Power frequency secondary arc current in uncompensated EHV AC mixed cable-overhead lines

The paper deals with the secondary arc current and recovery voltage in Extra-High Voltage (EHV) ac mixed lines (MLs), i.e. solidly series-connected overhead line (OHL) and underground cable (CL) sections, during the dead time of single-pole auto-reclosure (SPAR) cycles, following single-phase-to-ground faults in the overhead line sections. Secondary arc current and recovery voltage are investigated at first by an analytical approach, assessing power-frequency quantities on a simplified equivalent circuit model of MLs. A parametric analysis of uncompensated 400 kV-50 Hz mixed lines and the comparison with overhead lines of equal length have been subsequently carried out by a multiconductor simulation program. Results show that the inclusion of short underground cable stretches in mixed lines has two main effects: an increase of secondary arc current and a concurrent reduction of the recovery voltage in comparison to overhead lines. The study evidences possible criticalities during SPAR in some typical 400 kV-50 Hz mixed line configurations and operating conditions: secondary arc currents may be 50% higher than in 400 kV OHLs of equal length and can be greater than 50 ÷ 60 A.