Patent GB-348028-A (GB)
Title: Improvements in protective arrangements for electrical distribution systems
IPC: H02H3/17
Filed: 1930-02-01
Abstract: 348,028. Protective cutout systems. ASSOCIATED ELECTRICAL INDUSTRIES, Ltd., Bush House, Aldwych, London.-(Assignees of Ludwig, L. R.; 1310, Hay Street, Wilkinsburg, Pennsylvania, and McLaughlin, H. A.; 71, 38th Street, Irvington, New Jersey, both in U.S.A.) Feb. 1, 1930, No. 3503. Convention date, Feb. 4, 1929. [Class 38 (v).] In a protective system more especially for traction circuits of the type in which high-frequency control currents are superimposed upon the line currents for the operation of protective relays, in order that the application of the high-frequency current may entail no transformer losses the highfrequency generator, in parallel with a blocking shunt, is connected between ground and the neutral point of an inductance such as the secondary winding of a power transformer. As shown in the Figure the high-frequency generator 35 shunted by the filter 36 is connected between ground and the secondary mid-point tapping of a transformer 14 interposed between the A.C. generator 13 and transmission line 11. Distributing transformers 15, 16, 17 are connected to the transmission line, their earthed secondary windings supplying the sections 27, 28 of the traction system 10 through pairs of parallel feeders controlled by circuitbreakers 21 .. 26. High-frequency currents are transmitted to the load circuits through conductors joining the mid-points of the primary windings of transformers 15, 16 to the insulated ends of their secondary windings, these connections being made through blocking impedances 37 tuned to pass the H.F. current but to limit the flow of L.F. power current. The parallel feeders supplying the traction circuits each contain branched impedances 38, 39 which offer between terminals little resistance to the passage of H.F. or L.F. currents. The righthand branch of each grouped impedances, however, is designed to have a high impedance to H.F. currents and the left-hand, includes a condenser and relay winding which forms a resonant circuit only at the frequency of the generator 35. When an earth fault occurs on any feeder it causes a flow of H.F. current in the series resonant circuit so causing the operation of one of the tripping relays 29 .. 34 corresponding to the affected feeder, which is thereby cut out. In cases where long transmission lines are employed in which the H.F. currents at the end of the line may be insufficiently strong to effect operation of the relay, the points remote from the source of power may be provided with an H.F. amplifying arrangement as shown in connection with the transformer 17 in which the superimposed H.F. currents energize the field 41 of an A.C. commutator generator 43 driven by a motor 42 connected to the load circuit. The L.F. blocking impedance is placed in series with the armature 40 of the generator 43 and a condenser 41<1> neutralizes the inductance of the winding 41. In a modification (Fig. 2, not shown) the supply of the so-called three-wire type, the transmission circuit being connected to the distribution circuit through auto-transformers, relays in the supply and distribution circuits being engaged in a manner similarly to that previously described. In this type of system the auxiliary H.F. generator is connected between the mid-point or other suitable point of the secondary of the high-voltage supply transformer and earth, the H.F. generator being shunted as before by a resonant circuit designed to offer maximum impedance to current of high-frequency. This high-voltage supply winding is grounded at a relatively low voltage tap through the impedance and to prevent L.F. currents circulating between this tapping point and the central tapping a suitable series resonant circuit is included between the H.F. generator and the mid-point tapping, through which H.F. currents may pass unimpeded but through which the passage of L.F. is prevented. The distributing auto-transformers have similar midpoint tappings grounded through corresponding H.F. blocking shunts. Additional series resonant circuits for preventing L.F. current flow between tappings are provided for the distributing transformers, arranged and connected in a manner similar to that corresponding to the winding of the supply transformer.