Patent GB-210294-A (GB)
Title: Improvements in and relating to electro responsive devices for the protection of electric systems
IPC: H02H3/38
Filed: 1922-07-26
Abstract: 210,294. British Thomson-Houston Co., Ltd., and Fitzgerald, A. S. July 26, 1922. Cut-out arrangements.-The biassing transformer described in Specification 193,434 is employed, with relays of ordinary pattern, to afford protection in the event of reverse power in the case of a single circuit or on disturbance of relative distribution of power in the case of two more circuits connected in parallel. The secondary current of the biassing transformer, which controls the relay, is related in phase with the primary exciting current and operation of the relay may be arranged to take place in accordange with any predetermined vectorial relationship between the current in the circuit and any predetermined potential difference associated with the circuit. In the arrangement shown in Fig. 1, for the protection of a single feeder 2 supplying, or being supplied by, the bus-bar 1, the biassing winding 5 and the operating winding 6 are connected in series to the current transformer 4. An additional operating winding 29 is connected to a source of potential difference derived directly or by transformation from that due to which current flows in the feeder 2; in the arrangement shown, the winding 29 is connected to the secondary of a transformer, the primary of which is connected between the feeder and earth. A relatively large resistance 30 is connected in the circuit of winding 29, the current in which is in phase with the potential at 24 and practically constant. The secondary winding 7 is connected to the relay 9. with forward power, the winding 29 opposes the winding 6, but, with reverse power, 29 aids 6 and the relay operates. In the arrangement shown in Fig. 2, the transformer 24 is arranged to be excited bv a source of current derived from some point of the system, for example, the other two phases of the system, in quadrature with that in the conductor 1, in the manner described in Specification 4595/15. The current transformer supplies the biassing and operating windings 5, 6 of one biassing transformer and the potential transformer 24 supplies the similar windings 20, 21 of another. The secondaries 7, 22 are connected to the relay so that they are in opposition when the direction of power is positive and in addition, so that the relay operates, when negative. The current in the relay 9 is in accordance with the vectorial relation between the potential and current. In order to correct for phase displacements owing to copper and iron losses in the transformer and to ensure that the electromotive-force impressed on 22 is in quadrature with that derived from 24, a resistance 23 is connected across the operating winding 21. The same result may be obtained by connecting the secondary 7 through an ordinary small transformer 25, 26 to the secondary 22, as shown in Fig. 8; this, the preferred arrangement, by reason of the copper losses in the subsidiary transformer and the saturation of the core has the advantage that there is no tendency to trip with heavy lagging currents associated with forward power. An arrangement for differential protection of parallel feeders is shown in Fig. 11, the switch 32 being employed to leave the healthy feeders under protection when a faulty feeder has been isolated. To deal with lack of balance of current transformers on heavy overloads additional biassing transformers mav be utilized. Balanced current and balanced power protection may be compounded. In the protection of two parallel feeders it is possible to use one potential biassing transformer 20, 21, 22, only, as shown in Fig. 13, thereby reducing the energy consumption by the potential tranformers to a minimum. The interlock arrangements between the switches 32, Fig. 11, and the circuit-breakers as described in Specification 205,866 may be employed.