Abstract
The short rise time observed in the PWM voltages generated by ultrafast 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 aging, 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 analyzed by a fully coupled multiconductor transmission line model, which includes the damping effect of electromagnetic losses in the litz wire and 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 modeling process are presented.
Highlights
In recent years, the need for highly efficient interfaces between the Medium-Voltage (MV) AC grid and the LowVoltage (LV) DC buses has significantly increased hand in hand with the rapidly growing amount of high-power LV DC loads and sources [1]–[7]
These steps can be summarized as follows: 1) The harmonic spectrum of the source voltages applied to the MediumFrequency Transformers (MFTs) is obtained by the Fourier transform; 2) The Multi-conductor Transmission Lines (MTLs) model of the MFT is defined by computing its parameters: a) The matrices K, H and T; b) The matrices R and L for every frequency ω of the source voltage spectrum; c) The matrix C; 3) The harmonic spectrum of the turn voltages y is obtained from (5); 4) The waveform of each turn voltage is synthesized in the time domain by the inverse Fourier transform of the corresponding spectrum
This paper highlights the implications of using wide bandgap devices on the electric stress in the insulation of medium-frequency transformers, by analysing the occurrence of overvoltages inside the windings resulting from the fast rise time in the Pulse-Width Modulation (PWM) voltage
Summary
The need for highly efficient interfaces between the Medium-Voltage (MV) AC grid and the LowVoltage (LV) DC buses has significantly increased hand in hand with the rapidly growing amount of high-power LV DC loads and sources [1]–[7]. If the design of an MFT does not account for these effects, the high-frequency harmonic of the PWM can excite the internal resonances of the MFT windings, resulting in repetitive ringing overvoltages during normal operation. These overvoltages can intensify the electric field beyond the dielectric strength of the insulation, leading to the inception of PD and reducing the lifetime of the transformer [28], [30], [42], [43].
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