With the advance of superconducting wires for a. c. power use, researches on superconducting power transformers are increasing. These transformers can be divided into two types; iron-core type(1) (2) and air-core type(3)_??_(9). The latter type has advantages such as absence of iron losses and magnetic saturation, and it has more possibility of reduction of size and weight. But the air-core transformer has a large magnetizing current due to the absence of iron core. Thus, researches have been carried out on the possibility of using the air-core transformer as a transformer that can also serve as a shunt reactor in a power transmission system. However, the operating characteristics of the air-core transformer, such as voltage regulation and reactance voltage, are not sure at present.
In this paper, the equivalent circuit without losses is first proposed. Since this equivalent circuit is expressed by means of the magnetic coupling factor and self-inductances of windings, the effect of these parameters on the transformer characteristics can be investigated with ease. Then, based on this equivalent circuit, the per-unit expressions for the air-core transformer characteristics are derived and the characteristics are analyzed in detail. The validity of the theoretical results are confirmed by experimental results obtained by the use of an experimental superconducting transformer.
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