Insulation Design of Wireless Auxiliary Power Supply for Medium Voltage Converters

Auxiliary power supply (APS) that provides reliable power and voltage for gate drivers, controllers, and sensors is a critical component in a medium voltage (MV) converter. Compared to being fed from the power-cell dc-link, APS fed from an external earthed system has its own benefits. However, with the power-cell number scaled up, the insulation design of the externally fed APS becomes a significant challenge. Partial discharge (PD) can cause accumulative irreversible damages of insulation which further lead to auxiliary power circuit fault and eventually cascaded failures of the whole converter system. Therefore, a PD-free APS system ensuring the reliability of the MV converter is required. Among different technologies for building APS, wireless power transfer (WPT) becomes an attractive method because of its mechanical design flexibility and inborn insulation capability. In this article, in order to design a PD-free WPT converter, the insulation design criterion and electric field distribution under different coil geometries are analyzed, compared, and tested. Subsequently, a comprehensive insulation design including the influence of the ferrite shielding layer is shown. Finally, an optimized WPT prototype with 120-W output power, 92.78% full-power efficiency, 2.76-pF isolation capacitance, and 27-kV insulation capability is validated experimentally.