Application of Apparent Impedance Method at Variable Specific Resistance of Phase Conductor

The apparent impedance to single-phase earth fault place is obtained applying the classical algorithm of distance protection. Thanks to a complex character of this impedance, the expression for the algorithm decomposes into two equations, allowing two unknowns to be calculated: the reactance to a fault place and the fault resistance, provided the faulty phase voltage and current as well as ground current or zero sequence current are known. To do this, a special mathematical procedure called here the apparent impedance method is employed, which implies the use of specific line parameters. For high voltage two-terminal and radial lines of distribution networks the iterative procedure could be applied under the condition that the faulty phase voltage is measured in a distribution network with sufficient precision. Among other specific line parameters, the inconsistency of specific resistance of the phase conductor deserves special attention because of its unpredictable nature and wide range of deviations causing inadmissible errors in distribution networks. To cope with this shortcoming, the temperature of the phase conductor wire should be known at the moment of fault inception. Otherwise, the single-phase earth fault current should be increased to the level above the half the line load current.