Patent GB-554667-A (GB)
Title: Improvements in and relating to electric protective systems employing distance relays
IPC: H02H1/00
Filed: 1942-01-21
Abstract: 554,667. Protective cut-out systems. BRITISH THOMSON-HOUSTON CO., Ltd. Jan. 21, 1942, No. 864. Convention date, Jan. 21, 1941. [Class 38 (v)] A distance protective system for a polyphase circuit comprises a number of relays with current and potential windings and a switching means adapted to alter the winding connections to the protected circuit so that the relays have the same reach for all kinds of faults regardless of the number of phases involved or of whether the fault includes an earth. The Specification sets out the theory underlying the invention, the gist of which is that for faults not involving earth and for double-earth faults, the positive phasesequence impedance equals the quantity (EAEB)/(IA - IB) where the voltage suffixes denote the line-to-neutral volts, whereas for singleearth faults (and also double-earth faults), that impedance equals EA/(IA + KIR) where IR is the residual current and K a constant. For faults not involving earth, therefore, the potential restraining windings are energized by line-voltages and the current operating windings by the difference between the corresponding linecurrents, whereas an earth-fault relay, when operated, changes the voltage connections to line-to neutral volts and the current connections to line, plus an amount of residual, current. In Fig. 1, three impedance relays 17A ... C of the beam type have their parallel-connected contacts 21 in series with contacts 22 of a directional relay in the trip circuit. The voltage windings 24 are normally connected to a potential transformer 28 so as to be energized by line-voltages, for example E A  - E B  in the case of relay 17A. The current windings 26, 27 are normally energized in opposition by, respectively, one linecurrent and the next lagging line-current, that is (IAIs) in the case of relay 17A. The neutral lead includes a relay 39 which, on earth fault, energizes a solenoid 37 against spring-action 38 to change over switches 35A ... 36C, whereby the potential connections are changed to line-toneutral and current connections to line plus residual current, the amount of residual current being adjustable by an autotransformer 41. Fig. 2 shows the application to a reactance relay system in which the reactance relays OVA . . . OUc are made responsive to Z. sin a by having their potential windings 59 in series with a resistance-capacity unit 63, 64 adjusted to give the correct phase-angle. These relays and the directional starting units SUA . . . SUc are structurally similar and each comprises a hollow stator, a cup-like rotor, and, enclosed by the latter, a further stator element. The contacts 58 and 46, 47 of these relays are in series in the trip circuit with normally-closed contacts 86, 87, and further contacts 48 of starting units SUA . . . SUc control the motor 70 of a timer unit TU which has contacts 74, 75 successively momentarily engaged by a contact 73 to provide stepped operating times. The units SUA &c. have current windings 53, 54 energized in parallel from the appropriate line-current transformers 32A &c., potential windings 49, 50 energized by the appropriate line-voltages, and further potential windings 51, 52 energized by line-voltages over transformers 60, which latter are also connected to energize the voltage windings 59 of the ohm units OUA &c. to an adjustable extent determined by the energization or otherwise of relays ARA &c. by the timer TU. The units OUA &c. also have current windings 61, 62 which are connected to current transformers 32A &c. by switches 35A &c. as in Fig. 1, but in Fig. 2 the solenoid 37 changes the connections only on single-earth faults, and is adapted to be de-energized and restore the normal connections if a single-earth fault changes to a double-earth, since the latter does not produce the same measurement in a reactance relay as in an impedance relay. For this purpose a system of relays FDA &c. are energized by currents and voltages and have normally-closed contacts 82, 84 and normallyopen contacts 83 arranged so that operation of only one relay energizes 37, but simultaneously operation of two de-energizes it. When one operates, winding 85 opens the trip circuit at contacts 86, 87 and a new trip circuit is made over contacts 88 and, when 90 has operated after a time lag, over contacts 89, the relay 90 having a time lag to ensure that faulty tripping will not take place due to 58 closing before the winding connections of the units OUA &c. have been restored due to a single-earth having changed to a double-earth. The relays ARA &c. may have seal-in contacts, or alternatively may be momentarily energized, an arrangement which allows legitimate tripping by momentarily lowering the setting of the ohm. unit to the trip point, but prevents power swings from causing erroneous tripping. Where the ohm. and starter units are arranged with their stator in parallel planes. one above the other, the stators are so arranged as to avoid magnetic interference upsetting the operation. The Specification refers also to Electrical Engineering, Vol. 53, p. 206.