Evaluation of distance relay performance

POWER swings which result in lin large angles between voltages at the ends of a system and faults cause distance relays to measure impedances tedious to evaluate by mathematical computation. Also the slow clearing of a fault may cause a power swing, so that there will be a combination of swing and fault conditions to analyze. This article describes a method of determining quickly what impedances all phase and ground distance relays measure during all of these fault and swing conditions. The method requires only a ruler and a protractor or a draftsman´s 30-degree-60-degree-90-degree triangle, and a knowledge of the positive- and zero-sequence impedances of the system.