Harmonic-current-restrained relays for transformer differential protection

Abstract

Differential protection, employing percentage-differential relays, has1 long been the accepted method of fault protection for large power transformers. This method, illustrated by the schematic diagram, figure 1, is quite satisfactory in most respects but is subject to false tripping on the transient magnetizing-inrush current which flows when the transformer is energized. This current, since it flows in only one winding of the transformer, causes a differential current to flow which, to an ordinary relay, “looks” the same as the current due to an internal fault. One method of preventing operation on magnetizing inrush in the past has been to use slow-speed induction-type relays with long time and high current settings. This is undesirable because of the slow-speed, insensitive protection it affords on internal faults. Other methods, applied to high-speed relays, involve desensitizing the relay during a time period covering the maximum inrush transient controlled by auxiliary relays operating on the transformer voltage.2 These methods have the disadvantage that they usually require the purchase of additional potential transformers, involve one or more auxiliary relays with the attendant circuit complication, and seriously reduce the sensitivity of the protection against faults present when the transformer is energized or which occur during the inrush period. In this paper a new type relay is described which, using the principle of harmonic-current restraint,3 is able to distinguish between the differential current due to an internal fault and that due to magnetizing inrush by their difference in wave form, operating with high speed on the fault current and being restrained from operating by the magnetizing-inrush current. Being entirely current-operated, this relay does not require the purchase of potential transformers. It is entirely self-contained and the only moving part is the single relay armature which operates the tripping contacts. Minimum desensitization during the inrush period is obtained, depending on the relative magnitudes of the inrush and the fault currents.