Selection of Windings
The winding configuration is mainly selected according to the voltage class and capacity of the winding. Meanwhile, consideration should also be given to the electrical strength (mainly impulse performance) and mechanical strength of various types of windings, as well as the characteristics of the manufactured transformer, such as the size of the heat dissipation surface and manufacturability.
Cylindrical windings feature easy winding, favorable impulse voltage distribution and high heat dissipation efficiency of oil ducts. However, their insulating parts are relatively complex. The stability of single-layer end support is poor, their mechanical strength is inferior to that of disc windings, and the temperature difference inside the winding can sometimes be large. In particular, cylindrical windings for high voltage with a large number of turns have complicated insulation and are rather difficult to wind. Helical windings also have the advantage of easy winding. They are mostly used for low-voltage windings and occasionally for tap windings. Helical windings cannot be adopted when the number of turns is slightly large due to the limitation of axial height, so their application has limitations. Continuous windings can meet the requirements of various voltages and different capacities in a wide range. They have high mechanical strength and good manufacturability for double-layer structures, yet their impulse voltage distribution is unsatisfactory. The cross-sectional shape of conductors and the number of parallel wires exert a great influence on manufacturability.
Interleaved continuous windings and inner-shielded interleaved continuous windings are widely applied in 60 kV and 110 kV classes. They boast high mechanical strength and good manufacturability, with slightly better impulse voltage distribution than ordinary continuous windings. Similarly, the cross-sectional shape of conductors and the number of parallel wires also greatly affect their manufacturability. Interleaved windings are mainly used in 220 kV transformers. Their advantages include good impulse voltage distribution and high mechanical strength, while the disadvantages are numerous conductor joints and cumbersome winding. At present, winding selection is generally carried out in accordance with standards. Nevertheless, many cases in practical design break the limits specified in the tables, and some types of windings not listed in the tables have also been put into use. Therefore, designers need to make flexible decisions according to specific conditions.
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