Insulation Design of Medium Frequency Transformers
Recent advances in the field of wide-band gap (WBG) semiconductors and the associated combination of high blocking voltages and fast switching speeds enable the application of power electronic converters up to the medium voltage (MV) range. Particularly, isolated medium voltage converter structures as for example solid state transformers (SSTs) are of interest as they provide a galvanic isolated power conversion. The high operational frequency allows for a significant reduction of the magnetic core size of the SST’s medium-frequency transformer (MFT). However, the required large insulation distances make the MFT insulation a crucial part in terms of volume and safety.
At HPE models for predicting the maximum electric field which allow a comprehensive design of the transformer insulation are investigated and developed. The emphasis is put on rapidly executable yet accurate models to incorporate them within the transformer optimisation. These models also need to consider potential internal overvoltages introduced by the pulse-width-modulated waveforms at the transformer windings.
The field models are based on analytical methods and can describe the 2D electric field at very small radii. The methods are combined with a complex high frequency model of the transformer, that is based on the frequency dependent transformer impedance matrix. Both models have been verified using elaborate finite element simulations and are tested on prototypes.