A Digital Relaying Algorithm for Detecting Transformer Winding Faults

Electric power utilities use differential relays for detecting faults in power transformers. Conventional designs of these relays use second harmonic, third harmonic or total harmonic current as a restraining quantity. This inhibits the relay operation during magnetizing inrush conditions. Initial designs of digital differential relays also used similar approaches. Most designs used the second harmonic component of the current to block trippings during the magnetizing inrush conditions. As the operating voltages and lengths of transmission lines have increased substantially, currents during internal transformer faults can contain large harmonic components. Therefore, the security of differential relays using harmonic restraint is a matter of concern. However, some recently reported algorithms do not use second harmonic restraint to block trippings during magnetizing inrush conditions. They use winding currents. In most situations, the currents in a delta winding are not readily available. The algorithms based on transformer models need equivalent circuit parameters of the transformer. The relay engineer must determine the equivalent parameters of the transformer before using these algorithms. This paper presents a new digital algorithm for detecting winding faults in single-phase and three-phase transformers. The proposed algorithm does not use harmonic component of current to block trippings during magnetizing inrush conditions. It is applicable to situations where it is and it is not possible to measure winding currents. The algorithm does not require the B-H curve data and takes hysteresis losses of the transformer into account. The algorithm was tested using a variety of operating conditions simulated on a digital computer.