Existing electromechanical ground differential protection relaysare impaired in the event of CT saturation. They might trip when thefault is external (or not trip when the fault is internal) unless theconfiguration and settings are designed very carefully. In addition,inrush effects can also cause wrong protection behavior. Simulations andfield observations have revealed that the phase angle difference betweenthe ground current and zero sequence current, in combination with theratio of their magnitudes, can be used to identify precisely atransformer ground fault. These observations were used for thedevelopment of a new numerical transformer differential protectiverelay. Simulations and test results have shown that the new solutioncorrectly detects a wider range of phenomena that would indicate aninternal fault, while remaining able to not trip in the event of anexternal fault
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