Tutorial on high impedance fault detection

High impedance faults are generally not detected by conventional protection functions like over current, ground fault, distance, differential etc. because of the magnitude of impedance involved in the fault path and the nature and characteristic of the fault current are special and different than the conventional fault current profiles. Each type of high impedance fault is unique in terms of magnitude of fault current, nature, characteristic and waveshape. Majority of the high impedance faults are single phase to ground faults but this can involve phase to phase elements as well. Because of the inability of the conventional protection functions to detect high impedance faults especially high impedance phase to ground faults, the electrical conductor remains live under such condition and as can be imagined, poses a huge and significant risk to wild life and more importantly human life. Atmospheric and geographical conditions have a significant role to play in high impedance phase to ground faults since they have a direct impact on the magnitude and characteristic of the fault current. This paper describes different techniques to detect high impedance phase to ground faults and focuses on the proven algorithms that have been implemented in protection relays, had been verified by real site tests and fault inception on live power lines. The paper is broadly organized as a tutorial. The characteristics of high impedance faults and the challenges involved in detecting them are described first. The paper then goes on to detail some of the important techniques in use for high impedance fault detection highlighting their strengths and weaknesses, and some modern approaches proposed to improve the dependability of protection schemes. In particular, a technique combining the fundamental and harmonic analysis of the fault waveform is presented, along with its performance in field trials carried out in co-operation with utilities.