Cable environment analysis and the probabilistic approach to cable rating

The steps undertaken to determine the ratings of four 32 yr-old, 115 kV, LPOF (low-pressure oil-filled) cable circuits supplying downtown Toronto, Canada are described. The refurbishment of the circuits involved extensive condition surveys and development of mathematical models employing probabilistic methods and finite-element techniques to obtain accurate predictions of cable ampacities. It is found that the condition of a cable and that of the environment in which it is buried are sensitive to several time-dependent factors with complex relationships. These factors cannot be detected by any kind of theoretical analysis and, therefore, before any life extension or current uprating of an aging cable circuit can be attempted, an adequate condition survey of the cable and of the cable route is essential. The probabilistic method used for thermal analysis of loaded cables combined with the information gained from the cable condition survey has the advantage over deterministic methods of quantifying the benefit/risk trade-offs and therefore allowing for more informed decision making