Research On Operation Parameters Of AC Distribution XLPE Cables In Different DC Topologies Based On The SCLC Theory

DC distribution technology has received a significant attention around the world due to the rapid development of new energy sources and DC loads.This can be achieved using two options i.e.new infrastructure development and upgradation of the present infrastructure.First,it is very difficult to construct new DC distribution lines in large cities due to high capital cost investment.However,the existing AC distribution lines can be converted or upgraded to the DC operation and seems promising due to less capital cost.However,in the latter case,the optimized operational parameters are not available due to the lack of theoretical research and practical experience.This may lead to under-utilization of the power supply capacity of the original transmission line.Therefore,this thesis mainly studies the DC operational parameters of AC cross-linked polyethylene(XLPE)distribution cable based on the space charge limited current(SCLC)theory.The critical findings are as follows:A design method of DC operating voltage for the AC distribution XLPE cable in DC operation was proposed.To minimize the effect of space charge accumulation on the cable insulation,the concept of electric field threshold for the space charge accumulation was applied to the field of DC transformation for the AC distribution cable.The DC conduction current measurement using three-electrode setup was developed.The conduction current characteristics of the 10 kV AC XLPE,35 kV AC XLPE,and silicon rubber(SiR)samples are obtained.The electric field threshold of space charge accumulation for the aforementioned samples were obtained using the SCLC theory.The electric field in the AC XLPE cable insulation was kept below the electric field threshold and the DC operating voltages of the 10 kV and 35 kV AC XLPE cables under different conditions are obtained.The results show that the DC operating voltage of the two XLPE cables decreases with the increase of the conductor operating temperature.The variation of DC transmission power with conductor operating temperature for three-core 10 kV and 35 kV AC XLPE cables under three DC topologies were analyzed.The simulation model of these cables was developed using COMSOL-Multiphysics software.The distribution and variation of thermal field,steady electric field,and transient electric field of the cable were analyzed,and the DC transmission power was calculated for 10 kV and 35 kV AC XLPE cables in the three different DC operation modes.It is found that the ratio of DC transmission power to AC transmission power for the 35 kV AC XLPE cable is lower than that of 10 kV AC XLPE cable.Whereas,for the 10 kV AC XLPE cable,the transmission power under the monopole DC operation is the highest at each conductor operating temperature considering the safety margin of 1.2.Moreover,the DC transmission power is the largest at the conductor operating temperature of 60 oC in all three DC operation modes considering the safety margin of 1.2.The influence of AC service times on insulation properties and DC operational parameters of the 10 kV AC XLPE cable under the DC operation was analyzed.The variation of insulation properties with AC service time were obtained for 10 kV AC XLPE samples with different AC service times by using the electrical insulation and characterization tests.It is found that the thermal oxidation increases with the increase of AC service time.Moreover,the crystallinity is reduced,thereby resulting in the increase of the trap density and trap energy level of the XLPE samples.This increases the activation energy and electric field threshold.Moreover,the thermo-electric coupling simulations of the 10 kV AC XLPE cables with different AC service times were carried out.According to the results,the increase of activation energy leads to the decrease of maximum electric field in the insulation with the increase of AC service time.Whereas,the DC operating voltage of the cable increases with the increase of AC service time due to the decrease of maximum electric field and the increase of electric field threshold.Furthermore,The DC transmission powers of the 10 kV AC XLPE cables under different conditions were determined.It is found that the DC transmission powers are higher than the AC transmission powers at different temperatures,and the DC transmission power increases with the increase of AC service time.A steady DC electric field calculation and optimization algorithm for the stress cone of the XLPE cable joint was proposed based on the coaxial double-layer insulation model.The variations of the stress cone curve under different design conditions were studied by the algorithm and analyzed.For the optimization of the stress cone curve in the DC cable joint,it is more appropriate to use the algorithm due to its accuracy and efficiency.The thermo-electric coupling simulation model was developed for the 10 kV AC XLPE cable joint.The DC electric field distribution and issue of the space charge accumulation in the cable joint were studied.It is found that the maximum electric field near the stress cone of the cable joint is below 0.35 MV/m.Whereas,the maximum electric field near the high voltage screen tube is lower than the electric field threshold of SiR when the insulation temperature difference is large.Moreover,the steady DC electric field of the 10 kV AC XLPE cable joint was optimized through the algorithm and simulation model.It can be seen from the optimization results that increasing the thickness of the high voltage screen tube can effectively reduce the maximum electric field near the tube.Furthermore,the optimized stress cone curve achieves the expected objective of axial electric field and effectively improves the DC electric field distribution in the stress cone region.