A new inrush detection algorithm for transformer differential protection

Some power transformers, especially new designs with the core material improved for lower losses, produce low levels of second harmonic in their inrush magnetizing currents. Transformer differential relays face security problems when the second harmonic falls below the traditional 15 or 20 percent setting level. This paper derives a new protection method for detecting inrush conditions. The new algorithm is based on dwell-time periods in the inrush current. In each power cycle of a true inrush current, a period occurs where the differential current is both small and flat (dwell time). The new algorithm combines the absolute values of the instantaneous differential current with the absolute values of the differential current derivative to positively confirm both the low level and the flatness of the dwell-time currents. By utilizing the “flatness” feature of the waveshape, the algorithm performs well even under current transformer (CT) saturation. The new algorithm incorporates a separate method to cancel the inrush inhibit signal, allowing very fast operation for internal faults, particularly during inrush conditions. This dedicated element is based on a bidirectional instantaneous overcurrent principle.