Current summations with current transformers

The behaviour of current transformers in parallel, and of similar circuits employing summation transformers, is studied for different conditions of load distribution. After the existing theory concerning idle currents has been rejected it is demonstrated that the overall error is very nearly constant for a given total load, no matter how the current may be shared among the line transformers. A complete equipment may thus be regarded as a single current transformer and the circuit parameters are considerably simplified, enabling practical formulae to be established for determining the overall errors. In the latter part of the paper it is shown how the effective burdens on the individual transformers may be estimated and how test results at these burdens may be used to compute the overall errors. Worked examples are given, together with confirming test results.