DC magnetisation of transformers

A sufficiently high DC component in the current on the secondary side of a transformer causes distortion of the primary side current. Prediction of such distortion relies on the availability of magnetisation curves that extend deep enough into saturation. A simple test, carried out at rated voltage, is proposed which allows the magnetisation curve to be deduced from measured data. In addition, a systematic procedure has been developed to evaluate the primary-current distortion in both single- and three-phase transformers. Comparison between theoretical predictions and practical measurements are made, with reasonable agreement between the two.