Prediction Of DC Breakdown Strength For Polyethylene Composite Based On Trap Energy

Polyethylene（PE）is widely used as insulating material for high voltage direct current（HVDC）cable due to its excellent electrical properties,high working temperature,light weight and easy processing.Space charge is considered as main reason for the breakdown of cable insulation while trap energy of insulating material is regarded as an important factor affecting the space charge transport.Therefore,it is necessary to study relationship between the trap energy and DC breakdown strength,which is of great significance to understand the DC breakdown phenomenon and design structure of insulating materials.In this paper,PE nanocomposites were employed as samples and a method of the prediction of DC breakdown strength based on trap energy was proposed.In addition,the effect of ramping rate,thickness and temperature on the prediction of breakdown strength were also discussed.This work can provide an important reference for understanding the insulating material breakdown manner.The main work of this paper is as follows:1.Based on Poole Frankel effect,relationship between trap energy and intrinsic breakdown strength of polymeric insulating material was established.In order to obtain the trap distribution of the polymer,isothermal surface potential decay（ISPD）method was employed.The intrinsic breakdown strength of the material was calculated.DC breakdown strength of the material was predicted by bipolar charge transport（BCT）model according to the calculated intrinsic breakdown strength.2.Low density polyethylene（LDPE）and nano Si O₂ were selected as matrix and filler respectively so as to prepare the LDPE/Si O₂ nanocomposites.ISPD method was used to extract the trap distribution of the sample while the intrinsic breakdown strength was calculated.The DC breakdown strength of the sample under different ramping rates was predicted by single trap BCT model and dual trap BCT model respectively,and DC test was carried out in order to compare the predicted results.The results showed that the predicted DC breakdown strength increased with the increase of ramping rate.With the growth of nano content,it first increased and then decreased,which was consistent with the measured results.Compared with single trap model,the dual trap BCT model was better for the prediction of DC breakdown strength.3.Taking 10,15,20 and 25 k V/mm as initial electric field strength,the trap distribution of LDPE nanocomposites was extracted by ISPD method.The DC breakdown strength of samples with various thicknesses was predicted and experimental research was carried out.The results showed that with the increase of sample thickness,the predicted breakdown strength decreased,which fit inverse power law and was also consistent with the test results.When the initial electric field strength was 20 k V/mm,the accurancy of DC breakdown strength prediction was the best.4.With the purpose of extracting the trap distribution of LDPE nanocomposites under different temperatures,an equivalent time method was proposed to normalize the trap distribution of samples.The DC breakdown strength of samples at different temperatures was predicted based on the trap energy after the equivalence.The obtained results showed that the equivalent trap energy decreased with the increase of temperature,and the predicted breakdown strength decreased with the increase of temperature,which was in good agreement with the tested results.