Measuring accuracy of distance protection with particular reference to earth-fault conditions on 400kv looped-circuit interconnections

The paper provides a detailed evaluation of the errors arising in the measurement of impedance by distance protection, when applied to 400kV double-circuit lines formed in the widely used looped interconnection. An accurate separation of the errors arising from primary-circuit interphase and intercircuit mutual coupling, nontransposition of phase conductors, and fault resistance, is developed; and the fault and prefault operating conditions for which the errors have their greatest individual and combined effects are formulated. Extensive computer evaluations of errors under single-phase-to-earth fault conditions are tabulated, on the basis of which their consequences in relation to the fault operating performance of distance protection are assessed.