Emergency Ampacities of Direct Buried Three Phase Underground Cable Systems

A thermal model is formulated for the determination of the emergency transient ampacities of a three phase, horizontally spaced underground cable system. The model is based on a finite difference heat transfer analysis and the principle of superposition which permits the summation of the temperature rise of individual heat sources to eventually provide for the temperature rise of a multiple cable system. The thermal model is transformed into a complex computer program that is capable of calculating the realtime cable temperature for any change in conductor current The program is used to calculate the response of conductor temperatures caused by both ramp and step changes in currents for four conductor sizes between 4/0 and 1500 kc mil. The amount of time to reach conductor temperatures of 100??C and 120??C from a steady conductor temperature of 75??C is calculated forthe different conductor sizes and for different emergency current levels. The results show that underground cable systems possess significant thermal inertia and that larger cable sizes can operate long periods of time under emergency conditions before reaching temperatures that will damage cable insulation materials. For the cable system considered, an emergency transient, which doubles the 75??C ampacity, produces a conductor temperature of 100??C within 4 mm for a 4/0 cable and within 17 min for a 1500 kc mil cable. The emergency ampacity curves can be used by a cable engineer to determine how long a given emergency can remain on the system before the conductor reaches a predetermined temperature.