An improved transformer differential relay

POWER TRANSFORMERS usually are protected against damage due to internal faults by percentage differential relays, which compare currents on opposite sides of the transformer. However, when a transformer is energized, there is often an inrush of magnetizing current, which flows in only one winding of the transformer, producing a differential current which tends to operate conventional percentage differential relays. Special means, therefore, are necessary to prevent transformer relays from operating falsely on magnetizing inrush currents.