Application Considerations for Distance Relays on Impedance-Grounded Systems

Widespread use of digital distance relays has increased the application of distance protection to lower voltages, including subtransmission and distribution circuits. Many systems may be impedance grounded. In this paper, we look at some unique problems that may arise on single-source, distribution systems, such as might occur at the 26-kV voltage level. A source of concern for distance relays on these systems is the inclusion of a current-limiting impedance in the neutral of the source transformer. The voltage drop across this impedance during fault conditions affects the voltages seen by the relay and may affect the polarizing voltage used by the distance relay. We will explain the problem and offer solutions that will allow the application of distance elements.