Underground urban distribution cables, ampacity analysis and capacity improvements in Seattle City Light system

In analyzing an underground power system, one of the most important, but frequently unknown or assumed values, is the derated ampacity of underground feeder cables. Power engineers customarily refer to tables and select a previously calculated ampacity de-rating based on specified duct bank configurations and specified load factors. However, for actual duct bank configurations with different numbers of cables installed, as well as for cases where several ducts and heat sources coexist, de-rated ampacity numbers from tables are inaccurate. This presents concerns for utilities that use ampacity values in load flow calculations. Since 1996, Seattle City Light (USA) has been using ampacity software (AMPSOFT) to model and understand the feeder ampacity for locations such as substation feeder getaways or other congestion in distribution system. This paper presents the results of the application of ampacity evaluation to one part of a utility distribution system, network distribution. Topics covered include impacts of steam and transmission lines on ratings, planning additions, site evaluations for new substations, field calibrations, soil thermal resistivity, emergency ratings and suggestions for future work. While this study is limited to network distribution, the results have general applicability to all underground distribution