Effect of water trees ′string of pearls′ configuration on the distribution of electric field, current and stressed volume in XLPE insulation

The processes originating in local areas of cross-linked polyethylene (XLPE) insulation of extra-high voltage cables during water tree germination between closely disposed water micro-inclusions in insulation have been studied. According to current experimental data, water trees in XLPE have not solid cylindrical shape, as it was thought previously, but they consist of closely spaced nanoscale inclusions of spheroid form and nanoscale thin water channels between them (so-called "string of pearls" confi­guration). The electric field disturbances near such trees can significantly differ from the disturbances near solid ones. Therefore, the paper presents the results of mathematical modeling and analysis of the electric field strength distribution, changes in stressed volumes and densities of currents in XLPE insulation near the micro-inclusions connected by the tree of "string of pearls" configuration. The cases of nano-cracks appearing in a dielectric gap between micro-inclusions, their partial or complete filling with water, that is, the water tree formation with branches of different conductivity were simulated. It was shown that water tree germination between inclusions and the increase in the conductivity of its branches leads to connecting inclusions into a single conductive structure that disturbs the electric field stronger than single micro-inclusions. The regularities of increasing the electric field strength (which describes the rapid determined degradation mechanisms), increasing the stress volume (which describes the slow stochastic degradation mechanisms) and increasing current density in local areas (which describes the insulation overheating) were determined. The obtained results are useful for the analysis of interrelated processes of XLPE degradation and for the evaluation of insulation resource during its long operation.