Patent GB-445147-A (GB)
Title: Improvements in and relating to protective apparatus for electric systems
IPC: H02H7/30
Filed: 1935-04-26
Abstract: 445,147. Protective cut-out systems. BRITISH THOMSON - HOUSTON CO., Ltd., Crown House, Aldwych, London. April 26, 1935, No. 12563. Convention date, April 26, 1934. [Class 38 (v)] Induction relays ; electromagnetic switches; retarding and time-limit devices.-In protective systems wherein selective action is obtained on a time-graduated basis dependent on the fault-position, in order to prevent undesired operation of the protective relay by a power swing after a fault, or by other conditions which substantially reproduce the operating characteristics of the relay, but only after a time-delay, the relay is arranged so that if the conditions to which it is desired to respond appear initially, its characteristic will be altered so that it is at first more sensitive during the remainder of the fault, and then is made less sensitive. If a fault occurs, there is at first substantially no arc thereat, but when an arc develops, the power-factor increases and the current diminishes owing to an increased resistance component of the impedance; whereas with a power swing, the power-factor is high and the current is low initially, and the relay is enabled to distinguish between the two conditions by being made more sensitive to arc-lengthening until the expiry of the maximum fault-duration to be expected. For controlling the breaker 13, there is employed a relay of the character described in Specification 393,577, comprising a timer 18, an ohmmeter 19, and a starting unit 20. The timer 18 comprises a solenoid 31 which, when energized, drives in the direction of arrow 35 a rack-and-pinion 33, 34, so rotating a shaft 23 which is connected by a spring 24 to a shaft 22. A pinion 29, loose on the latter, is connected to an escapement 26 by means of a ratchet-and-pawl 28, 27, which allows free return of the parts under spring action 32 independently of the escapement. The shaft 22 carries a contact-member 21 which, when moved in the direction of the arrow 36, engages a contact 37 and, immediately after, a contact 38. The winding 43 of the ohmmeter 19 always biasses the disc 41 in the direction of arrow 65, whereby the movable member 44 is held against contact 45 corresponding to minimum ohm setting. Magnet 42 is directional and when it overcomes 43 the member 44 takes up a position corresponding to the impedance of the line. A condenser 52 ensures the correct phaseangle between the currents in the current and voltage windings 48, 49, the latter winding being normally shorted at the closed contacts 56 of the starting unit 20. The magnet 59 of unit 20 normally biasses the movable contact 60 against contacts 56, but when the bias is overcome by the directional magnet 58, member 60 is moved to close contacts 61 and energize the solenoid 31. A phase-displacing resistance 65 is connected across the current winding 63. To ensure sufficient voltage across winding 64 with low impedance faults, a condenser 66 is adjusted to resonate with 64 at the power frequency. On normal voltage, a glow-discharge valve 67 byepasses the condenser and destroys the resonant feature, but when the voltage is inadequate to break down the valve, resonance occurs. To provide the necessary discriminative feature, either the torque in unit 20 can be increased by disconnecting a resistance 68 at contacts 9 when solenoid 31 is energized, so as to give the ohmmeter time to determine if the fault is within its range, or the input to winding 64 may be increased, or a combination of both features may be employed, as by putting 67 in series with contacts 9 so as to produce resonance when contacts 9 open or a drop in voltage occurs, Fig. 2 (not shown). If a fault occurs between A, B, Fig. 3, within the range of the minimum setting of 19, operation of starter 20 trips 13 in time t by closing contacts 61 and making thereover a circuit from battery 15, over 44, 45, contacts 79, and trip relay 74 which lifts and energizes the trip coil. If the fault occurs between B, D such that 44 lies between 45, 46, the solenoid 31 is energized by closure of 61 to cause 21 to engage contact 37 after a time t<1>, thereby energizing the trip relay 74 over the contacts 61, 21 and 37, and 79. With a fault beyond D the member 44 engages contact 46 to energize a relay 80 which opens contacts 79 and prevents tripping while contacts 61 remain closed. If the starting unit fails to reset because of a swing initiated by a fault, member 21 closes contacts 38 to reduce the sensitivity of that unit. Contacts 61 are thereby opened and 18, 19 are de-energized, except in the event of a severe swing, or an immediately-following fault, or a sustained fault, in which events the unit 20 retains its contacts 61 closed to prevent tripping.