GB767188A - Improvements in or relating to electric protective systems, and control circuits therefor - Google Patents

Abstract

767,188. Protective cut-out systems. GENERAL ELECTRIC CO. Ltd. Dec. 9, 1953 [Dec. 9, 1952], No. 31186/52. Class 38(5) In order to render a circulating-current protective system for a power transformer immune to operation by magnetizing currents and through-faults, a variableimpedance circuit is employed which is applied to transformers arranged in the circulating-current circuits so as to lower the output to the protective relays. The Figure shows the arrangement for one of the phases of a delta-star transformer, the primary 14 of a load-bias transformer 21 in one of the circulating-current lines being connected through the primary 17 of a differential-circuit transformer to the star point of the line current transformer on the delta side of the power transformer. The secondary 24 supplies an input winding 2 of a transformer 1 in a circuit tuned to the supply frequency, and also a bridge rectifier 33 in. a circuit tuned to the second harmonic of that frequency. A control winding 4 of transformer 1 is connected across an impedance 5 and in series with a pair of opposed rectifiers 6, 7. The secondary 20, of transformer 21 supplies a bridge rectifier 22 which is connected in the same sense as 33 between the midpoint of 5 and the rectifiers 6, 7. An output winding 3 of transformer 1 supplies the protective relay 27. On through-fault, the rectifiers 22, or on magnetizing inrush, the rectifiers 33, supply a D.C. component to lower the impedance of the variableimpedance circuit, and therefore the output from the winding 3 to the protective relay 27. The Specification refers also to feeding a single control circuit by summation transformers from all three phases, to combining the windings 4, 5 into a single winding, and to connecting the control circuit across the A.C. input terminals of a bridge rectifier.