Patent GB-293811-A (GB)
Title: Improvements in electric power transmission
IPC: H02H7/26
Filed: 1927-03-12
Abstract: 293,811. Taylor, A. M. March 12, 1927. Addition to 277,049. Transmission conductor arrangements.-In high-tension three-phase transmission systems with one or more pairs of three-core feeders paralleled only through the low-tension windings of transformers proper to each feeder and in which similar phases are balanced in split-conductor transformers SCT, whether one feeder transformer is reversed, as in the parent Specification, or not, the secondary winding of each splitconductor transformer feeds two poralized relays PR1, PR2, in series and these relays discriminate between sound and faulty feeders to trip the latter only. The polarized relays each have contacts SCC which short-circuit the associated relay as soon as one moves, so that the relay on the sound feeder is then irresponsive to out-ofbalance currents due to the switching operation. The feeder circuit-breakers may operate auxiliary contacts CM2 which place the sound feeder on leakeage and overload protection. The split-conductor transformers may be placed on the neutral side of the feeder transformers to reduce insulation requirements and extend protection to their windings. A transformer NT1 may be inserted in each earth connection at the neutral point of the high-tension transformer windings, the secondaries of these transformers being mutually reversed and in series with the relay coils so that any return fault current adds an e.m.f., say ten times that due to the split-conductor transformer, which increases the sensitiveness of the relays to such faults and reduces their sensitiveness to unimportant out-ofbalance currents in the feeders, such as those due to "through" faults. These arrangements may be repeated at the substation end, but switches in the neutral-earth connection would be closed only at the power-transmitting end under the control of reverse-power relays. Automatic circuit-breakers on the feeders may be dispensed with at the distant end and the relays at the generating end made correctly responsive to a distant-end fault by means of a substation relay and a potential transformer arranged to vary the potential of the neutral point at the substation. A low-tension transformer PT1 feeds a step-up transformer PT2 connected to a feeder terminal and, through a spark-gap SG, with earth. A potential is applied across this gap and an earth fault simulated by a relay AER1 which short-circuits the primary of the transformer PT2 when fault conditions arise in the substation transformers. The operating coils of the relay AER1 are fed by windings on the central limbs of two three-limbed transformers ST1, ST2 the outer limbs of which are excited by the sum of the currents induced in windings RT1, rt1 primary and secondary transformer circuits and by the same of the currents in windings lt1, lt2 respectively; other windings on the outer limbs carry currents representing the difference between aforesaid currents and the unbalancing of these currents due to a fault causes a flux in the central limb of either transformer which operates the relay. The potential transformer PT2 may comprise three separate series-connected transformers, so that a fault therein may not operate the generating station relay. The generating station arrangements include relays AR1 such that the closure of a trip coil current by an attendant to cut-out one of a pair of feeders will short-circuit the polarized relay PR1 of the other feeder in advance and will put that feeder on overload and leakage protection. The substation arrangements provide that the closure by an attendant of the trip coil circuit of a low-tension switch will operate a relay AER1 to cause the simulation of an earth fault and thereby open the appropriate feeder switch at the generating station. According to the first Provisional Specification the means of altering the operation of the polarized relays under reverse-power conditions include means for damping the action of the relays, such as a series inductance enclosed in a movable copper cylinder, and means, for example, motor operated, controlled by a reverse power relav to reverse the connections of the polarized relays to the split-conductor transformer and simultaneously vary the damping of the said relays so that those at the sending end shall always act in advance of those at the other end.