Impact of Foreign Objects on the Performance of Capacitive Wireless Charging Systems for Electric Vehicles

This paper presents the impact of foreign objects on the performance of capacitive wireless power transfer (WPT) systems for electric vehicle (EV) charging. For this study, a prototype 6.78-MHz 12-cm air-gap capacitive WPT system utilizing 12.25-cm x 12.25-cm coupling plates is designed, built and tested. With no foreign objects in its vicinity, the prototype system transfers 507 W with a peak efficiency of nearly 90%. In the presence of foreign objects (including metal, wood, plastic and water), the system's performance is found to be significantly impacted only when the object is in very close proximity (< 3 cm) to the coupling plates. It is found that this performance degradation is mostly due to detuning, that is, a drift between the system's operating and resonant frequency. This digest evaluates two approaches to re-tune the system and recover its performance. The first approach, which involves changing the system's operating frequency, is shown to partially restore system performance. However, this approach is not suited to multi-MHz capacitive WPT systems, which need to operate within a narrow ISM frequency band. The second approach involves utilizing an active variable reactance (AVR) rectifier, which operates at a fixed frequency and can efficiently provide the incremental reactance required to re-tune the system in the presence of foreign objects.