Analysis of Transient Behavior of Mixed High Voltage DC Transmission Line under Lightning Strikes

Growing urbanization coupled with increased power demands have led to increasing use of mixed power transmission lines with sections of overhead lines (OHL) and underground cables. Due to differences in surge impedance of cables and OHL, voltage surges experience reflections and refractions at their boundaries which make the transient behavior of mixed high voltage direct current (HVDC) line quite peculiar. Lightning strikes on overhead sections of lines induce voltage surges that travel along OHL and enter the cable section. Lightning overvoltage can cause OHL insulators to flashover and stress cable insulation or cause its permanent breakdown. In this paper, we simulated a fast front model of a mixed HVDC transmission line using an electromagnetic transient simulation program (PSCAD) to analyze its transient behavior under a lightning strike. The leader progression model has been used to predict the dielectric performance of OHL insulators. It has been shown that transition towers adjacent to the cable section are much less vulnerable to flashover than subsequent towers. The length of a riser section (connecting OHL and cable) and tower footing impedance have shown to significantly influence the flashover performance of OHL insulators. In addition, the length of cable segments and sheath grounding impedance has been found to influence cable overvoltage. This paper can be used to evaluate the insulation coordination and overvoltage protection requirements for a mixed HVDC transmission line.