1. Department of Power System Engineering, Chonnam National University, 50 Daehak-ro, Yeosu 59626, South Korea
2. Department of Electrical Engineering, Chungnam National University, 99 Daehak-ro, Yuseong-gu, Daejeon 34134, South Korea
3. Department of Electric, Electronic and Comm. Eng. Edu., Chungnam National University, 99 Daehak-ro, Yuseong-gu, Daejeon 34134, South Korea
4. Department of Electrical and Computer Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA