TROUE PHASE SELECTOR

PROBLEM TO BE SOLVED: To enhance faulty phase selection performance, including the case of multiple troubles in a multi-terminal system. SOLUTION: The trouble phase selector comprises a first operating means 11 for calculating the differential current of each phase of each transmission line; a second operating means 12 for calculating the line differential current of a faulty line, based on the differential current of each phase obtained from the first operating means and faulty line information; a third operating means 13 for determining the ratio of a minimum value to a maximum value obtained from the second operating means; a fourth operating means 14 for making a decision that a single phase fault has occurred, if ratio obtained from the third operating means is smaller than a specified value; a fifth operating means 15 for determining the ratio of a differential current in the phase not constituting a maximum value to a smaller differential current, with regard to differential currents of two phases constituting the maximum value of line differential current; a sixth operating means 152 for making a decision that a two-phase fault has occurred if a ratio obtained from the fifth operating means is smaller than a specified value; and a seventh operating means 17 for deciding that a three-phase fault has occurred, when the ratio obtained from the fifth operating means is not smaller than the specified value.