Investigation On Temperature Characteristics Of High-Voltage Cable In Tunnel

Recently,the production and living condition in our country are gradually enhanced.The demand for energy is rising,and the requirement for power supply reliability is increasingly strict.Cable tunnel has the advantages of being little affected by the external environment,accommodating a large number of cable circuits,convenient operation and maintenance,etc.A large number of completed cable tunnels have been in operation and it will become the main laying method in the future.The cable temperature not only determines the ampacity,but also concerns the safety of power system.Therefore,providing an accurate model to calculate the cable temperature is the focus of current research at home and abroad.In this thesis,to address the shortcomings of existing studies,the temperature characteristics of high-voltage cable laid in tunnel is mainly investigated from two aspects.Existing thermal model is modified,considering air flow,circumferential heat transfer and tunnel curvature.The impact of humid environment on heat transfer is considered,and the heat and moisture coupling model is developed based on the finite element method.Firstly,the principles of analytical method and finite element method are introduced.The heat sources of power cable are studied.Based on the electro-thermal analogy and thermal conductivity law,the cable thermal resistance is obtained and the governing equation of temperature field is derived.The complex heat transfer modes in cable tunnel are analyzed.The steady-state temperature simulation of 500 k V highvoltage cable is conducted using both the analytical method and the finite element method to analyze the agreement of these two methods.Secondly,the thermal network model of tunnel cable is investigated.The airdomain triangular thermal model is established by taking into account the impact of airdomain heat transfer on tunnel cable.Considering the circumferential heat transfer and tunnel curvature,the cable cross-section is suggested to be divided into two parts,the windward side and the leeward side.Thermal network model for high-voltage cable in tunnel is established to investigate the temperature characteristics.As a result,the circumferential temperature difference exists in all layers of the cable,and the temperature on the windward side is lower than that on the leeward side.The larger the wind angle,the smaller the circumferential temperature difference.The state of air flow is changed by the tunnel curvature,reducing the temperature of cable in the curved part,and the impact on the cable surface temperature is greater.By comparison,it is observed that the computation formula in IEC-60287 is too conservative since it does not consider the properties of airflow.The transient temperature characteristics are examined to verify the applicability of thermal network model for transient studies.In addition,the coupling of heat and moisture based on finite element method is proposed to investigate the cable temperature characteristics under humid environment in tunnel.The main physical processes involved in the humid tunnel are analyzed,and the corresponding finite element models are developed.Governing equations are used to describe the turbulence,moisture transport,and heat transfer.The dynamic coupling of moisture and heat is explored.Considering both the potential heat change caused by evaporation and the variation of air thermal properties with humidity,the heat and moisture coupling model of tunnel cable is established.The simulation is implemented.It is demonstrated that,compared with the situation without considering humidity,the humid environment will lead to a reduction in cable temperature.The extent of reduction is closely related to the moisture distribution,the relative humidity of air,etc.The temperature distribution is also changed.The cable has temperature gradients in the radial,circumferential and longitudinal directions.Therefore,the temperature variation due to the humidity in tunnel cannot be ignored and needs to be considered comprehensively when developing the calculation model.