Phase Correction

Correct operation of transformer differential protection requires that the HV & LV secondary currents, measured by the relay, are in phase. If the transformer is Delta / Star connected, then balanced three phase load / through fault current suffers a phase shift of 300. If left un-corrected, this phase difference would lead to relay this through / load current as unbalanced fault current and result in relay operation. Electromechanical and static relays use appropriate CT / ICT connections to ensure that the primary and secondary currents applied to relay are in phase. For digital and numeric relays, it is common to use star connected line CTs on both HV & LV windings of the transformer and compensate for the winding phase shift in software. Depending on relay type / design only data required for the relay is vector group designation of the transformer. Phase compensation is then done automatically by the relay.