Issues on optimization of operating modes of power transformers A.N. Alyunov, O.S. Vyatkina, I.G. Akhmetova, R.D. Pentiuc and K.E. Sakipov E3S Web Conf., 124 (2019) 02015 Published online: 25 October 2019 DOI: 10.1051/e3sconf/201912402015