A Novel Solution for Clearance of High Resistive Faults in High Voltage Transmission Lines

As per grid code (CEA – Central Electric Authority) EHV & UHV faults should be cleared in 100ms for better power system stability and availability. Generally EHV and UHV line will be longer enough to give Distance protection to clear any type of fault in the line with numerical relay within 100ms using scheme communication. But single phase faults can be high resistive in nature and frequent also. Failure of Distance protection relay for such High Resistive Faults (HRF’s) is observed in many places at Indian substations. In this paper, reasons for failure of Distance protection relay during HRF’s are explained using simulations carried out in Mipower software with low, medium and high resistive faults in a 400kV transmission line. Distance protection fails even if fault resistance is equal to load resistance. At present, protection system in India is depending on Directional earth fault protection for HRF’s and load resistive faults by compromising on the delayed trip since fault current cannot be very high during HRF's. But delayed trip in 400kV and above systems is generally not a good practice because of higher voltages in healthy phases with HRF in one phase. A novel solution is prepared in PCM (Protection and Control Manager) software using Directional earth fault protection and scheme communication of Directional earth fault protection to clear HRF’s and load resistive faults instantaneously without no major wiring and scheme changes for easy implementation. Prepared logic in PCM is tested in the laboratory using ABB REL670 numerical relay and omicron injection unit. This testing is carried out for a disturbance took place in a 400kV transmission line HRF where Distance protection failed to clear the fault instantaneously.