Frequency Dependence of Transformer Losses

Abstract

In this thesis, theory concerning skin effect, eddy current and magnetic hysteresis in transformers has been investigated. An electric equivalent model of a transformer based on this theory is proposed. The validity of this theory and the usefulness of the model have been tested through laboratory measurements. In these measurements three scenarios where studied: a no load test of a transformer with varying frequency, a transformer loaded with a diode rectifier and last, the transformer was loaded with a pulse generating load. The model has been used to perform a prediction of the energy savings available through the use of amorphous transformer cores, and active filters. Finally a comparison of the two most widespread down rating standards for transformers has been made. Analytical calculations suggest that the core impedance of a transformer increases as the square root of the frequency. A no load test with varying frequency supports this. The analysis also shows that at high frequencies the resistance of a copper winding increases as the square root of the frequency. The proposed model proved more accurate at higher frequencies compared to lower frequencies. A comparison of the European down rating standard with the standard proposed in this report shows that the two standards behaves similarly for lower frequencies, but that the European standard is stricter for the higher frequencies. The model suggests that the use of amorphous steel in the transformer core decreases the core losses by 48% using assumptions stated in the report; the model also shows that the use of amorphous steel decreases the losses that are generated by load harmonics by 8%.