Increasing cable rating by distributed fiber optic temperature monitoring and ampacity analysis

The allowable current that can be carried by underground cables is limited by the hottest location along the circuit. Deratings of 10% or more can result from deep burial, nearby distribution duct banks or steam mains, or areas of high soil thermal resistivity. Accurate knowledge of thermal conditions along the route will permit confidently rating the cables right up to their maximum ampacity. Distributed fiber optic temperature monitoring uses an enhanced optical time domain reflectometer to inject a laser pulse down a conventional multimode fiber, and develop a temperature profile with one centigrade degree accuracy along every meter of a cable circuit. This paper describes an application where a temperature profile combined with methods developed to relate optical fiber temperature to cable conductor temperature, plus sophisticated ampacity analyses, permitted one utility to obtain an 8% ampacity increase and defer reinforcing a 69 kV circuit by two years.