Underground Transmission Cable Ratings

Tangibl developed component and facility ratings for the Transmission Planning and Transmission & Substation Engineering organizations of one of the original PJM transmission-owning utilities.  As the Transmission Owner implemented its T&S capital program, Tangibl updated facilities ratings to reflect component changes and additions.  One of the projects was the replacement of two 138kV Low Pressure Fluid Filled (LPFF) underground transmission cables supplying each of two 138-13kV 2-winding distribution transformers.  These 1968-vintage oil-filled cables were generally accepted by the client (and the industry) to be at the end of their useful lives.

The project was complicated by existing transite ducts which ultimately were replaced to accommodate the new 3-1×3156 KCMIL CU XLPE cables. Tangibl reviewed plan and profile drawings for the project and identified crossings in the station with 34.5kV distribution duct banks.  The cabling in the duct banks and their proximity to the relocated transmission cable had the potential to compromise the intended normal and emergency cable ratings requested by Transmission Planning for that 138kV line.  Tangibl established ratings for these cables using the ETAP cable analysis module.

While this planned project was in progress, a failure occurred at another substation involving one of two parallel 138kV LPFF cables used by a 230-138kV autotransformer to supply the 138kV yard at this substation.  The subsequent derating of this transformer due to the loss of one if its secondary cables resulted in an n-1 condition for supply to another substation serving significant load in the center of a major U.S. city.  The same project team on the first, planned project was redeployed to plan and execute this second emergent project while maintaining schedule on the first project.  Here again, Tangibl established ratings for these cables as components of the autotransformer transmission facility using the ETAP cable analysis module.