Patent ID: 7319576

Claim:
An apparatus for providing differential protection for a transformer having a load side and a source side in a three-phase power system, the apparatus comprising: a microcontroller including a microprocessor, a memory operatively coupled to the microprocessor, a positive sequence differential element and a negative sequence differential element, the microprocessor adapted to: calculate a first positive sequence component and a first negative sequence component associated with incoming secondary currents detected on the source side, and calculate a second positive sequence component and a second negative sequence component associated with outgoing secondary currents detected on the load side; apply a phase angle shift to the second positive sequence component and the second negative sequence component to form a compensated positive sequence component and a compensated negative sequence component, respectively, to compensate for a phase angle shift introduced by operation of the transformer; and determine an operate current value and a restraint current value for each of the positive and negative sequence differential elements based on the compensated positive sequence component and the compensated negative sequence component.