Simulation And Application Of The Space Cahrge Dynamics In Two Dimension Model For High Voltage Direct Current Cable And Joint

The space charge accumulation in the insulation of the extruded insulated high-voltage DC cable and its accessory under DC voltage is a key issue that limits its development to higher voltage levels and further application.When the space charge accumulates inside the insulation or at the interface,the electric field distortion caused by it may cause or accelerate the deterioration of the insulation,and may even cause breakdown in a short time during the transient operating condition.The present space charge measurement technology,however,still has some limitations in the application of high-voltage DC systems: first,the measurable thickness is limited,and at the cost of reducing spatial resolution;second,the armor,metal shield,etc.should be removed during measurement,which are not easy to recover.Thus the possibility of application in actual operation and maintenance is small,and the cable cannot be wholly evaluated;in addition,Irregular insulation structure cannot be directly measured by current technology.With the development of computer technology and polymer charge transport theory,numerical simulation is a new method to study the charge transport in dielectrics in recent years.However,whether in flat or coaxial specimens,the charge transport simulation and corresponding algorithm are based on one-dimensional assumptions.Considering the axisymmetric structure of the solid insulation which is commonly exists in cable transmission systems,in which the spatial relationship between the charge and the electric field is more complicated,a simple one-dimensional charge transport model is difficult to describe in such a condition.In order to study the space charge dynamics in the insulation of cable and its accessory,the corresponding algorithm for the charge transport in the solid insulation with two-dimensional model was studied in this paper,aiming at the key issue of the correlation between charge and electric field dynamic distribution.In this paper,based on the finite element method(FEM),the Poisson equation in two-dimensional cylindrical coordinates was derived and solved.And the continuous equation was solved based on the finite volume method(FVM),with the variable distributed in the lattice center.By cyclically solving the Poisson equation and continuous equation,the distribution of charge dynamic and corresponding electric field could be obtained.In order to solve the problem that the numerical divergence and oscillation of the traditional upwinding scheme in the calculation of continuous equations which may affect the accuracy or even cause a divergent solution,this paper adopted a high-order upwinding scheme with limiter in calculation.With this scheme,the accuracy of charge transport calculation in the two-dimensional model could be guaranteed without affecting the computational efficiency.Based on the bipolar charge transport model,the calculation procedure of the charge transport and electric field distribution in the insulation of high-voltage DC cable and its accessory was programed,and the corresponding numerical software platform was developed.Based on the developed software platform,the charge transport in the cross-linked polyethylene(XLPE)cables of different voltage levels(10,320 kV)were firstly simulated under DC voltage.The relationship between the space charge and the electric field near the electric field concentration region caused by the insulation screen discontinuity was analyzed.The results show that the insulation screen discontinuity will cause significant distortion of the space charge and electric field along the axial direction of the cable which suggests that attention should be paid to the treatment of the insulation screen section during the cable fabrication and operation;The homocharge injection can weaken the electric field near the electrodes,while it could significantly increase the electric field in the middle of the insulation.Thus the electric field distortion in the insulation increases the radial average electric field which may threaten the insulation safety.In addition,the resolution of two-dimensional numerical simulation based on refined mesh is discussed and analyzed.The results show that mesh refinement can improve the resolution of numerical results,but at the same time it will occupy a lot of computing resources.Although space charge measurement still has limitations in engineering applications,it is still irreplaceable as a method for quantitative analysis of the space charge distribution in insulation under non-operating conditions such as laboratory studies or type tests.And experimental quantitative analysis is essential for the verification of numerical calculation accuracy.Therefore,this paper developed a full-sized cable space charge measurement system based on pulsed electro acoustic method for cables of different voltage levels.The difficulty in the development of this system lies in the impedance matching of high voltage nanosecond pulses and the optimization of local high electric field under DC high voltage.In this paper,high voltage nanosecond pulse generators based on forming line and resistance-capacitance were developed respectively,aiming at space charge measurement of cables with different insulation thicknesses.In order to solve the problem of impedance matching,this paper proposed an impedance matching method for the pulse circuit based on the lumped elements.The matching results show that this method can conveniently adjust space charge peak at the outer electrode.In addition,tests have shown that the pulse generator based on this matching method can provide discharging channels in extreme cases such as breakdown and flashover of the system to maintain normal operation.In order to solve the local electric field concentration under DC high voltage during space charge measurement,this paper optimized the design of grading ring and the stress cone by simulation,which ensured that the breakdown and flashover will not happen under the target voltage.For comparison with the simulation results,space charge measurements were performed on 10,320 and 500 kV XLPE cables.The space charge measurement results of 10 and 320 kV cables show that the two-dimensional numerical simulation is basically consistent with the measured results in terms of values and trends,which proves the feasibility and accuracy of the two-dimensional numerical algorithm.Based on the comparison between the simulated results and the experimental results,the two-dimensional charge transport model and the corresponding algorithm are preliminarily applied to a 320 kV overall prefabricated cable joint and its model which composite insulation are XLPE/ethylene-propylene-diene monomer(EPDM).The results of the joint indicate that the electric field at the end of high-voltage screen does not increase with the charge injection,while the electric field in the middle of the insulation is obviously enhanced with the charge injection;Besides,after a polarization period,the electric field that originally decreases with the charge injection near the interface begins to reverse.This indicates that the injected charge is an important factor in the electric field distortion both in the insulation and at the interface.Therefore,in addition to the electric field distortion caused by the insulation structure in the design,attention should also be paid to the suppression of the injected charge;This two-dimensional model can determine the maximum value and the changing trend of the electric field in the space charge transport process,which could offer a reference in determining the allowable value of electric field in the insulation design.In order to balance the computational efficiency and the spatial resolution of charge distribution,the possibility of using the scale model for surrogate calculation was explored.The results show that the charge transport calculation with scale model can improve the spatial resolution of charge distribution.Besides,the electric field changing trend inside the insulation is basically the same as the original size.