Experimental method to summarize the effect of transformer inrush currents on a digital overcurrent relay

This article shows an experimental method to summarize the effect of transformer inrush currents on a digital overcurrent relay. The effect of these currents on instantaneous and definite-time overcurrent functions is shown by simple time-current curves. In the case of inverse-time overcurrent functions, an equivalent current is obtained for each test point, from the operation time and the curve equation. Limit-curves in the time-current graph, which summarize the way to avoid the operation of inverse-time overcurrent functions for transformer inrush currents, are found for each type of inverse curve. Three different cases for waveshapes of transformer inrush currents were injected into a commercial relay in order to assess their effect on phase and ground overcurrent functions. Instantaneous and definite-time functions, and seven types of inverse-time functions were tested. For the sake of simplicity, the effect of transformer inrush currents on inverse-time overcurrent functions can be grouped for each case of waveshape. A brief theoretical description of some factors that influence the dynamic behavior of overcurrent functions is also included.