The Zero-Flux Current Transformer [includes discussion]

A method is described of reducing the core flux of a current transformer by supplying the necessary voltage to the secondary circuit. For one load current, the flux may be eliminated, giving zero errors, and at other loads the ratio and phase angle are so small as to be difficult to detect in many cases. Applied to reasonably good transformers, extremely high accuracies are obtained, with small weight and bulk. The scope of low ratio bushing-type transformers is also increased, but the addition of triple-frequency auxiliary flux still further extends the use of the method, and test results are given for ratios down to 20/5 amperes.