Distance Relay Response to Transformer Energization: Problems and Solutions

Modern distance relays use various filtering algorithms to extract the fundamental frequency component of power system voltage and current waveforms. The relays then use the filtered voltages and currents in such numerous algorithms as distance, overcurrent, under/overvoltage, and frequency. Use of filtered voltages and currents generally provides a protective relay immunity to power system harmonics such as those resulting from transformer energization. This paper focuses on the application of distance relays on or near power transformers. It reviews digital filtering algorithms and shows both digital filter and distance element algorithm responses to power transformer inrush. Without careful setting of digital relays in such applications, power transformer inrush can cause distance relay measurement errors and unwanted operations. This paper provides guidelines for setting distance relays applied directly on a power transformer and for applications of relays to a line on which transformers are connected. The paper demonstrates these guidelines through the use of actual power system events and events modeled on a real-time digital simulator