Effects of Spike Voltages Coupling With High dV/dt Square Wave on Dielectric Loss and Electric-Thermal Field of High-Frequency Transformer

The insulation of high-frequency transformers (HFTs) has a significant impact on the safety and reliability of high voltage power electronic transformers (PETs). The transient voltages from the rapid turn-off and turn-on of the high voltage power semiconducting device increase the insulation stress and loss, resulting in thermal stress distortion. However, few studies have presented an accurate evaluation of the dielectric loss, especially under high dV/dt square waves. This paper focuses on the dielectric loss calculation in HFT insulation by analyzing the loss from a high-frequency square wave with spike voltages. A step function that is equivalent to spike voltage superposition is proposed for the quantitative calculation of the dielectric loss of epoxy resin. A cutoff frequency ( $f_{\mathrm {c}}$ ) is defined to indicate high dV/dt characteristics. A similar narrow step function is used for simulating spike voltage at the rising/falling edge. The additional dielectric loss increases with the spike voltage and the duration. The electric field and temperature distributions are simulated in a 10 kW, 10 kHz HFT by considering the calculation of dielectric loss. The spike voltages enhance the electric field and temperature in the insulation. The increase in the external temperature (close to the glass transition temperature ( $T_{\mathrm {g}}$ ) of epoxy resin) causes a significant increase in the insulation temperature. This paper is of great importance to the design and application of HFT insulation.