Large Temporary Overvoltages in MV Network due to a Series Fault in the HV Subtransmission System

The paper deals with the potential occurrence of large temporary overvoltages in a medium voltage (MV) distribution system, due to a phase conductor interruption along a high voltage (HV) subtransmission line. In meshed HV systems, the chance of overvoltages is quite small and often the dissymmetry due to the series fault is tolerated for a relatively long time. Radial configurations, liable to incur severe overvoltages in case of series faults, can occur at HV level due to contingencies and/or maintenance. The explanation of the phenomenon and an extensive parametric study of one such case involving a 180 km long, 220 kV overhead line radially supplying a HV/MV substation are presented, evidencing the potential onset of large power frequency overvoltages and severe dissymmetry at both HV and MV level. EMTP-type time domain simulations are also reported, confirming the possible extent of the temporary overvoltages.