Design and operation of EHV transmission lines including long insulated cable and overhead sections

Application of EHV lines formed by series connected overhead sections and XLPE-insulated cables sections is planned in various projects in Europe. At first the maximum feasible length of 380kV-50Hz and of 500kV-60Hz XLPE cable lines is calculated as a function of the power carrying capacity derating due to charging current. Then shunt compensation and optimal voltage-reactive power control are analysed with use of shunt reactors with tapped windings and on-load tap changer for regulation of Mvar output. The authors' ATP-EMTP electromagnetic transient analysis of the long mixed EHV lines has revealed the risk of sustained overvoltages due to resonance on 3rd harmonics. The phenomenon is described and countermeasures are proposed. The feasibility of the single-pole high speed reclosure of mixed EHV lines is analysed, and means for limitation of the secondary arc current are examined. A protection scheme is proposed for fast selective detection of faults and for implementation of the single-pole reclosure only in the cases of 1-φ-to-Gr faults in the overhead sections of mixed EHV lines.