Investigation of alternative residual current compensation for improving series compensated line distance protection

The effect of the residual compensation factor on the measuring accuracy of distance protection measurements when an earth fault occurs on a series compensated line is investigated. It is shown that, when conventional residual compensation is used, there will be an error in the impedance measurement which depends on the ratio I_sa (I_sa+K_cI_res ) where I_sa is the faulted phase current at the relaying point, K_c is the conventional residual compensation factor, and I_res is the residual current. It is also shown that this expression depends on load current and source conditions at the line ends. In consequence, errors of measurement and concomitant overreaching/underreaching can occur when conventional residual compensation methods, as applied to plain feeders, are used in series compensated applications. An alternative method of compensation has therefore been developed. This has been found to improve accuracy of measurement at the boundary and relay measurement integrity for other fault positions in series compensated line applications