1. Lithuanian Energy Institute, Breslaujos 3, Kaunas, LT-44403, Lithuania
2. School of Electrical and Computer Engineering, National Technical University of Athens, Heroon Polytechniou 9, GR-15780 Zografou Campus, Athens, Greece
3. Czestochowa University of Technology, Faculty of Management, Armii Krajowej 19B, 42-200 Czestochowa, Poland and North-West University, Trade and Development; Potchefstroom Campus, South Africa