Measurement of Earth Fault Current and Earth Potential Rise on Live HV Systems

Abstract

Tests were carried out on a live 11kV network to measure the earth-fault level. A pole mounted transformer and LV load were configured to provide a load between phase and earth which could be switched on and off to produce a low level HV fault. The resulting HV current magnitude is below, or for shorter duration than, that which would normally cause earth-fault protection to operate. The resulting phase-to-earth voltage depression and currents were measured at the load point; analysis yields the overall zero sequence impedance of the system (including soil-return paths) and resulting maximum earth-fault current at that point. This value differs from conventional analysis based on computer modelling which often provides a ‘zero-ohm’ fault impedance. Consequently measurement provides a real-world figure that can be used more efficiently for design purposes. It was found that measurement/analysis based on a series of measurements was better than that based on single events.The earth-potential-rise (EPR) on the transformer tank was also measured relative to a remote earth reference some 200m away. The results were entirely as predicted for a HV earth rod resistance of just under 9Ω. The measurement of EPR is something that is rarely, if ever carried out by network operators in UK, and the results provide additional confidence in the validity of the testing.