Patent GB-309780-A (GB)
Title: Improvements in or relating to tap-changing switches for electric transformers
IPC: H01F29/04
Filed: 1928-06-16
Abstract: 309,780. Siemens - Schuckertwerke Akt.-Ges., Kurda, K., Wernicke, W., and Oertel, C. June 16, 1928. Regulating transformers by tap - changing- switches.-The contacts of the contact rows of two step selectors are connected in pairs to the tapping points of the regulating winding, and the step selectors are connected by gearing which, when the voltage is being regulated, first switches one selector one stage and then the other selector in the same direction to the same stage, the selector being connected to the load by a switch which renders each selector alternately dead upon each switching step. The tappings 11 - - 13 (Fig. 1) are connected each to a pair of contacts in two rows of contacts 111 - - 113, 211 - - 213 of two step selectors 100, 200, the contacts being connected through two normally-opposing primary windings 507, 508 of a transformer to contacts 502, 503 of a load switch 500. The switch 500 and selectors are connected by gearing that automatically causes the correct sequence of tapchanging operations. The switch 500 moves in the direction of the arrow to leave the contact 502 and simultaneously the selector 100 moves to contact 112. Continuing rotation in the same direction, the switch 500 connects contacts 502, 503 and the resulting current now flowing in the local circuit through the primaries 507, 508 gives rise to a current in the secondary 509 which is connected to an equalizing resistance 510, the resistance and transformation ratio being so chosen that the equalizing current does not exceed half the normal load current. The switch 500 is then rotated to leave the contact 503 and simultaneously the selector 200 is moved onto contact 212, the switch 500 finishing up in the original position shown. In Fig. 2, the equalizing resistance 505 is connected directly in the load circuit and is made equal to the quotient of the step voltage and load current. Two further contacts 501, 504 are provided on the switch 500 and are bridged when the resistance is taking equalizing current, the operation being in other respects similar to Fig. 1. In the event of a short-circuit, the danger of the resistance being burnt out is minimized by shunting with a saturated choke, so that a small rise of voltage across its terminals causes it to take the larger part of the current increase. Two modifications of this form of invention are shown, in one of which the resistance is connected across the secondary of a transformer, the primary of which is connected across the terminals 503, 504, whereas in the other, another primary is connected across the terminals 501, 502, normally opposing the first primary. The resistance may in such cases be located outside the transformer oil or even outside the transformer tank, so that the heat generated thereby due to short-circuits is kept from the oil and replacement of the resistance is made easy, but protection of the resistance may be ensured by arranging for the transformer feeding it to be saturated at normal load. If a circuit is connected to a regulating winding, which can work as a primary or secondary, the equalizing resistance should be divided equally in the circuits of both selectors, or switching devices should be arranged to switch the resistance, or its feeding transformer, into the appropriate selector circuit (Figs. 6-10, not shown). The switching-over may be effected manually or by an energy flow directional relay, and ensures that the resistance is always in the branch of the load switch 500 that carries the difference of the equalizing and load currents. Further relief (Fig. 11, not shown) can be got by dividing the switch output between two synchronously-moving load switches in parallel, these switches being connected to the load circuit through opposing windings of a choke, the mid-point of which windings is the load terminal. 'To increase the range of regulation (Fig. 12), there is employed in addition to a number of small voltage tappings a number of large voltage tappings, the voltage across which equals the sum of the voltages across the small voltage tappings. These small voltage tappings 11 - - 19, forming a close-step winding 1, are connected to the contacts of two rotary selectors 100, 200 operated from the switch 500 by maltesecross gearing 120, 220. The large voltage tappings 21 - - 28, forming a big-step winding 2, are similarly connected to selectors 300, 400, which are operated from the first pair by maltese-cross gearing 310, 410. As shown, the windings 1, 2 are in series with one another and with a winding 3, and to reduce potential from a maximum, the switch 500 is operated to cut out the parts of the winding 1 in succession until the selectors rest on contacts 119, 219 respectively. The next movement of the switch 500 restores the selector 100 to contact 112, but by means of the gear 310 the selector 300 is moved to contact 322. The continuation of the movement of the switch 500 brings the selector 200 to contact 212 and, by means of the gear 410, the selector 400 to contact 422. The section between tappings 21, 22 is cut out, but the winding 1 is re-introduced into the circuit. The cycle of operations is now repeated, the steps of the winding 1 being again successively cut out until the selectors 100, 200 again reach contact 119, 219, the next step, by means of gearing 310, 410, advancing the selectors 300, 400 to the next contacts 323, 423, and cutting out the second step of the big-step winding 2. This process goes on until the whole of the winding 2 is cut out, the selectors 300, 400 being on the contacts 328, 428, respectively. The windings are next coupled in opposition, the next movement of the switch 500 moving the selectors 100, 300 to contacts 112, 321, and changing over reversing switch 6 from contact 61 to contact 62 and reversing switch 7 from contact 71 to contact 72. The next part of the movement of switch 500 restores selectors 200, 400 to contacts 212, 421 and changes the intermediate switch 5 from contact 51 to contact 52. This cuts the winding 4 into circuit between contacts 119, 219 and the selector 400, the winding 4 being of equal voltage to that of the winding 1 and in opposition to the winding 3. This winding 4 is normally dead, but is put in circuit at the contacts 119, 210 to carry current instead of the winding 1 when the latter is being moved from one step to the next of the winding 2. The process of opposition switching is similar to that of the series switching, the parts of the winding 1 being successively inserted in opposition to the winding 3, these being then replaced by winding 4; and finally one step of the winding 2 and one step of the winding replaces the winding 4. In another modification (Fig. 13, not shown), the close-step winding is divided into two parts by omitting one step, and the start of one part is connected to one selector, and the finish of the other part to the other selector, of the by-step winding. The steps of one part of the close-step winding are preferably sandwiched between those of the other part, and a high resistance preferably connects the two parts to prevent capacity effects. A constructional example of a switch is shown in Fig. 14, in which the load switch 500 is driven by bevel gearing 513. The disc 512, which is coupled to the switch 500, carries pins 120, 220 which mesh with maltese-crosses 121, 221, connected by means of a shaft 122 and sleeve 222, respectively, with the brush 223 and collector ring 224 of the selector 200, and with the brush 123 and ring 124 of the selector 100, their co-operating contacts being connected to the contacts of the load switch.