Underground Cable Thermal Analysis Case Study Authors - Chulanga Niriella - B.Sc (Eng) Hons, AMIESL chulanga.niriella@gmail.com Nadeera Wijesinghe - M.Sc., B.Sc (Eng) Hons, PMP, AMIESL nadeerawije@gmail.com Introduction The use of underground cables for the purpose of distribution and transmission of electricity has been rapidly increased in the past decades throughout the world specially in the urban areas. With this development electrical related authorities are interested in knowing the maximum possible power that can be transmitted via an underground cable to optimize the use of the cable. This arises the question at what ampacity the cable reaches its maximum possible operating temperature. The Ampacity is the Current Carrying Capacity (CCC) of the Cable, a term which was first used by Del Mar in 1951. If the temperature is exceeded the maximum allowable limit, the reliability of the cable will reduce and also may risk of having a premature permanent failure. The maximum temperature which a cable can withstand is defined by the type of the insulation the cable is having. Determination of the safe conditions for the cable which is buried becomes a significant factor in a gulf country like United Arab of Emirates or State of Qatar where the ambient air temperature rises up to 50 0 C during the summer which leads to a soil temperature of 40 Deg. C. In addition, that soil in this region is at a relatively higher dry state (low moisture content) which affects a higher thermal resistance, resulting lower CCC. Actual Underground Cable Laying in a construction site Image Courtesy of /www.ep-entel.com Heat is generated when a current is passed through the conductor which is governed by the simple equation, Heat Generation (Watts) = I 2 R Where R is the resistance of the conductor and I is the current. To prevent the cable undergoing a thermal failure, it is a must to extract this heat in a manner such that cable does not reach the maximum permissible temperature limit (90 0 C as specified by most manufactures). Therefore the ampacity of the cable is governed by the capacity of the installation to extract heat from the cable and dissipate the heat to the soil and the atmosphere.
When considering ampacity of a cable, the ampacity is defined at three conditions. 1. Steady state 2. Transient 3. Short Circuit In this article, the cables are analyzed only with respect to the first condition - steady state condition using the electrical/power system simulation software called - ETAP (14.1 version). Methodology & Standards There are two applicable approaches when it comes to thermal analysis of underground cables. 1. Analytical methods based on Neher-McGrath method (IEC 60287 & NEC) 2. Numerical method (Finite Element Method) In this paper the ETAP V14.1 electrical simulation software has been used to simulate the thermal behavior of the underground cable system according to the method specified in the IEC method. An underground cable system having 4 number of cables 11kV 3core cables having cross section of 240 mm 2 has been analyzed. The current rating I (ampere) used in the IEC 60287 method is governed by the following equation, where Δθ [ o C] is temperature rise between ambient temperature and cable conductor temperature, W d [W/m] is dielectric loss of cable insulation, T 1, T 2, T 3 [K.m/W] are equivalent thermal resistances calculated from the cable material s thermal properties, T 4 [K.m/W] is the cable external thermal resistance, R ac [Ω/m] is the AC electrical resistance of the cable conductor at maximum temperature and λ 1 and λ 2 are the ratio of losses in the metal sheath to total losses in all conductors and the ratio of losses in the armouring to total losses in all conductors, respectively. Cable Underground Thermal Simulation Generally manufacturers provide cable data for specific conditions when it comes to under ground ampacity. But to use the manufacturer data practically for a site installation, we need to adjust the given conditions for the site condition where there are nearby cables (which emits heat and effects one another) as well as the thermal resistivity and temperature of the soil varies. The effect of the nearby power cables as well as the increased ground temperature together with the increased resistance for heat transfer comes with the increased thermal resistivity has to be accounted when it comes to a typical design calculation in Gulf region where the soil temperature reaches 40 0 C. The simulation was first done for the running current, 436 Amperes (the rated current given in the manufacturer cable catalogue - Ducab) in each cable. First the load flow analysis has been done.
Conditions given in the cable catalogue Actual Site Conditions Ground temperature - 30 0 C 40 0 C Maximum steady state temperature of the cable - 90 0 C 90 0 C Spacing between 11kV cables Singly laid 750mm Depth of the cable (from earth surface) 800mm 800mm Thermal Resistivity of the back filling material 1.2 0 C m/w 2 0 C m/w Current Carrying Capacity (CCC) Ampacity 436 Amps??(Need to find) ETAP Single Line Diagram Cross Section of the Cable System simulated For the above cable arrangement ETAP model has been built and the simulation has been run. For the actual site conditions the temperature of the cable was simulated using the running current of 436Amps (the ampacity given in the cable manufacturer catalogue). Screen shot of the ETAP simulation Temp of cables Since the site conditions are worse than that of the manufacturer specified conditions, the cables heat beyond 165 0 C (when carrying 436Amps) as per the ETAP simulation results.
Neher-McGrath method has been used to find the uniform current which each cable can carry without exceeding the 90 0 C limit in any cable. This can be done with IEC 60287 method using a trial and error iterations. But for the purpose of reducing the iterations, Neher-McGrath method has been used. However after finding the maximum current from Neher McGrath method, the simulation was run using the maximum possible current of 285.5 Amperes with the IEC 60287 method and the result is verified with the IEC methodology. Using the ETAP simulation, it has been found that 285.5 Amperes running in each cable is the maximum current each cable can carry without exceeding the cable temperature of 90 0 C under normal laying conditions. Summary of the ETAP Simulation Results Etap model has been simulated to find the ampacity of each cable system in different site conditions. The cross section of the cable has been kept constant 240 mm 2 three core 11kV cable system. The following factors has been varied to analyze the effect 1. Cable laying depth 2. Thermal resistivity of the backfilling material 3. Cable spacing The summary is given in the below table. Conclusion Determining the ampacity or Current Carrying Capacity (CCC) of the underground buried cables plays a vital role when it comes determining the optimum power which the cable system can transmit. This become far important when the application is in a gulf country where the soil temperature reaches 40 0 C. The manufacturer catalogues typically gives ampacity at specific conditions and most of the time these values can not be directly applied to the prevailing site conditions.
The summary of the results has been produced in the earlier section. It has been noted that by decreasing the cable depth, using lesser thermal resistive back filling material or by increasing the spacing between the cables, it is possible to increase the ampacity of the cable. When there is restriction on the depth and also when there are space restrictions, it is possible to increase the ampacity with using of the special backfilling material which have been engineered to have lesser thermal resistance.