Analysis of Extruded HVDC Cable Systems Exposed to Lightning Strokes

In high-voltage direct current (HVDC) transmission systems comprising overhead line (OHL) sections as well as cable sections, the dc cable system is exposed indirectly to lightning strokes. In case of HVDC cable systems with extruded insulation the present test recommendations do not specify lightning impulse test levels related to rated system voltage. Instead, preliminary electromagnetic transient studies are required to determine the absolute maximum lightning impulse voltage the cable might experience. Within this paper, the general lightning behavior of OHL-cable mixed systems subsequent to the occurrence of a backflashover as well as a shielding failure is investigated for a 525 kV HVDC transmission scheme. This paper presents a modeling method to calculate occurring lightning overvoltages along a dc cable embedded between OHL sections. Based on a detailed parametric study using time-domain simulation, maximum lightning impulse voltages along the cable system are determined, and evaluated with regard to impulse level and polarity. The impact of project related parameter such as cable length, tower design, and tower grounding conditions on lightning overvoltage affecting the cable is demonstrated.