302 / 2018-09-25 22:23:35

Simulation of Space Charge Distribution in the Needle-Plate Electrode

Space charge,needle-plate electrode,simulation,stepped DC voltage,electrical tree

At DC voltage, space charge is the main factor to affect the electrical tree growth. On the one hand, the injected homo-charge moderates the electric field near the needle; on the other hand, the injected charge would be trapped in the dielectric, which is accompanied by the release of energy. The energy may motivate the electrons to be hot electrons which could bombard the molecule chains to generate dissociated molecules. Lots of dissociated molecules form the low density-region and it may promote the electrical tree growth. However, limited by the detection sensitivity of the space charge measurement device, the space charge distribution in the needle-plate electrode is difficult to be determined. Normally, the space charge characteristics in plane-parallel electrode system could be a reference to the needle-plate electrode logically, but it is not comprehensive enough. Hence, in order to obtain the possible distribution of space charge in the needle-plate electrode, a model based on a finite element simulation software is applied in this paper to simulate the density and distribution of trapped charge in XLPE under stepped DC voltage, where the tip radius of needle is 5 μm and the distance between the needle tip and the plate is about 1.5 mm. The needle tip is set to the positive polarity and the plate is grounded. The stepped DC voltage is carried out as follows: from 0 V, voltage rises 5 kV every five minutes until the voltage reaches at 40 kV, where the rising rate of voltage is 500 V/s. The bipolar charge transport model has been widely used for the simulation of space charge behavior in polymer. The needle-plate electrode model in this paper takes into account following process: charge injection, trapping, detrapping, transport, recombination, diffusion and extraction. The results show that there is positive trapped charge near the needle, and the trapped charge region is in shape of a water drop. The density of positive charge near the needle tip increases with the voltage, whereas at a certain voltage the density of positive charge decreases with time. At 40 kV, the maximum positive trapped charge near the needle is 200 C/m3 and the injection depth of the charge is about 100 μm. Based on the simulation model, an electrical tree growth experiment under stepped DC voltage is carried out. The experiment results show that the length increment of the tree whose initial length is less than 100 μm is larger than the length increment of the tree whose initial length is more than 100 μm. The experiment results could be explained by the simulation results of the space charge (positive charge near the needle) distribution. As the short initial tree defect is within the area of the trapped positive charge, the front of the initial defect may form a large low-density area, which causes the more growth for the short tree defect; however, the long initial tree defect may exceed the trapped charge area, thus the growth for the long tree defect may not occur in the low-density area, which may restrict the tree growth.