Experimental Study On DC Breakdown Mechanism Of XLPE For High Voltage DC Cables

DC cables are important equipment in DC transmission.The insulation reliability of DC cables is a key factor that determines the safe operation of high voltage transmission grid.Cross-linked polyethylene(XLPE)has been used as one of the main insulation material for HVDC cables because of its excellent performance.As we know,there is an inverse power attenuation relationship between breakdown strength and thickness of insulation materials.Determining the insulation thickness of the cable at the designed voltage by DC breakdown strength of XLPE sheet samples of different thicknesses is a commonly used insulation electric field design method in engineering.However,in the traditional DC breakdown test method,the true breakdown strength data could not be obtained due to the effect of surface flashover for samples with a thickness of more than 1 mm.Therefore,the traditional breakdown test method which uses sheet sample placed in insulating medium has great limitations.In this paper,electrode-embedded fully extruded molding sample is proposed.Verify the no surface flashover characteristic based on the finite element method.DC breakdown experiments on XLPE-A and XLPE-B samples with insulation thicknesses of hundreds of microns to several millimeters at different temperature are conducted and compared with the traditional DC breakdown experiment at the same time.The range of thickness could be measured up to 1.5 mm at normal temperature and 2.5mm at high temperature(70?).The sample thickness range of DC breakdown test is greatly improved,and the thickness limitation of the traditional DC breakdown method is made up.The breakdown strength of both sheet and fully extruded molding XLPE samples decreases with increasing thickness.The relationship between breakdown strength and thickness is fit according to an inverse power function.The following conclusions are drawn: At the same thickness,the characteristic breakdown strength of fully extruded molding samples is significantly higher than that of sheet samples.The breakdown strength and thickness of fully extruded molding samples still follow the inverse power law at high temperature.However,the breakdown strength is significantly lower than that at normal temperature.This shows that the increase in temperature reduces the thickness effect of the breakdown strength.Extrapolation of the fitting results could be used to estimate the true value of the insulation breakdown strength under operating condition of actual cable.In order to verify the experimental phenomenon that the breakdown strength tends to be stable when the thickness of the fully extruded molding sample in the breakdown test reaches 2 mm and above,the paper studies the DC breakdown strength of model cables with different insulation thicknesses and designs different terminal processing schemes.It is found that install a simple stress cone on the section of the outer shield and insulation layer,and cooperate with water and transformer oil medium to maximize the breakdown strength of the model cable.In the breakdown experiment of model cables with different thicknesses,all breakdown locations of model cables are still near the section.The breakdown strength of model cable bodies with different insulation thicknesses is higher than70 k V/mm at 70?.There is no significant difference in breakdown strength of cables with different thicknesses.This indicates that the breakdown strength has reached a stable stage in this thickness range,and the thickness dependence is not obvious.This verifies the correctness of the breakdown test results of the fully extruded molding sample.To study the effect of space charge on breakdown strength in DC breakdown process,the space charge distribution of XLPE sample is measured from the beginning of polarization to breakdown when the voltage rises uniformly.The studies show that positive charge is injected from anode in XLPE sample under high electric field.Positive charge completely occupies the sample and the amount of charge increases sharply when breakdown happens.In the step-up method,when the electric field is above 90 k V/mm,the increase of polarization time would lead to an increase in mean volume density of space charge and field distortion.Construct an inverse power function to describe the relationship between maximum electric field distortion and sample thickness.Both maximum electric field distortion and breakdown strength have similar inverse power relationship with thickness.The thickness effect of breakdown strength could be explained from the secondary effect of space charge.Experimental research on the breakdown strength test method of XLPE in millimeter insulation thickness range is carried out.And explore the effect of space charge on the breakdown strength during DC breakdown process to provide accurate breakdown parameters for the insulation structure design of HVDC cables and similar equipment.