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| Study on Calculation Model of Submarine Cable Ampacity in J-Tube Section of Offshore Wind Power Transmission |
| Li Gen1, Du Zhiye1, Xiao Pai1, Hao Zhaoyang1, Cui Rongjian2 |
1. School of Electrical Engineering and Automation Wuhan University Wuhan 430072 China;
2. Central Southern China Electric Power Design Institute Co. Ltd of China Power Engineering Consulting Group Wuhan 430071 China |
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Abstract The transmission line of offshore wind farm generally passes through the J-tube section with poor heat dissipation conditions,which is divided into three sections: platform section, tube air section and tube water section. The tube air section is often the weak point of ampacity of the submarine cable due to the closed air interlayer between the J-tube and the cable. There is no standard formula for calculating the ampacity of submarine cables in this laying section, which mainly depends on the numerical simulation. However, 3-D fluid simulation is time-consuming. This paper proposes an analytical model of radial-axial heat transfer of submarine cables in J-tube section, which takes into account the axial temperature distribution difference of submarine cable conductors and realizes the rapid calculation of ampacity of submarine cables in J-tube section.
Firstly, the radial thermal path model of the submarine cable in J-tube section is built according to the thermoelectric analogy method. The Nusselt number correlation formula is used to calculate the convection heat transfer coefficient between the submarine cable surface and the inner surface of J-tube. And the radiation heat transfer between the submarine cable surface and the inner surface of J-tube is considered. Secondly, the axial heat transfer of submarine cable conductor in J-tube section is approximated by the heat transfer model of medium cross-section straight rib, and the axial heat transfer model is built on the basis of its radial model. Finally, with the radial-axial thermal path model of the J-section submarine cable, the conductor temperature distribution of the J-tube submarine cable under the given current load can be quickly calculated by the dichotomy method.
Taking high voltage DC submarine cable as an example, a comparative calculation is carried out with the analytical model and the 3-D numerical model. The results show that when the length of tube air section is 5 m, the error between the maximum temperature of the conductor calculated by the axial heat transfer model and that calculated by the numerical method is 3.0%; the error is 1.6% when the length of tube air section is 10m; the error is only 1.1% when the length of tube air section is 15 m. The numerical simulation results show that the maximum temperature change is less than 0.4℃ when the outer diameter of J-tube increases by 50 mm, while the analytical model results show that the temperature change is smaller. The maximum temperature of the conductor increases linearly with the increase of solar radiation intensity. And the trend and law of the calculation results of the analytical method and the numerical method are basically consistent. With the increase of the radiation intensity, the error of the calculation results of the analytical method is slightly increased compared with the numerical method, and the maximum error is 2.8%. In addition, the maximum temperature of the conductor increases approximately linearly with the increase of ambient air temperature. The external ambient air temperature increases by 5℃, and the maximum temperature of the conductor also increases by about 5℃. The maximum conductor temperature calculated by the analytical method at different air temperatures is basically consistent with the numerical simulation method, with an error of about 1%.
The following conclusions can be drawn from the comparative calculation and analysis: (1) When the length of tube air section is greater than 10m, the maximum temperature of the axial temperature distribution of the cable conductor has formed an obvious stable section. (2) With the increase of the outer diameter of J-tube, the maximum temperature of the conductor decreases slightly; besides, the reduction of the seawater temperature directly affects the temperature of the conductor in tube water section, but has little effect on the maximum temperature of the conductor in tube air section. (3) The maximum temperature of the conductor in J-tube section increases linearly with the increase of solar radiation intensity and ambient air temperature. However, when the solar radiation intensity increases, the increasing trend of the maximum temperature of the conductor decreases slightly with the increase of air temperature.
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Received: 04 May 2022
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2023, Vol. 38 
