Transformer Windings Part2

5. Transposition
(a) For helical windings, usually three transpositions are provided. The complete transpositions Fig. 8(a) is provided in the middle of the windings.Tow partial transpositions are provided, one at 25% of turns Fig. 8(b) and the other at 75% of turns Fig. 8(c). In complete transposition, each conductor position is varied symmetrically, relative to the middle point, whereas in partial transpositions, tow halves of parallel conductors are interchanged in the positions: The upper have becomes the lower, and vice versa. Such a transposition needs extra space in the height of the coil.


(b) Wit a multi-start helical winding, the transposition can be achieved by using rotary transposition. Fig. 9 shows transposition in a double -start helical winding. By this arrangement, every conductor occupies every position by turn and thereby complete equalization of impedance is possible. Also, there is no need for extra space in the coil height.

(c) For disc windings having more than one conductor in parallel, transposition is made between the conductors by changing their mutual position at each cross-over from one section to another Fig. (10).

6. Interleaved Disc Winding
       A disadvantage with the continuous disc winding is that their strength against impulse voltages is not adequate for voltages above, say, 145 KV class. The impulse voltage withstand behavior of disc coils can be increased if the turns are interleaved in such a fashion that tow adjacent conductors belong to tow different turns. Fig. (11) shows such a winding in which interleaving has been done in each pair of discs. It will be noticed that it is necessary to have 2n conductors in hand for winding when n is the number of conductors in parallel. Conductors of turns 8 and 9 are joined by brazing. A cross-over is given at the bottom of the disc.

       Apart from interleaving between every double-disc, it is also possible to have more number of discs (say four) in each interleaved group (Fig. 12).

        This gives further improved behaviour against impulse voltage, though there are concomitant increased complexities.
       Interleaved windings require more skill and labour than plain continuous disc windings. Sometimes a part of the winding is interleaved while the remaining part is plain disc, so as to combine the advantage of better impulse withstand at the high voltage end of the winding and reasonable labour cost for the winding as a whole. These are known as partially interleave windings.

7. Rib Shielded Windings
       An alternate way of increasing the series capacitance without actually interleaving is achieved in Rib shielded windings. Floating shields are provided inside continuous disc windings, and are comparatively easier to manufacturer, when compared to interleaved disc windings. The shield wires are not conductively connected to circuit (Fig. 13).

8. Shield Layer Windings
       This type of winding is generally used for star-connected transformers having graded insulation and for voltages greater than 132 KV class. The winding consists of a number of concentric spiral coils  arranged in layers. The layers are graded in lengths from the longest at the neutral end (innermost layer) to the shortest at the line end (outermost layer). The layer are arranged between two concentric cylindrical shields, connected one to each end of the winding ( Fig. 14). All these layers are connected in series, wherein tow schemes are possible, viz. Parallel layer type (Fig. 14) and tapered-layer type (Fig. 15).
       The layers are separated by oil ducts and unbonded paper cylinders. During winding, the latter are arranged to extend well beyond the turns of the layer and afterwords these extensions are petalled and bent over at right angles to form insulating flanges between succeeding layers. These flanges provide an insulation system to ground, which increases progressively from the neutral end to a maximum for the line-end of the coil.

       The winding layers and the shields form a series of capacitors and are so dimensioned that it results in substantially equal capacitances in series. This ensure a relatively uniform distribution of surge voltages throughout the winding. When the winding current and the density of the leakage flux are not very high, the winding can be wound with conductors of rectangular cross-section. For large currents and higher density of leakage flux, transposed conductors are used.

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