[1] Yuri Gagarin State Tech Univ Saratov, Inst Elect & Mech Engn, Saratov 410054, Russia
[2] Yuri Gagarin State Tech Univ Saratov, Inst Energy & Transport Syst, Saratov 410054, Russia
[3] Yuri Gagarin State Tech Univ Saratov, Phys Tech Inst, Saratov 410054, Russia