Modeling of losses in power transformers from dc to the low kilohertz range

Sammanfattning: The development of HVDC converters, local generation and tractions systems in-creases the demand for power converters. While a lot of studies have been conducted on the power electronics, there are still open questions concerning the contained power transformer that operates at a few kilohertz. As power and loss increase with frequency, a smaller transformer can be constructed supposing cooling is increased or losses are reduced.New modeling approaches have been studied and compared with existing models. This concerns the electrical, magnetic and thermal behavior of the transformer. Common transformer models are based on inductances with effective permeabilities. Such mod-els cannot use hysteresis models. A new transformer model based on the lumped ele-ment approach has been derived. The model combines magnetic hysteresis, eddy cur-rents and thermal diffusion such that their interaction is simulated correctly. The model has been optimized with respect to accuracy and the required number of lumped elements. A field homogenization procedure for a magnetic lamination with eddy currents is presented. The previously suggested magnetic hysteresis model has been further developed and numerically optimized and the agreement with the one-dimensional measurements increased. The overall transformer model was run for a realistic geometry. A novel 2D measurement set-up for characterization of magnetic sheets has been designed and constructed to operate up to a few kilohertz. The ex-perimental verification of the set-up proves that the magnetic field amplitude and es-pecially homogeneity increased considerably. The latter allows larger pick-up coils. The set-up is much smaller than the previous and allows small samples, which is important in the medium frequency range, where a core material often is manufactured on small rolls. In addition, the necessary electric excitation decreased substantially. Measure-ments are required to characterize and evaluate prospective transformer core materials such that they can be used as a material database for a simulator program. A measure-ment program including automatic demagnetization has been written. One- and two-dimensional magnetic measurements on non-oriented and grain-oriented materials were performed. Rotational power loss was measured for different axis ratio of the rotating magnetic field. The resulting hysteresis loops were explained. The hysteresis model has been extended for isotropic materials and the result agrees with the two-dimensional measurements.