GB902998A - Improvements in and relating to electrical protective relay systems - Google Patents

Abstract

902,998. Protective cut-out arrangements. ENGLISH ELECTRIC CO. Ltd. March 11, 1959 [March 11, 1958], No. 7787/58. Class 38 (5). A phase comparison protective system includes at each of two separated positions of a three-phase circuit to be protected, circuit means for separately deriving the positive and negative phase sequence components of the current in the circuit, circuit means for combining the components in a predetermined ratio and means responsive to the phase relationship between the combinations at the two positions for performing a protective function. Fig. 1 indicates the equipment at one of the positions and in this Figure delta-connected current transformers supply the above circuit means which are those of Specification 902,999, and are shown in Fig. 10. This Figure shows transformers T3 and T4 having their primary windings energized respectively by the negative and positive sequence outputs of the sequence network. The secondary winding of the transformer T4 is connected between a centretapping of the secondary winding of transformer T3 and the junction between resistors R3, the resulting bridge network combining the positive and negative sequence signals in the predetermined ratio to produce a combined signal for application through a D.C. filter network, such as that described in Specification 903,000, to a squaring circuit, Fig. 1, which supplies square-wave signals to a phase comparator and to a modulated carrier-current transmitter. The sequence network also supplies signals dependent on the positive and negative phase sequence components to a limiter and starting circuit, part of which is shown in Fig. 3. This circuit comprises fullwave rectifiers 20, 21 which supply potentiometers 24 ... 27. Capacitors 28, 35 are associated with the potentiometers 24, 25 so that the signals derived from the positive sequence input are dependent on the rate of rise of the positive phase sequence components. The potentiometers are connected through isolating diodes 29, 30 and 32, 33 to two transistorized trigger circuits (not shown) which provide low and high set control signals, the high set signals being slightly delayed with respect to the low set control signals. Under fault conditions, the low set signals control the operation of the squaring circuit and the operation of the modulated transmitter, while the high set signals trigger the operation of the phase comparator and a slave circuit. Thus the phase comparator compares the phase relationship of the square-wave signal produced by the squaring circuit with that received from the other position by means of a modulated receiver when permitted to do so by a high set control signal. If the phase relationship is that corresponding to an internal fault, the comparator supplies a signal to the slave circuit which trips the circuit-breaker and energizes a steady signal transmitter to transmit an intertripping signal to the other position. Should an intertripping signal be received through a steady signal receiver from the other position before the phase comparator functions, the signals from the steady signal receiver cause the operation of the slave circuit and prevent the operation of the steady signal transmitter. The Complete Specification fully describes and illustrates the various units shown in Fig. 1. Where appropriate these units are transistorized, the slave circuit being the transistor trigger circuit of Specification 903,001.