A transformer protective relaying algorithm using the ratio of induced voltages

This paper proposes a transformer protective relaying algorithm using the ratio of induced voltages (RIV) of the primary and secondary windings. The RIV is equal to the turn ratio during magnetic inrush and over-excitation as well as in the steady state, while it differs from the turn ratio when internal winding faults occur. In case of a single-phase and a three-phase Y-Y transformer, the induced voltages of windings are calculated and their ratios are compared with the turn ratio. On the other hand, in case of a three-phase Y-/spl Delta/ transformer, induced voltage´s differences of the windings are calculated using line currents instead of winding currents of a delta winding which are practically unavailable in many situations; then, their ratios are compared with the turn ratio. The algorithm not only detects internal winding faults, but also does not maloperate even in a large inrush current due to a high remanent flux at the instant of energization. The results of various tests show that the algorithm successfully discriminates between internal winding faults and normal operation conditions including inrush and can reduce the relay´s operating time.