The effect of capacitance on the design of toroidal current-transformers

The design of high-accuracy current-transformers for operation at rated currents of less than five amperes or at frequencies higher than 50 c/s should include consideration of the effects of capacitance. The performance of transformers is shown to be dependent not only on the magnitudes of the self-capacitances of the windings and the mutual capacitance between the windings, but also upon the distribution of the capacitance between windings. With the normal method of winding a transformer layer by layer, the capacitance between windings is concentrated between the outer turns of the inner winding and the inner turns of the outer winding. This is shown to be an unsatisfactory distribution, and the performance is improved if the capacitance between windings is symmetrical and distributed as much as possible. A satisfactory distribution of capacitance between windings and also low self-capacitances are obtained by adopting a winding technique well known in radio work, in which the order of winding the turns is such that adjacent turns are as nearly as possible consecutive turns. Further improvement of the transformer performance may be obtained by increasing the thickness of insulation between windings and using insulation of low permittivity. It is shown that the thickness of this insulation may, with advantage, be about one-eighth of the mean depth of the secondary winding. Formulae are given for estimating the capacitances and leakage reactances of the transformer. It is shown that the leakage reactance of the secondary, when both windings are uniformly distributed round the core, is negative. The performance of multi-ratio transformers, especially of the tapped primary type, is necessarily less satisfactory than that of single-ratio transformers, but considerable improvements may be made by adopting an arrangement of the winding which is symmetrical with regard to the secondary for all ratios. Good winding arrangements for multi-ratio transformers with primary wind- ings of the series-parallel type or with secondary windings of the tapped type are also suggested.