Restricted earth fault protection for transformers using a directional element

摘要

The residual phase currents for each selected winding of a transformer are first determined and then summed to provide a value designated IX. The neutral current for the transformer is also determined, with the neutral current being designated IY. A directional element calculates a torque value T according to the following formula: T=Re(IXxIY*). This torque value will be negative for an external (reverse) fault, and positive for an internal (forward) fault. The quantity of the torque is compared against threshold values before a forward or reverse fault for the transformer is declared.