Effect of space charge layers on the electric field enhancement at the physical interfaces in power cable insulation

This paper intends to investigate the effect of two vented water trees w₁ and w₂ growing simultaneously from both conductor and insulation screens of medium voltage cable in the presence or not of space charge, on the electric field distribution. +12 kV DC voltage has been applied. Such simulation has been implemented under Comsol Multiphysics^® environment using the finite element method. Particular interest is assigned to the effect of the polarity, density and layers thickness of the space charge as well as to the behavior of this charge initially accumulated at water trees tips. Note that for the polarity influence, the four polarities configurations related to the space charge accumulated at w₁ and w₂ have been considered. As results, the electric field enhancement at the semiconductor layers especially close to the water tree defect is strongly dependent not only upon the relative permittivity distribution inside water trees, but also on the amount, density and layers thickness of the space charge. In the absence of space charge, we show that, with regard to the inhomogeneous (linear and parabolic) distribution of the permittivity, the homogeneous one causes much more local enhancement of electric field at the trees tips. Moreover, the electric field distribution depends on the geometrical parameters (length and width) and shape of trees; the spherical form engenders a higher electric field than elliptical one. In addition, the association of the space charge could lead to the initiation of the electric trees even at lower voltages. Furthermore, the initiation of electrical trees from the physical interfaces is more probable in the case of opposite polarities especially for negative-positive configuration. Such electrical trees start not only from the tip but also from the water tree root (interface of treed region, un-treed one and semiconductor) which is particularly more vulnerable than inside.

1 Introduction

The presence of water trees in the insulation of power cables is still a major problem. XLPE based materials are subject to treeing which is an important cause of the electrical aging of polymeric insulations and cable failures [1], [2]. The electrical charges which tend to accumulate in the insulation accelerate these processes. One of the ways of contribution to understand the causes of such phenomena consists in determining the distribution of the electric field inside the cable insulation in the presence of the most dangerous water trees case with accumulation of space charge.