Patent GB-1408908-A (GB)
Title: Onload tap-changers for very high voltages comprising selector  switches and changeover selectors
IPC: H01H9/00
Filed: 1972-09-07
Abstract: 1408908 Transformer tap-changing arrangements SMIT NIJMEGEN ELECTROTECHNISCHE FABRIEKEN NV 7 Sept 1972 [9 Sept 1971] 41634/72 Heading H1T An on-load tap-changer, for a high-voltage regulating transformer, comprises a selector switch A having a series of fixed contacts 1 to 7 equally spaced in an arc and connected to the tappings of a fine regulation winding 14, the distance outside the series between the last and first contacts 7 and 1 being greater than that between any pair of contacts within the series; at least two rigidly coupled switching contacts 8, 9, 10 being arranged to sweep around the series of fixed contacts stepwise continuously in either direction; at least one transition impedance 11, 12, interconnecting the switching contacts, the switching contacts making at least two steps in passing beyond the series from the last to the first contact (or vice versa) and being currentless in, or adjacent to this intermediate position; this being achieved by the association with the selector switch of a further switching arrangement which enables the output potential to be maintained. As shown in Fig. 1 the further switching arrangement comprises a switch B having ganged movable contacts 20, 21, which are capable of engaging fixed contacts 17, 18, 19 in turn. The end of a main winding 15 is permanently connected by a contact 16 to the contacts 20, 21. With the moving contacts 8, 9, 10 of the selector switch in the position shown and the windings 14 and 15 connected in the same direction, these have a common terminal connected through the switch contacts to the network conductor 13. At this stage the potential on 13 is that at the top of winding 15 via 8, 1, 17, 20 and 16. To reduce voltage by the steps of winding 14 the bridging switch contacts 20, 21 are first moved to contact 18, thus maintaining the potential at 13 at the same level via 18, 20 and 16, whilst disconnecting the winding 14. Then selector switch A is moved two steps counter clockwise into the intermediate position between 1 and 7 and then on to contact 7, wherupon the potential at the top of winding 14 is brought to that of line 13 and hence contact 16. The bridging contacts 20, 21 are moved to contact 19, so that the potential at 16 is now connected to the top of winding 14 via 16, 20, 21 and 19, and thereafter the voltage on 13 can be reduced from the level at 16, by the steps of the fine winding, as the selector switch A moves counter-clockwise from 7 to 1. The procedure is reversed to raise the voltage. In the embodiment of Fig. 2, in which a coarse regulation winding is associated with the fine winding 14, the selector-switch A is as before but the switching arrangement comprises a changeover-selector C1 separate from but rigidly mechanically coupled with a coaxially-mounted switch C2. Coarse regulation windings 22 and 23 each produce a voltage equivalent to that produced by the total winding 14. To raise the voltage the switch A is first moved around its series of contacts 1-7 thus providing 6 fine steps. Thus the combined switch C1, C2 is rotated counter-clockwise until contact 32 reaches contact 30, contact 28 being near the end of contact 25, so that the network conductor 13 is connected with the end of winding 14, which is in series with winding 15, and with bridges winding 14 so that its moving contacts may be brought into the intermediate position between 7 and 1. The combined switch C1, C2 is then rotated further until 28 reaches 26, 32 still being on 30. Selector switch A is then moved to contact 1 and C1, C2 is moved further so that 28 is in the middle of 26 and 32 is intermediate 30 and 31, thus connecting the fine winding 14 in series with the end of winding 22 for a further series of fine-step voltage rises. In a simplified embodiment (Fig. 3, not shown), the second coarse regulating winding is omitted and the construction of the combined change-over selector and bridging switch is altered so that a single set of fixed contacts cooperating with two pairs of cross-connected moving contacts all in a single plane is used. To avoid the voltage of the fine winding 14 floating during the disconnected period a further contact (42, Fig. 4, not shown) is provided in the intermediate position and is connected to the mid-point of the winding via a high resistance.