There are several places in a power system where connections from one voltage level to another do not entail large transformer ratios, for example, 400/275, 500/345, 725/500 kV, and then the autotransformer is used (Figure 2.12).
In the autotransformer only one winding is used per phase, the secondary voltage being tapped off the primary winding. There is obviously a saving in size, weight and cost over a two-windings per phase transformer. It may be shown that the ratio of the weight of conductor in an autotransformer to that in a double-wound one is given by (1 — N2/N1). Hence, maximum advantage is obtained with a relatively small difference between the voltages on the two sides. The effective reactance is reduced compared with the equivalent two winding transformer and this can give
Figure 2.12 One phase of an autotransformer, - -
rise to high short-circuit currents. The general constructional features of the core and tank are similar to those of double-wound transformers, but the primary and secondary voltages are now in-phase.