Calculation of the lowest currents caused by turn-to-turn short-circuits in power transformers

Among all internal faults in transformers the most difficult to detect are turn-to-turn short-circuits with few turns involved. In such cases the short-circuit currents flowing in the affected turns are high, but at the terminals they are very low, thus making operation of the differential relays doubtful. If large number of turns are short-circuited, the currents are not that low and are mainly determined by the reactance, which is a consequence of the leakage flux. However, when very few turns are affected, the influence of the winding resistance may become dominant, reducing the terminal currents to a small fraction of the rated current. Since those are critical cases from the point of view of protection operation, calculation of the resistance and fault currents in such cases is important. The paper presents a simple equivalent circuit applicable for small number of short-circuited turns, along with the method of calculation the equivalent resistance for the turn-to-turn short-circuits between the parallel conductors.