State-space model for availability assessment of EHV OHL–UGC mixed power transmission link

In the process of reinforcement and modernisation of the grid, state-of-the-art EHV XLPE circuits gain growing attention as an attractive alternative to OHLs, above all in the context of intensely urbanized districts. Mixed (or composite or hybrid) (OHL–UGC) lines often represent the most cost-effective solution; however, reliability concerns deserve careful analysis for both cases of inclusion of short cable stretches, consisting of few major sections, and of links with “long” siphons needing reactive power compensation. Typical circuit configurations are presented and their expected availability evaluated on the basis of the most recent return-of-experience from existing installations. The focus of the paper is on the use of state-space models for assessing the mixed line availability and the calculations have been performed on the basis of existing statistics.

► Availability of typical circuit configurations is evaluated on the basis of the most recent statistics from existing installations.
► Flexible model able to handle all systems features and operative constraints.
► The inclusion of a single circuit siphon with limited number of major sections has a tolerable effect on the overall link availability.
► For an 8.1 km long siphon, the unavailability ranges between 24 and 77 h/year, depending on the referred failure statistics.
► Partially redundant cables in electrical parallel allow limiting the link unavailability, at more than half capacity, to only few hours per year.