A new approach for transformer ground differential protection

Existing electromechanical ground differential protection relays are impaired in the event of CT saturation. They might trip when the fault is external (or not trip when the fault is internal) unless the configuration and settings are designed very carefully. In addition, inrush effects can also cause wrong protection behavior. Simulations and field observations have revealed that the phase angle difference between the ground current and zero sequence current, in combination with the ratio of their magnitudes, can be used to identify precisely a transformer ground fault. These observations were used for the development of a new numerical transformer differential protective relay. Simulations and test results have shown that the new solution correctly detects a wider range of phenomena that would indicate an internal fault, while remaining able to not trip in the event of an external fault.