For Ydelta connected transformer

the currents in delta winding are not easily measurable. So the problem of current phase compensation has to be considered in the differential protection. However

using the present current phase compensation methods

the calculated differential current cannot really reflect the characteristics of the inrush. At the same time

the application based on the equivalent instantaneous inductance scheme is limited to identify the inrush. In this study

a constructing algorithm of circulating current in delta windings is proposed based on the voltage equations of two side windings. And the relationship between the circulating current and zerosequence voltage and current in Y side windings is also obtained. The algorithm has nothing to do with the neutral grounding

so that its application is wide. The simulation and experimental results verify that the algorithm is correct and effective to identify the inrush.