I. Introduction
To mitigate the global climate and energy issues, electric energy obtained from nuclear, wind and solar energies is occupying an increasing ratio in energy consumption. However, the wearing, aging, and sealing issues of wire power transfer methods have seriously limited the employment of electric energy in severe environments. Wireless power transfer (WPT) technology with excellent airtightness was first invented by Tesla in 1889 [1]. Then, until MIT proposes the magnetic coupling resonance WPT technology in 2007, which successfully lighted the lamp using two contactless coils over the distances in excess of 2 m, this technology is being paid an increasing attention [2].