Thermal and electrical combined analysis of a high voltage underground cable under different operating conditions

To meeting the energy demand which increasing day by day, necessity for the electrical energy is increasing too. On the purpose of deliver of electrical energy to the necessary places, underground cable's importance and usage is growing up. When considering this kind of situations, reducing the technical losses and faults on underground cable system is becoming more important. Therefore, following right methods and implementing efficient analysis for taking well results on underground cable system has a critical importance at the whole process of underground cable's from production to end user. Especially in big cities which has complex network, the choice of overhead lines is becoming less popular because of visual pollution and lack of space. Overhead lines has critical disadvantages. They are unprotectable to some natural events such as lightning etc. which can lead to interruption. Overhead lines have bare conductor. Due to this reason when the breakage failure occurs at system, it will bring about safety hazard. In addition, when compared with respect to voltage drops, voltage drops experienced in overhead lines are higher than underground cables. Ampacity of underground cables are calculated by implementing thermal analysis. İn thermal analysis, conductor temperature is significant for the ampacity.Every cable has maximum operating temperature. This temperature identifies by the kind of insulation material. The ampacity of cable is the current value that reachs the cable's conductor to its maximum operating temperature. İf there is more than one cable, the hottest cable sets as reference cable and current carrying capacity calculations are done for that cable. In the literature, underground cables have been studied in terms of their thermal and electrical behavior according to different ambient conditions. When the studies are examined, it is aimed to reduce the temperature for the current value passing through the cables. İn this manner, current carrying capacity will be increased. At the same time, it will reduce the possibility of cable breakdown in case of overload. In this thesis, the behaviors of high voltage underground cables are analyzed thermally and electrically according to different ambient conditions. Current carrying capacities are calculated according to the maximum allowable operating temperature for modeled cables. It has been observed the affect of changing ambient conditions to the temperature distributions of the high voltage cables and made a relation between these changes and current carrying capacities. Hereby, helpful suggestions are given about the ambient conditions and laying conditions for the high voltage cable to purpose of operated more efficiently. In the first part of the thesis, an introduction has been made to the study in general. After that, literature review has been made, numerical and analytical studies on this subject have been discussed in detail. In the second part of the thesis, the advantages and disadvantages of underground cables are listed briefly. Also in the same section, the general structure of high voltage underground cables, the layers and materials that make up the cable are mentioned. In the third part of the thesis, the calculation of the direct current and alternating current resistances of underground cables, the losses occurring in the cables and the calculation of these losses , the methods used to determine the current carrying capacity of the underground power cable was mentioned. In the fourth part of the thesis, the fundamentals of thermal analysis is mentioned for calculating the current carrying capacity of the cables. Be touched on how the thermal resistances are calculated in the cables. Also the types of thermal conduction and its calculation methods was examined. In the fifth section, which contains the numerical applications of the thesis, thermal and electrical analyzes were implemented on 89/154 kV high voltage underground cable using Comsol Multiphysics program which can perform multiple physical analyzes according to the finite element method. Firstly, Comsol Multiphysics program was introduced, and the methodology of analysis was mentioned. Then, directly buried 89/154 kV high voltage cable was modeled separately as one and three cables on flat formation. Temperature distribution and current carrying capacity analyzes were implemented for these cables. Then, thermal and electrical analyzes of the cable models was performed according to the specified laying model. The laying model is contained three cables and these cables are laying with flat formation. During the analysis are being made, some changes which can be affect the cable's ampacity and temperature distribution were observed. These changes are; the effects of the thermal conductivity of the filling material, changing the horizontal distance between cables and the laying depth of the cables, the vertical dimension of brick and the thermal conductivity of the brick material;the surface temperature and wind speed. Using the obtained current values and temperature variations, lifetime analysis was performed under the diverse conditions of the cables with the life equation. In addition to the Comsol Multiphysics program, thermal and electrical analyzes were also performed with Cymcap software. Cymcap is a program which can calculate ampacity values for underground cables based on IEC 60287 method. The same cable model which created in Comsol Multiphysics program before, is also created in Cymcap software. The effect of diverse parameters on the cable ampacity has also been observed in the Cymcap program. The results obtained in the Cymcap program and the results obtained in the Comsol Multiphysics program were compared. In the conclusion part of the study, the analyzes were interpreted and the suggestions for operating the cables according to changing ambient conditions are given.