Apparatus and method for providing differential protection for a phase angle regulating transformer in a power system

A method for providing differential protection for a transformer having a load side and a source side in a three-phase power system, the differential protection including a positive sequence differential element and a negative sequence differential element, the method comprising:

• deriving a first set of three phasors from incoming secondary currents detected on the source side and a second set of three phasors from outgoing secondary currents detected on the load side;
• based on the first set of three phasors, calculating a first positive sequence component and a first negative sequence component;
• based on the second set of three phasors, calculating a second positive sequence component and a second negative sequence component;
• determining a phase angle shift based on a tap position of at least one tap changer and a predetermined number of degrees per step of the transformer, the phase angle shift introduced by operation of the transformer;
• applying the phase angle shift to the second positive sequence component and the second negative sequence component to generate a compensated positive sequence component and a compensated negative sequence component, respectively;
• and
• determining 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.

A method for providing differential protection for a transformer having a first side and a second side in a three-phase power system, the differential protection including a positive sequence differential element and a negative sequence differential element, the method comprising:

• calculating a first positive sequence component and a first negative sequence component associated with secondary currents detected on the first side, and calculating a second positive sequence component and a second negative sequence component associated with secondary currents detected on the second side;
• applying 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
• determining 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.