle and main features. Research result shows that, under the nearby large current fault, the differential current induced by the big difference of U-I characteristics between the current transformer of common winding and that of 500 kV or 220 kV presents big value, which will cause the protection misoperation. The counter-measure is to increase the protective settings by adjusting the variable resistors in RADHA differential circuit, which avoids the misoperation caused by the unbalanced currents and voltages generated in RADHA by the through currents during external fault. Field operation shows its effectiveness.