DC-DC Intersystem Faults in Parallel Partially-Cabled HVDC Links

Abstract:
Within the next decades, an increasing number of HVDC systems based on Modular Multilevel Converters is planned to support existing AC grids. For both socio-economical and technical reasons, recent network development plans suggest re-using infrastructure not only of AC, but also of existing DC systems. Therefore, partially-cabled HVDC systems might be routed in parallel on the same right of way. On the systems’ overhead line sections, intersystem faults between conductors of two independent HVDC links can arise. However, the impact of these new fault scenarios has not yet been investigated. This paper provides an in-depth analysis of voltage stresses imposed by DC-DC intersystem faults, taking into account different transmission system topologies of two parallel HVDC links. The results indicate that cable sections based on cross-linked polyethylene are exposed to transient polarity reversals exceeding the amplitudes reached under pure DC faults. In particular, asymmetries between the HVDC transmission systems lead to overvoltages on short cable sections that are not covered by the existing cable testing recommendations. Therefore, DC-DC intersystem faults need to be considered, when developing future cable test specifications applicable to parallel, partially-cabled DC links.