Simulation And Experimental Study On Electrical Tree And Space Charge Distribution Under Non-uniform Electric Field In Polymer Insulation

With the rapid development of HVDC technology,as the most important insulation material for HVDC cables,cross-linked polyethylene(XLPE)has gained more and more extensive application.Recent studies have shown that there is obvious space charge accumulation in XLPE under DC voltage.The accumulation of space charge will lead to serious electric field distortions in XLPE.The excessive electric field may cause local breakdown at the weak point of XLPE,and then gradually develop into dendritic discharge channel.When the dendritic channel reached the opposite end of the insulation,the entire cable insulation failed.In order to study the relationship between the growth characteristics of grounded DC tree and the distribution of space charge in the XLPE,periodic grounded DC tree experiments with both positive and negative polarities under various voltage were carried out on a commercial grade high voltage DC XLPE cable insulation.On the basis of the experiment,a modified 3D simulation model of needle-plate electrode is established based on the bipolar charge transport mechanism,which has considered the injection,migration,diffusion,trapping and detrapping of space charge.A new concept,based on the comparison of results of simulation and experiment,the effective injection depth has been proposed to characterize space charge distribution range,at which the density of space charge is large enough to initiate electrical tree.With the help of this concept,a reasonable explanation has been given to the characteristics found in the grounded DC tree experiment.From the grounded DC tree experiment,the results show that with the increase of voltage,both of the initiation rate and length of electrical tree increase.The length of electrical tree grows with the increase of short circuit times under both polarities.At the same time,the length of electrical tree rises and then decreases under positive polarity with the increase of single DCprepressing time,while it shows little effect under the negative polarity;the expanding coefficient of electric tree under negative polarity is greater than that under positive polarity.For the 3D rotation model of the needle-plate electrode,the traditional differential methods using in 1D model is no longer suitable for solving the 3D model because of the complexity of the boundary.Therefore this paper chooses Finite element method(FEM)to solve the model with an appropriate mesh.The simulation results shows that the effective injection depth of space charge increases and then decreases with the increase of single prepressing time,which is similar to the experiment results of grounded DC tree.This match indicates that the effective injection depth of space charge in the XLPE can reflect the growth characteristics of grounded DC tree to a certain extent.When the needle tip is grounded,the trapped charge is deprived from the trap due to the stimulation,and the electrical mechanical energy is quickly released in the trapped area,causing the breakage of the molecular chain of XLPE and the formation of low density area,even leads to the initiation of electrical tree in the range of effective injection depth.As the number of short circuit times increase,this process repeats and progress toward the plate electrode,resulting in different characters of electrical tree at different prepressing time.