Voltage Distribution in the Windings of Medium-Frequency Transformers Operated With Wide Bandgap Devices

Abstract

The short rise time observed in the PWM voltages generated by ultra-fast wide bandgap devices increases the amplitude of voltage harmonics at higher frequencies. These harmonics can excite the resonances of Medium-Frequency Transformers (MFTs), resulting in overvoltages inside the windings during continuous operation. Without further measures, these overvoltages can lead to unexpectedly high electric fields in the insulation material, which can result in partial discharge, accelerated ageing and possible failure of the MFT. To avoid these effects, the mechanism causing the overvoltages has to be understood and quantified during the design process. Based on this, the MFT can be designed in a way that the overvoltages vanish or are tolerable. Therefore, the voltage distribution inside the MFT windings is analysed by a fully-coupled multi-conductor transmission line model, which includes the damping effect of electromagnetic losses in the litz wire and in the core. This method is verified by measuring the transfer functions of the voltage to ground of individual turns and their voltage waveforms during continuous operation. The waveforms indicate repeating overvoltages inside the windings. A guideline for the design verification and a simplified approach to speed-up the modelling process are presented.

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