Mathematical model of a part of an opened extra-high voltage electrical grid Marek Lis, Andriy Chaban, Andrzej Szafraniec, Radosław Figura and Vitaliy Levoniuk E3S Web Conf., 84 (2019) 02005 Published online: 11 February 2019 DOI: 10.1051/e3sconf/20198402005