1. Graduate School of Integrative Science and Engineering Tokyo City University 1‐28‐1 Tamazutsumi, Setagaya‐ku, Tokyo 158‐8557 Japan
2. Electrical Energy Technology Section, Corporate R&D Headquarters Advanced Technology Laboratory, Fuji Electric Co., Ltd 1‐5‐45, Minami, Konosu‐shi, Saitama‐ken 369‐0192 Japan
3. Faculty of Science and Engineering Tokyo City University 1‐28‐1 Tamazutsumi, Setagaya‐ku, Tokyo 158‐8557 Japan