GB238911A - Protective arrangement for a polyphase alternating electric current circuit and the like - Google Patents

Abstract

238,911. British Thomson-Houston Co., Ltd., and Fitzgerald, A. S. Feb. 25, 1924. Electric protective arrangements.-In balanced protective arrangements for star-delta transformers &c. windings are provided the energization of which is due only to earth faults on either side of the transformer so that greater sensitivity is obtained with earth than with inter-phase faults. As shown in Fig. 1 in which the neutral point 14 of the star-connected transformer winding 12 is earthed the delta-connected current transformers 18 on this side of the transformer are connected by leads 19 to the starconnected current-transformers 15 in the leads 13 passing to the delta-connected transformer windings 11. The primary windings 25 and series-connected restraining windings 24 of biassing transformers, of the type described in Specifications 193,434 and 216,254 are connected between equi-potential points of the protective system represented by the leads 19 and a common lead 23. Further series-connected windings 22 are wound in opposition to the windings 24 and are connected at one end to the lead 23 and at the other end through lead 21 to the neutral point of the transformers 15 which is also earthed at 16. The windings 22 may also be fed by a transformer 26 in the lead between the neutral point 14 and earth. This arrangement is such that with inter-phase faults the windings 22 are not energized and the restraint produced by the windings 24 on the action of the primary windings 25 is fully operative but with earth faults on either side of the transformer the consequent energization of the windings 23 destroys the restraint in one of the biassing transformers so that a comparatively sensitive operation of the relay is produced. In the modified arrangement shown in Fig. 2 in which the transformer star point is not earthed the windings 18 are connected to the leads 19 through windings 28 on the core 27 of a summation transformer, which also has a winding 29 between the leads 21 and 23. The windings 22 are fed from a winding 31 on the transformer core 27 which is only magnetized when an earth fault occurs. In a similar modification also described the above arrangement is combined with restraining means for preventing operation on balanced overloads due to unequal characteristics of the current transformers. In a further modification shown in Fig. 4 for use in cases where the neutral point may or may not be earthed the secondary winding 31 of the summation transformer is connected to a sensitive earth fault relay 42, an additional winding 41 being fed from the current transformer 26 and wound in opposition to the primary windings 28. In this form the interphase fault relays 25 are relatively insensitive.