1. T&D Systems CenterMitsubishi Electric Corp, 8‐1‐1, Tsukaguchi‐Honmachi Amagasaki Hyogo 661‐8661 Japan
2. Department of Electrical Engineering, Graduate School of EngineeringNagoya University (NU) Furo‐cho, Chikusa‐ku Nagoya 464‐8603 Japan
3. Advanced Technology R&D CenterMitsubishi Electric Corp Amagasaki Hyogo 661‐8661 Japan