Design and Evaluation of 70 kV Isolated Excitation Coil for Transformerless Multilevel Inductive Power Transfer System

A transformerless multilevel inductive power transfer (IPT) system converts a medium-voltage (MV) to a low-voltage (LV) using a multilevel active rectifier and resonant inverters. Excitation coils deliver the output power of the multilevel converter to a common transmitter coil. The transmitter coil then generates an augmented magnetic field to induce load voltages on receiver coils, which can be attached to the bottom of electric vehicles. However, MV isolation between the excitation and transmitter coils should be guaranteed for safe operation. This study presents a new design methodology for MV-isolated excitation coils. Electric fields are analyzed to evaluate the insulation performance of the excitation coil. Additionally, magnetic fields are discussed to design an efficient excitation coil. Based on the analysis, a multi-objective optimization is conducted on the excitation coil with respect to the insulation voltage, core loss, and volume. A 6 kV/6-level IPT system was developed for experimental evaluations. Grid-to-load and coil-to-coil efficiencies of 86% and 90% were achieved using the testbed. Finally, the developed excitation coil passed a 70 kV over-voltage test.