A novel selective earth fault detection method using residual current

In neutral point non-directly grounded medium voltage distribution system, to selectively detect single phase to ground fault remains as a challenge for protection relays. Conventional selective / directional detection method can be used in such cases, but they cannot be implemented without voltage measurements. If the radial connected feeders are grounded via resistor at neutral point on busbar, a novel selective earth fault detection scheme without voltage measurements which only uses residual current can be applicable and it is presented in this paper. The basic idea is the comparison of residual current´s 3^rd harmonic with fundamental component. The targeted feature is the harmonic signature of earth fault current. It is an improvement of selectivity and sensitivity for overcurrent relays in such system grounding condition. The effectiveness has been proved by EMTP test. A prototype relay has been produced and physical model tests have been done with positive results.