1. Department of Electrical Engineering, Graduate School of Engineering, Tohoku University 6‐6‐05, Azaaoba, Aramaki, Aoba‐ku Sendai 980‐8579 Japan
2. Department of Electrical and Electronic Engineering, College of Engineering, Chubu University 1200 Matsumoto‐cho, Kasugai Aichi 487‐8501 Japan
3. Chubu Electric Power Grid Co., Inc. 1, Higashi‐shincho, Higashi‐ku, Nagoya 461‐8680 Japan