Experimental and Numerical Investigation of Flux Density Distribution in the Individual Packets of a 100 kVA Transformer Core

The accurate measurement and calculation of transformer no-load losses is of high importance to transformer users and manufacturers alike. There are various ways of calculating no-load losses of transformers based on empirical formulas and experimentally obtained coefficients but these tend to mostly apply to known and tested core designs. The most important factors that contribute to the no-load loss of transformer cores are the grade of steel used, the core and corner geometry and stacking configuration. This paper presents results and analysis for a 100 kVA, 3-phase model transformer core, consisting of 11 packets of different width laminations, over a range of flux densities up to 1.8 T. The results show the average magnetic flux desnity across the core cross section as well as the magnetic flux density of the individual packets. Using this information in combination with the magnetising current the no-load loss can be derived for the whole core as well as for the individual packets. The paper also shows numerical calculations of the excitation current required to magnetise the core. The FEA software used was developed in ALSTOM and allows the calculation of flux density distribution as well as the determination of the magnetising current necessary to magnetise the core at different flux density levels. It is believed that the combination of measurements and FE modelling will enable the calculation of local power loss thus allowing for optimisation of the various core parameters.