The influence of water trees on permittivity and loss factor of medium voltage cables polyethylene insulation

This paper presents an experimental study regarding the variation of permittivity and loss factor of power cables polyethylene insulation, due to the change of the frequency (f_a) and the duration (τ) of the applied electrical field. The experiments were performed on XLPE flat disc samples from a medium voltage cable insulation. The samples were aged in an electric field of E = 4 kV/mm and f_a = 3-5 kHz in the presence of a NaCl solution (0.1 mole/l) for a period τ = 24-96 hours. After ageing, water trees density and dimensions (lengths and diameters), the real and imaginary parts of the permittivity and loss factor were measured. The results show that the all quantities increase with f_a and τ and the permittivity and loss factor increase with the decrease of the measurement frequency.