Double-circuit transmission lines on the same tower are widely applied recently bringing challenges to operation and value setting due to errors in earth-fault distance protection. Through calculation of measured impedance and trip boundary of zone Ⅰ of earth-fault distance protection in the double-circuit normal operation mode

single line operation mode and single side operation mode

the factors influencing trip boundary of zone Ⅰ of distance protection magnitude of errors are analyzed in different operation modes. Based on actual ultra-high voltage alternating current system parameters

the line protection device is tested with a real-time digital simulator (RTDS) to verify the calculation and analysis above

providing a basis for value setting of distance protection. It is shown by analysis that

influenced by double-circuit mutual inductance

traditional value setting of zone Ⅰ of earth-fault distance protection should be reduced to a rational extent to ensure the selectivity of protection.