Leakage phenomena in ring-type current transformers

The paper describes an investigation of the effects of leakage flux on the performance of ring-type current transformers with uniformly-distributed secondary windings. If the primary ampere-turns are concentrated on one part of the core, large leakage fluxes are superposed on the working flux, and these cause premature saturation of parts of the core, with a resultant rapid rise in the ratio of primary to secondary current. A copper shield of special design was made for a 10000/5 transformer, and when fitted it was found to reduce the leakage fluxes to one-frith of the value existing without the shield. Saturation of the Gore, which started at about two-fifths of full load without the shield in position, did not begin until twice full load was reached when the shield was fitted. The ratio error of this transformer was 35 per cent at full load without the shield in position, and was 1 part in 10000 at full load with the shield in position. A shield of similar design fitted to a 20000/5 transformer reduced the ratio error at load from 67 per cent to less than 1 part in 10 000.