The Effects of Steep Voltage Slopes on Insulation Systems of Coil Windings caused by Next Generation Power Semiconductor Devices

Next generation silicon carbide (SiC) and gallium nitride (GaN) semiconductors offer much faster switching characteristics in power electronic devices. The faster switching speed allows higher switching frequencies in combination with lower losses, which leads to smaller components and thus, a higher power density. However, the high voltage slopes associated with fast switching cause exceeding stress on the insulation systems, especially of coil windings. This increased stress results in accelerated aging and thus, premature failure of the insulation system. This paper focuses on the design and execution of experiments to quantify the detrimental effects of fast switching on the insulation system of coil windings. To investigate these effects, a dv/dt- generator is developed to excite various specimens with steep voltage slopes. Commercial products are not suitable for this purpose as they are either not available on the market or do not fully exploit the potential of the next generation semiconductors. A SiC-based inverter with an H-bridge topology is selected. The dielectric strength of twisted pair enameled wires are tested using a standardized insulation tester and the proposed dv/dt-generator. While the specimen withstands the standardized insulation test, partial discharges occur during excitation with high dv/dt, which leads to a premature breakdown after short time, even though the dv/dt test voltage is less than a sixth of the standardized test. It is experimentally observed that the steepness of the voltage slope has a major impact on the insulation system.