Why does the earth-fault detection method based on third harmonic work in large meshed 110 kV networks

Abstract

Due to today's increased demands on grid operation management, new methods for earth fault localisation and detection are needed. The fault localisation should be performed as quickly as possible and under the condition that the fault current at the fault location will not be significantly increased. Up to now, the third harmonic was never used as a directional earth fault indicator, as the third harmonic does, more or less, not exist in the line–line voltage of a healthy network. In today's meshed compensated 110 kV networks with increased length, only the transient relays work correctly. However, these relays give a signalisation only during the beginning of an earth fault. They are not able to give correct information after a change of the network configuration. One effect that was observed in the past was that the harmonics in the line–line voltages are changing during an earth fault. In this study, the advantages of the use of the third harmonics will be presented.