TRANSFORMER LONGITUDINAL DIFFERENCE PROTECTION METHOD WITH ZERO-SEQUENCE RATIO BRAKING

摘要

The present invention discloses an improved relay protection method for the electrical power system. According to the method, the weighted sum is obtained by adding the brake current denoting the through current in the conventional transformer longitudinal difference protection and the zero-sequence current on the Y0 side of the transformer to get the to get the brake current including the zero-sequence current. The obtained brake current constitutes the ratio brake unit of the corresponding phase along with the difference current of the phase. The ratio brake unit constitutes the ratio brake protection along with the excitation surge current detection and the overexcitation detection unit. When the difference current is more than the brake current including the zero-sequence current and is also more than the minimum action current, the action signal of the ratio brake unit of the phase is outputted. Furthermore, the weighted sum is obtained by adding the maximal value or the weighted sum value of the zero-sequence currents of the Y0 sides and the brake current in the conventional longitudinal difference protection to get the brake current including the zero-sequence current in the case that the transformer includes several Y0 sides. The advantage of the invention is that the transformer differential protection accidental operation resulted from groundfault outside the Y0 side area is prevented when the errors of the three-phase current transformer are in inconformity. It is suitable for the electrical power transformer longitudinal difference protection.