[1] Univ Cambridge, Dept Engn, Cambridge, England
[2] Univ Pretoria, Grad Sch Technol Management, Pretoria, South Africa
[3] Slovak Univ Technol Bratislava, Inst Power & Appl Elect Engn, Bratislava, Slovakia
[4] Univ Leicester, Dept Engn, Leicester LE1 7RH, Leics, England
[5] Univ Greenwich, Fac Engn, London SE10 9LS, England