In high voltage direct current (HVDC) cable systems, study of electric field distortion created by space charge accumulation is very important for examining insulation performance. Therefore, in this paper the influence of space charge on short-time breakdown characteristics of cross-linked polyethylene (XLPE) are studied. Trap energy distribution is an important factor affecting space charge behavior in dielectrics. Dielectrics with various thicknesses exhibit different space charge phenomena due to various trap distribution characteristics, which regulate the injection, migration and accumulation processes of charges. Consequently, trap characteristics can affect many parameters, such as charge accumulation depth and injection barrier. XLPE with thickness less than 100 µm show lower density of deep traps which can accumulate homogeneous bulk charges as compared to thick XLPE with thickness greater than 100 µm. This phenomenon determines the internal field strength distortion rate which is 20% larger for thin XLPE and it is much higher than that for thick XLPE. The external factor which is considered for analysis is applied field strength and it shows a linear proportional affect on the electric field distortion. A quantitative relationship model between applied field strength and distortion field strength of XLPE with different thicknesses was established. Linear extrapolation was used to obtain the trend of applied, distortion, and actual field strengths at different XLPE thicknesses when breakdown occurred. When the thickness of XLPE is less than 100 µm, distortion field strengths is higher than 50 kV/mm and increases with decreasing thickness, resulting in a significant effect on the breakdown phenomenon and breakdown strength increases with increasing thickness. And when the thickness of XLPE is greater than 100 µm, the distortion field strengths is less than 50 kV/mm which also confirms the volume effect on breakdown phenomenon and breakdown strength decreases with increasing thickness. INDEX TERMS Breakdown, thickness, space charge accumulation, XLPE.
Inverse power model (IPM) is often used in the engineering field to estimate the electrical life of cross-linked polyethylene (XLPE) cable insulation and describe the relationship between applied voltage and insulation failure time. The voltage tolerance index n in IPM is also used as an important factor to select appropriate AC cable thickness and pre-evaluation test voltage. However, under the influence of DC electric field, space charge accumulation changes the internal field strength of XLPE. As a result, the actual electric field strength is quite different from the applied value. Hence, the existing IPM model exhibits a large error in evaluating the electrical life of DC cables. In this paper, an improved IPM is proposed. The correction parameters α and β are introduced into the cumulative loss parameter C and voltage tolerance index n of the existing IPM to quantify the effect of space charge on distorting the electric field in electrical insulation life. Correlation between applied field strength (E a), maximum endurance field strength (E rm) and insulation failure time t of several XLPE with different thicknesses is obtained from DC voltage withstand test and pulsed electro acoustic test. In addition, the validity of the improved IPM is preliminarily verified. α and β predicted by using the space charge characteristic parameter matrix [P] is proposed to efficiently obtain the actual parameters. According to experimental analysis, the improved IPM with a certain thickness is predicted by using the neural network fitting method. INDEX TERMS Cross-linked polyethylene, space charge, inverse power model, voltage tolerance index, pulsed electro acoustic.
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