A Water Cooling System to Improve Ampacity in Underground Urban Distribution Cables

Seattle City Light (SCL) identified a thermal bottleneck that limited ampacity of 13 kV cables inside duct banks near a major substation supplying downtown Seattle. Depth of burial limited the seventy-two cables to 310 amps each instead of 510 Amps. Other cables nearby limited ampacity further. Thus the existing cables and ducts wouldn't meet near-term projected load increases. New substation construction was not feasible. Nor were customer interruptions for cable capacity improvement work. A chilled-water heat-removal system solves these problems. A prototype designed by SCL is under construction with commissioning planned for 2005. Ampacity increases by about 60% for the 13 kV cables and for 26 kV and 115 kV distribution and transmission cables near the substation. This paper describes the cooling system design, operation, environmental and safety features, and construction methods. The cooling water approach to ampacity improvement has potential application elsewhere at SCL and at other utilities