GB732654A - Improvements in or relating to on-load electric transformer tap changing systems - Google Patents

Abstract

732,654. Transformers. JANSEN, B. March 22, 1951, No. 6934/51. Class 38 (2). An on load transformer tap changing system comprises a step selector having a series of contacts connected to each transformer tapping, a load change-over switch having a plurality of switch blades co-operating with main and auxiliary contacts and operable simultaneously with the step selector, and at least one inductive current divider having a number of cores corresponding to the number of blades, of the load change-over switch, each core being supplied with a pair of magnetically opposed windings, the connections between the step selector, the load change-over switch and the windings on the inductive current dividers being such that the load current divides equally between the plurality of switch blades thus reducing the wear on each blade. As the step selector Wal... Wa3; Wbl ... Wb3 is moved to change from tap B to tap A on the transformer winding the load change-over switch blades L1 ... L3 move from contacts Hb1 ... Hb3 to contacts Ha1 ... Ha3. If during the movement of the blades L1 ... L3 one blade leaves its associated contact before the other blades (in the Fig. 1 the dotted lines indicate blade L1 leaving contact Hb1 before the blades L2, L3 leave contacts Hb2, hb3) the current path b10 from the step selector would tend to be interrupted, thus the windings b32, 11 on the current divider Stb would tend to be de-energized, thereby upsetting the normal magnetic balance which exists in the cores when the current is equally divided. Thus a voltage drop will exist in windings b31, b12 and thus voltages will be induced in windings b32, b11 which will produce a potential difference across the break between switch blade L1 and contact Hb1 sufficient to cause an arc to be struck across the break, thus to restore the current flow and the equality of current division. The cores of the current dividers may be separate as shown to the left of the Figure or have common yokes as shown on the right. The load change-over switch may be provided with additional contacts so connected that current only flows in the current dividers during the switching periods, or if the divider is connected between the load change-over switch and the current take-off AL only one divider is necessary. The normal auxiliary resistors and auxiliary contacts ha1 ... ha3 ; hb1 ... hb3 are provided as is normal in the tap changing art.