Patent GB-205866-A (GB)
Title: Improvements in and relating to alternating current protective apparatus
IPC: H02H7/26
Filed: 1922-07-22
Abstract: 205,866. British Thomson - Houston Co., Ltd., and Fitzgerald, A. S. July 22, 1922. Protective arrangements for parallel circuits or windings comprise a biassing transformer as described in Specification 193,434 with three windings; a biassing winding 5. Fig. 1, which serves to magnetize the transformer approximately to the. saturation point, a primary operating coil 6 and a secondary coil 7 in connection with an ordinary relay, the transformer being connected up so as to give biassed and discriminating protection against line faults and earth faults. In the arrangement shown in Fig 4 for a pair of parallel feeders, the current transformer 4 produce equal currents in two circuits which comprise the biassing winding 5 of one biassing transformer and the operating winding 6 of another. The secondaries 7 are connected to ordinary relays 10 controlling the tripping circuits of the circuitbreakers 3 of the respective feeders. In the event of a fault on one of the feeders, the resulting unbalancing causes the operating winding of the corresponding biassing transformer to receive more current while its biassing current remains the same, whereas the biassing current of the biassing transformer on the sound feeder increases. As a result, the relay in connection with the biassing transformer of the faulty feeder operates while the biassing transformer of the sound feeder is further restrained. The application to a circulating-current transformer system is shown in Fig. 5 in which the current transformers circulate current through the restraining windings 5, and the operating windings 6 are connected across normally equi-potential points. The relays do not normally operate on heavy balanced currents by reason of the heavy restraint imposed by the windings 5. In order, however, to deal with the possibility of the insulation of a healthy feeder due to the magnification of slight differences between the current transformers on verv heavy overloads, an additional winding having a restraining action may be added to each biassing transformer and be energized from the other. An arrangement for more than two feeders is shown in Fig. 6 with provision for leaving the remaining feeders under protection when a faulty feeder is cut out. Auxiliary switches 12, operated to the upper position, directly or indirectly by the main circuit-breakers immediately before they open, serve to short-circuit windings 4 and 5 in association with the dead feeder or, alternatively, to open the circuit of 4 and 5 at the point 13. Fig. 7 shows the application to protection against earth or leakage currents where load currents are not normally equally shared. A null balance is normally maintained between the current transformers in one feeder (which are connected so that the vectorial sum of the phase currents is zero) and those of the other feeder. In the event of a fault to earth on, for example, one of the phases of the right-hand feeder, current due to the leakage current divides through the righthand windings 5, 6 and a branch traverses the left-hand winding 6 while the left-hand winding 5 carries the united currents of the two branches with the result that, while the right-hand relay operates to isolate the faulty feeder the transformer in association with the left-hand relay is further restrained due to the extra current in its biassing winding 5. In order to deal with inequalities of the current transformers which manifest themselves on very heavy overloads the winding 7, in any of the arrangements referred to, instead of being connected directly to the relays may be connected to the operating winding of an auxiliary biassing transformer, the biassing winding of which is in two parts connected to the ends of the individual current transformers. With this arrangement the fault may be augmented, automatically, in accordance with the increase of the load. In the arrangement shown in Fig. 9, relays 10 are employed to deal with earth faults in the manner described in connection with Fig. 7, and separate relays 14 in association with separate biassing transformers 21, 22, 23 are used for faults between phases which give rise to heavier fault currents. The windings 21 are connected in a circulating-current circuit with the secondaries 16 of ordinary small transformers, the primaries 15 of which have a midpoint tapping and are connected to the secondaries of the current transformers as shown.