Electrical shielding of MV/MF transformers subjected to high dv/dt PWM voltages

The introduction of Medium-Voltage (MV) Silicon-Carbide (SiC) devices enables the usage of higher power converter operating voltages, switching frequencies, and commutation speeds. This implies that Medium-Frequency (MF) and High-Frequency (HF) transients are applied to passive components, and particularly to inductors and transformers. Together with the operation at medium voltage, this leads to challenging situations with respect to Common-Mode (CM) currents, parasitic resonances, insulation coordination, and EMI. The electrical field is the key parameter for the aforementioned effects. Therefore, this paper analyzes the electric field distribution (in the insulation, at the surface, and in the air) for a ±3.5kV/±400V, 50kHz, 25kW MV/MF transformer employed in a Solid-State Transformer (SST) demonstrator. For reducing the field, a suitable shield is designed. It is found that the shield drastically reduces the field at the surface of the transformer and in the air without increasing the losses.