Space Charge and Associated Electric Field Distribution in Presence of Water Trees in XLPE Insulation Under DC and AC Voltages

The aim of this work is to investigate the effect of frequency and the physical properties of water trees, namely, conductivity and permittivity, on the evolution of the space charge and the associated electric field distribution in the cross-linked polyethylene (XLPE) insulation under direct current (dc)and ac voltages. For this purpose, we have considered a 12-kV electric cable model with XLPE internal insulation, coated with two semiconductor layers, one inside and one outside. A pair of vented water trees, denoted as w1 and w2, is developed from these semiconductor layers. It is assumed that the permittivity and the conductivity are homogeneously distributed within the two water trees. Our study was carried out under COMSOL MULTIPHYSICS environment, using the finite element method. As main results under ac voltage, the increase of applied frequency blocks the ability of the space charge to move through the insulation, affecting both space charge density dynamic motion and electric field distribution. On the contrary, a water trees conductivity increase releases the space charge accumulated at the interfaces of the two semiconductor layers. In fact, this process amplifies the activity and dynamic behavior of such charge, facilitating its penetration into the insulation through the semiconductor layers before completely moving to two water tree tips. The same phenomenon, resulting in the complete migration of the accumulated charge from the semiconductor layers to the tips of the water trees, has also been observed in dc. The amount of this accumulated space charge leads to the electric field reinforcement especially at the XLPE insulation/water trees tips critical interfaces. This situation could generate an ac or dc electrical tree. This latter can only be initiated from the water tree tip.