Application Of Space Charge Characteristics In Ageing Assessment Of Extruded Hvdc Xlpe Cables

As one of the key equipment of HVDC(high voltage direct current) transmission, the safe operation of extruded HVDC XLPE(cross-linked polyethylene) cables is of great significance to the stability of HVDC transmission networks. Therefore, it is a necessity to study the aging assessment of extruded HVDC XLPE cables. In this thesis, space charge measurement as well as physiochemical tests were performed on extruded HVDC XLPE cable specimens which were previously subjected to type test and thermal aging. The aging assessment of extruded HVDC XLPE cables is thus discussed based on space charge characteristics and correlation analysis to provide theoretical and technical support for the establishment of life management platform for extruded HVDC XLPE cables.It is demonstrated by physicochemical analysis that type test does not lead to the deterioration of cable insulation and may instead accelerate the re-crosslinking reactions of the XLPE material. Under thermal aging of 100℃ and 125℃ over the period of 0~336h, thermal crosslinking takes the leading position and the aging of cable insulation is not distinct. Under thermal aging of 140℃, thermal crosslinking is advantaged over thermal oxidation over the period of 0~144h. With the increase of thermal aging time, thermal degradation gradually takes the upper hand, thermal oxidation reacts more violently and cable insulation gradually deteriorates.It is demonstrated by space charge measurement that the surface area of domestically-made cable insulation is easy to accumulate a great deal of hetero-charge under high field, leading to a high rate of field distortion. After applying laboratorial degassing treatment to insulation specimens, it is found that the distinct hetero-charge accumulation is strongly associated with high levels of crosslinking byproducts left in the insulation due to incomplete industrial degassing treatment. After thermal treatment over the temperature range of 100℃~125℃, hetero-charge accumulation shows a clear decrease. Thermal treatment over the range of 100℃~125℃ can therefore be considered to be applied to extruded HVDC XLPE cables, which can help the diffusion of crosslinking byproducts, suppress the accumulation of hetero-charge and meanwhile improve the crystallinity of insulation and the quality of the cable. The heating temperature and time of the thermal treatment, however, still need to be further investigated. In the early stage of 140℃ thermal aging, hetero-charge rapidly disappears, indicating faster speed of degassing. However, distinct space charge accumulation appears in the later stage within the cable insulation, leading to the significant increase in field enhanced factor and mean volume charge density and the distinct decrease in threshold voltage. The conclusion is thus made that the cable insulation is seriously degraded.The following conclusions are drawn after completing the correlation analysis between physicochemical parameters and space charge characteristics. As it is well correlated with other evaluation parameters, the mean volume charge density can be adopted as the key parameter to assess the aging condition of extruded HVDC XLPE cables. Besides, space charge measurement and DSC test have a relatively strong correlation, indicating that the application of space charge measurement in aging assessment of extruded HVDC XLPE cables is of some effectiveness.