| In order to theoretically verify the feasibility of converting three-core AC cable to DC operation,and determine the voltage level and rated current for cable line after the conversion,a series of research of temperature field and electric field of the cable's insulation layer is carried out.The typical power cable YJV43-3×120mm~2 three-core AC XLPE cable line with rated voltage of 35kV is selected to do the research on the conversion of AC cable to DC operation.The cable is regarded as infinite long and the cable structure is simplified in order to build two-dimensional simplified model of three-core AC XLPE cable.Analytical calculation of ampacity using the classical thermal circuit method is carried out.The result of analytical calculation is 297A when the cable is laying in the air with temperature of 40?.Simulation of temperature field of the cable under steady-state operation by using FEM software COMSOL Multiphysics is carried out and the ampacity is turned out to be 302.1A.Change the ambient temperature,the different ampacity is obtained.The relationship between the air temperature and the corresponding ampacity is basically linear.With the increase of temperature by 1 ?,the ampacity decreased by 3.97A.Take comprehensive consideration of the analytical results and simulation results,the steady ampacity of the YJV43-3× 120mm~2 three-core AC XLPE cable with rated voltage of 35kV is recommended to be 300A.The difference of electric field distribution in the insulation layer when the cable is under AC or DC operation is analyzed.For the insulating material XLPE,the electric field intensity threshold of space charge to be accumulated is about 3?5kV/mm at 70?.Therefore,in order to avoid the space charge's accumulation in the insulating materials,the maximum electric field strength of insulation layer should lower than the threshold range of 3?5kV/mm.When the cable is under operation with no-load,the DC voltage need to be controlled lower than ±27 kV?±45kV.The specific operating voltage after AC cable converted to DC operation can be decided by the operation life and aging degree of the original cable.When the cable is steadily operated under ±30kV and superimposed with 210kV reverse polarity lightning impulse voltage,the maximum electric field strength of the insulation layer is about 29.58kV/mm,which means the cable can be operated safely.Considering a certain margin and the existing DC voltage level,it is recommended that the YJV43-3x 120mm~2 three-core AC XLPE cable of 35kV to be transformed to the rated voltage of 30kV after AC to DC conversion.The ampacity of the YJV43-3×120mm~2 three-core AC XLPE cable with rated voltage of 35kV in AC operation is about 290A.The power factor is taken as 0.85,so the maximum transmission capacity of the cable under AC state is15MW;after the conversion to DC operation,the transmission capacity can be expanded to 18MW,about 1.2 times the maximum AC transmission capacity.The research provides a theoretical reference for converting the existing cable lines from AC to DC operation in the follow-up engineering. |
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