DIGITAL PROTECTION OF DOUBLE-CIRCUIT LINES IN SOLIDLY EARTHED HIGH-VOLTAGE SYSTEMS

A distance-protection relay in three-phase a.c. systems is able to use the same X-R-operating characteristic for line-toline short circuits as for a balanced three-phase short circuit, if the line-to-line voltages and currents are used, independently of the neutral point connection in the system. In solidly earthed systems it is in addition to that necessary, to introduce a zero-current correction in the case of a line-toearth short circuit to use the same X-R-operating characteristic as for three-phase short circuits. Proposals are given for the zero-current correction in the case of double-circuit lines with T-branches in meshed, solidly earthed high-voltage systems.