[1] Politecn Torino, Dipartimento Ingn Elettr, I-10129 Turin, Italy
[2] CNR, CERIS, Inst Econ Res Firms & Growth, Moncalieri, TO, Italy
[3] Shanghai Jiao Tong Univ, Key Lab Control Power Transmiss & Transformat, Minist Educ, Dept Elect Engn, Shanghai 200240, Peoples R China
[4] Zhejiang Univ, Hangzhou 310027, Zhejiang, Peoples R China