Patent GB-421060-A (GB)
Title: Improvements in and relating to protective apparatus for electric systems
IPC: H02H7/30
Filed: 1933-06-10
Abstract: 421,060. Protective cut-out arrangements; induction relays. BRITISH THOMSON - HOUSTON CO., Ltd., Crown House, Aldwych, London.-(Assignees of Warrington, A. R. van C.; 1, Upper Providence Road, Media, Pennsylvania, U.S.A.) June 10, 1933, No. 16654. Convention date, June 11, 1932. [Class 38 (v)] In a distance relay for interrupting a circuit on fault conditions, in dependance on the impedance of the circuit between the point of interruption and the fault, comprising a timing unit 19, an ohmic unit 20 and a power directional starting unit 24 operable on flow of fault current in a predetermined direction to render the ohmic and timing units operable, the ohmic unit is arranged to interrupt the circuit substantially instantaneously for faults located within a predetermined distance, whilst for faults located at a greater distance but within a second predetermined distance the timing unit controls the ohmic unit to cause it to interrupt the circuit after a predetermined time, and for faults located beyond the second predetermined distance the timing unit effects the interruption of the circuit after a second and longer predetermined time. The timing unit 19 is similar in construction to that described in Specification 393,577 and carries a contact member 27 which co-operates with two adjustable contacts 29, 30 to provide two independently adjustable time delays. The starting unit 24 and ohmic unit 20 are preferably dynamometer relays of the form shown in Figs. 2 and 3, comprising an inductor ring 6 mounted to rotate in the air gaps between polepieces 4, 5 and a central enlargement 3 on a core 1 carrying current and voltage windings 7, 8 and 10. The ring 6 is connected by a spring 15, Fig. 5, to a pivoted contact member 14 movable between a fixed stop 16 and contacts 17, the spring giving a snap action. In the arrangement shown in Fig. 6, the windings 7, 8 of the starting unit 24 are energized from a potential transformer 31 connected across the phase conductors 21, 22, and the winding 10 is energized in accordance with the current in the conductor 21. Undesired operation of the starting unit by surges attendant on switching in and subsequent to the clearing of faults are prevented by shunting the winding 7 by a condenser 32 and including in series with the winding a voltage responsive device 33, such as a saturable reactance or a gaseous discharge tube, having a characteristic such as to pass a current which increases disproportionately with increasing voltage. The ohmic unit 20 is constructed as shown in Fig. 3, the windings 8, 9 on the polepieces being energized in accordance with the current in the conductors 21, 22, a portion 12 of each winding being wound on the central limb 3 of the core for the purpose of introducing a desired torque characteristic. The voltage windings 7 are energized from the potential transformer 31 through an adjustable resistance 37, a condenser 35 and reactance 36 being connected as shown in Fig. 6, to provide the desired power factor for the circuit of the winding 7. On the occurrence of a fault to which the starting unit responds and closes its contacts 14, 17, the circuit of the time unit winding 25 is completed, and the upper contact 17 of the ohmic unit is connected to the negative side of a control line 26. If the fault is within a predetermined distance, determined by the setting of the resistance 37, the ohmic unit closes its contacts 17 and completes the circuit of an annunciator 39 and relay 38 which operates the trip coil of the circuit-breaker 18. If the fault is more distant, the ohmic unit does not respond, but after a predetermined time the contact arm 27 engages the contact 29 and energizes a relay 40 which changes the setting of the resistance 37 to a higher value. If the fault is within the new setting of the ohmic unit 20, the latter responds to trip the circuitbreaker as before. The relay 40 short-circuits the annunciator 39 and removes a short-circuit from the annunciator 43. If the fault is still more distant, the ohmic unit does not respond, but after a further time interval, the contact arm 27 engages the contact 30 and the trip coil is operated, the operating circuit including an annunciator 45. In a modified arrangement, Fig. 7, two ohmic units 20, 20<1> are provided, each having a fixed setting on the resistance 37, and the annunciators 39, 43 are connected directly in series with the contacts 17 of the respective ohmic units. The unit 20 effects instantaneous tripping of the circuit breaker for faults up to a predetermined distance, while the unit 20<1> effects the release for faults beyond a second predetermined distance, when the contact arm 27 on the timing device reaches the contact 30. The winding 7 of the starting unit 24 in this arrangement is shunted by a current limiting device 33<1> whose resistance increases with increasing voltage, e.g. an ironhydrogen resistance, and the winding 8 is shunted by a condenser 32<1>. In a modified arrangement for protecting against faults between a phase conductor and earth, Fig. 8 (not shown), the voltage windings of the starting and ohmic units are energized from a potential transformer having an earthed neutral, and the current windings 8 of the ohmic units are energized in accordance with the residual current. The Specification as open to inspection under Sect. 91 describes other forms of induction relay. In some of these there is only one projecting polepiece and air gap for the inductor ring, and in one form, stated to act as a frequency relay, the winding 7 is short-circuited through a condenser. This subject-matter does not appear in the Specification as accepted.