1. Department of Electrical Engineering, University of Nottingham, Nottingham, U.K.
2. Institute of Automatics, Electronics and Electrical Engineering, University of Zielona Gora, Zielona Gora, Poland
3. Division of Mathematical Statistics and Econometrics, University of Zielona Gora, Zielona Gora, Poland
4. Department of Energy Technology, Aalborg University, Aalborg, Denmark