5.1.3.2 Fundamental frequency differential currentsThe fundamental frequency differential current is a vectorial sum (sum offundamental frequency phasors) of the individual phase currents from different sideof the protected power transformer.Before any differential current can be calculated, the power transformer phaseshift, and its transformation ratio, must be allowed for. Conversion of all currentsto a common reference is performed in two steps:• all current phasors are phase-shifted to (referred to) the phase-reference side,(whenever possible a first winding with star connection)• all currents magnitudes are always referred to the first winding of the powertransformer (typically transformer high-voltage side)The two steps of conversion are made simultaneously on-line by the pre-programmed coefficient matrices, as shown in equation 1 for a two-winding powertransformer, and in equation 2 for a three-winding power transformer.These are internal compensation within the differential function.The protected power transformer data are always entered as theyare given on the nameplate. Differential function will by it selfcorrelate nameplate data and select proper reference windings.1 1_ 1 1_ 2_ 22 2 _ 1 2 _ 2_ 13 3 _ 1 3 _ 2IDL IL W IL WUn WIDL A IL W B IL WUn WIDL IL W IL Wé ù é ù é ùê ú ê ú ê ú= × + × ×ê ú ê ú ê úê ú ê ú ê úë û ë û ë û1 2 3EQUATION1880 V1 EN (Equation 1)where:1. is Differential Currents2. is Differential current contribution from W1 side3. is Differential current contribution from W2 side1MRK 502 034-UEN - Section 5Differential protection71Technical Manual
