Electrical condition monitoring techniques for low-voltage cables used in nuclear power plants

In this paper, dielectric spectroscopy is adapted to investigate the dielectric response, in the frequency range 10^-2-10⁶ Hz, of a low-voltage cable for nuclear power plants. Cable samples were aged under accelerated stress conditions up to an equivalent aging time of 60 years. The aging procedure consisted of two phases: thermal aging by means of high temperature followed by gamma-irradiation at room temperature. Dielectric spectroscopy has been adapted to measure the dissipation factor Tan(δ) of inner insulation and outer sheath. A significant reduction of Tan(δ) can be observed in the frequency range 10¹-10⁴ Hz, indicating that accelerated aging process mainly affects the dipolar polarization. Furthermore, the shape of the dielectric response presents a global minimum which shifts to lower frequency as the aging time increases. Experimental results show that the frequency in correspondence of this minimum value, as well as Tan(δ) at the reference frequency of 100 Hz, can be used as aging indicators.