Simulation Study On Electric Conduction And Space Charge Characteristics In HVDC Cable Insulation

Cross-linked Polyethylene(XLPE)has excellent electrical properties,mechanical properties and heat resistance,based on which XLPE is widely used in HVDC cable insulation.But space charge accumulation easily occurs inside XLPE under DC field,causing electric field distortion or even dielectric breakdown.Therefore,it is important to study the conductivity and space charge properties of XLPE to guide the improvement and engineering application of insulation materials.But the charge transport process is complex in material and it is difficult to explore the interaction process between various microscopic particles by experimental means.The numerical simulation based on the bipolar charge transport model can obtain microscopic data that are difficult to obtain by experimental methods,and relate the microscopic parameters to the macroscopic performance,which can study the conductivity and space charge properties of polymer insulating materials more deeply from the microscopic perspective.In this thesis,based on the mathematical equations of space charge dynamics and heat transfer equations,a bipolar charge transport model and a cable thermal field model are established,and an ion transport model is introduced on this basis.The model is constructed and solved by finite element software,and the convergence of the whole model is improved by optimizing the mesh dissection and initial value setting.The space charge distribution and conductivity current characteristic curves of XLPE in a flat plate structure are simulated numerically by using the bipolar charge model alone and the model that combines bipolar charge model with the ion transport model.The simulation results are compared with the experimental results.The results show that the model that combines bipolar charge model with the ion transport model can better simulate the heteropolar charge accumulation and overall trend of conductivity characteristic curves.The conductivity curves are fitted in segments,and the curves are divided into four regions Part 1,Part 2,Part 3 and Part 4 by three turning points A,B and C.The slope of each region rises,falls and rises in turn.The conductivity characteristics of XLPE are analyzed based on the dynamic characteristics of the charge.The results show that when there is an obvious space charge accumulation inside XLPE,the conductivity curve passes through the turning point A into the Part 2 region;the ion density near the electrode decreases,which enhances the suppression effect of the homopolar charge on the electric field at the electrode,making the charge injection intensity of the electrode decrease,and the conductivity curve passes through the turning point B into the Part 3 region,when the charge injection of the electrode is completely controlled by the internal XLPE With the further increase of electric field strength,the charge injection intensity of the electrode increases,the effect of ion density reduction is weakened,and the conductivity curve enters the Part 4 region through the turning point C.Raising the temperature,the field strengths corresponding to the turning points A,B and C all become smaller.Increase the trap depth and trap density,the field strengths corresponding to the turning points A,B and C become larger,and the slope of the Part4 region decreases significantly;decrease the electrode injection potential,the field strength corresponding to the turning point A becomes larger,the field strength corresponding to the turning point B decreases,the turning point C disappears,and the slope of the Part 3 region decreases significantly.Numerical simulations of the electric field and space charge distribution of fullsize cable insulation under coaxial structure were carried out using macroscopic conductivity model and bipolar charge transport model,respectively,to compare the results of both.The results show that the simulation results obtained based on the macroscopic conductivity model are not accurate enough,and the charge distribution is unipolar charge distribution with only positive charge present,which has limited offsetting effect on the Laplace field.Finally,based on the bipolar charge model,the effects of structural effects,current-carrying capacity and composite coefficients on the spatio-temporal evolution process of electric field and space charge within the insulation of HVDC cables are analyzed.The results show that the space charge accumulated in the insulation can smooth out the field strength inhomogeneity formed by the geometric structure;the larger the cable current-carrying capacity,the faster the space charge and electric field distribution in the cable insulation reach the steady state,while the electric field reversal is more serious and the maximum value of electric field strength keeps increasing;the compound process has a great regulatory effect on the charge transport process.Reducing the compounding coefficient and weakening the compounding effect can effectively reduce the maximum value of electric field strength within the insulation and suppress the electric field distortion.