GB377975A - Improvements in or relating to electric protective cut-out arrangements - Google Patents

Abstract

377,975. Protective cut-out arrangement. WESTINGHOUSE ELECTRIC & MANUFACTURING CO., East Pittsburg, U.S.A.-(Assignees of Goldsborough, S. L.; 372, Park Avenue, East Orange, New Jersey, U.S.A.) May 1, 1931, No. 12835. Convention date, May 1, 1930. [Class 38 (v).] In a protective arrangement employing impedance-responsive relays, a first impedanceresponsive relay element actuates the circuitbreaker with faults at adjacent points in the line, a second impedance-responsive element responds to faults at more distant points in the line and a definite time-element relay controlled by the second relay element actuates the circuit-breaker after a predetermined time, the whole of the relays being under the control of a power-directional element so as to be operable only when the current is flowing in a predetermined direction. A third impedanceresponsive relay may also be provided with its associated definite time element relay, which is also subject to directional control, for "backing up " the operation of the second relay, in response to still more distant faults. As shown in Fig. 1 three impedance-responsive relays 11, 12, 13 are provided for each phase having series-connected current windings 14 fed from one phase of the current-transformer group 20 and having opposing parallel-connected voltage windings 15 fed from one phase of the deltaconnected voltage transformer group 19. The windings of the impedance-responsive relays are graded so that the relays are responsive to faults at different distances along the line, for example relays 11, 12, 13 reach their balance points and do not operate for faults beyond points PX1, Fig. 2. Relay 11 on operation instantaneously completes the tripping circuit of coil 31 at contacts 16, for faults between the substation 1 and the balance point PX1. Relay 12 for a point O between PX1 and PY1 opens its contacts 17 and causes the operation of a definite time-element relay 21 which after the expiration of a predetermined time interval completes the tripping circuit at contacts 23. For faults between PY1 and PZ1, if they have not already been cleared by the relays at substation 2 (sub 2) " back-up " protection is provided by relay 13 which by opening contacts 18 brings into operation a second definite timelimit relay 22 which completes the tripping circuit at its contacts 23 after a still longer time delay. The operations of these relays are under the control of a directional relay 24 so that tripping only takes place with power flow in a predetermined direction, this relay then closing contacts 27 in the tripping circuit and opening contacts 26 normally short-circuiting a transformer 25 supplying the time-limit relays. The tripping coil 31 is energized through a contactor 36, the energizing winding 35 of which is controlled by the impedance and timing relays, this circuit including a trip indicator 34. The balance points of the impedance relays indicated in Fig. 2 are to some extent varied by the type of fault occurring those corresponding to an interphase fault being shifted approximately 15 per cent to the right by a three-phase fault. The relays are graded as far as possible to prevent overlap between corresponding relays of succeeding sections especially when the sections are of differing lengths and the relay system is applicable with slight modifications to parallel feeder and ring- main systems. A line-to-line-to-ground fault will shift the balance points of the impedance relays in the same direction as a three-phase fault shifts it by an amount depending on the ratio of the negative to the zero phase-sequence impedance and to limit this effect, more especially in the case of a double ground fault, a filtering arrangement may be provided consisting of small 1 : 1 current transformers arranged so that zero phase-sequence components neutralize each other and have small impedance whereas a substantially infinite impedance is offered to positive and negative phase-sequence components. The constructions of the individual relays employed are also described, Fig. 5 showing the construction of each of the impedance-responsive relays 11, 12, 13. Each of these relays consists, as shown in Fig. 5, of a voltage coil 15 and current coil 14 within each of which is disposed a movable core 41, the cores being secured to the ends of a centrally-pivoted beam 40. Normally the voltage coil holds its armature against an adjustable stop-rod 51, the attraction of the core being initially set by an adjustable fixed core 49 but on preponderance of the attraction of the other core 41 by the current coil the beam is rocked and contacts 16 are brought into engagement. Adjustment of the value of current necessary to produce operation of the relay is made by varying the number of effective turns in the coil, tappings in the coil being brought out to screwed sockets 54 one of which may be connected to the outgoing lead by a screw connector 57. Adjustment of a relatively fixed magnetic core 43 within the current coil may also be effected, the latter adjustment being indicated on a scale 59 and arranged to produce intermediate values between the tapping adjustments. The directional relay is shown in Fig. 7 and consists of a pivoted copper loop 66 which straddles one limb of a closed core 65 carrying the voltage winding 63. Thus an induced current is produced in the loop by transformer action, and two limbs of the loop are arranged in air gaps formed in the magnetic circuit of the core 69 carrying the current winding 64. The reaction between the induced current in the loop 66 and the field in the core 69 produces rotation of the loop in one or other direction in accordance with the phase relationship of the currents in the current and voltage windings so opening or closing contacts 26, 27 by means of an insulating bar 75 attached to the loop. In the definite time-element relay shown in Fig. 8 the squirrel-cage rotor of the induction motor driving element rises by solenoid action on energization of the stator coil 83 and brings a gear wheel 87<1> on its shaft into mesh with a gear wheel 94. The latter wheel is secured on a shaft 92 carrying adjustably a contact arm 91 which is then driven against a spring 89 until the bridging member 23 engages the fixed contacts 27 after a predetermined interval. On de-energization of the stator coil the parts automatically resume the positions shown.