Application of RVM technique to study the polarisation and depolarisation characteristics associated with thermal ageing of epoxy mica composite
The Recovery voltage measurement technique was used to investigate the polarization processes in the dielectric to assess the insulation ageing. Two different types of insulation systems are evaluated, resin poor and resin rich mica composites. This paper presents and discusses the data on polarization and depolarization characteristics of thermally degraded epoxy mica composites. Beyond polarization spectrum, additional spectra slope spectrum and peak time spectrum of Recovery voltage measurement technique were used. The combined spectrum: a plot of initial slope (V/s) against Max. Recovered voltage, Vr(V) showed a clear peak in the form of “bun” and “nose” distinctly which probably could give more information on the bigger polar species and the polarization phenomena. Multiple peaks are seen in case of thermally aged samples which clearly exhibit the degradation phenomena.
Analysis of dielectric response of epoxy-mica insulation with temperature normalization
To decrease equipment costs and facilitate the simple and convenient on-site frequency dielectric spectroscopy (FDS) testing of epoxy-mica insulation of stator coils, specimens from a stator coil, subjected to 623 hours of 48 kV AC electrical ageing, were tested at temperatures from 0 to 120°C and frequencies from 10−1 to 107 Hz. The master curves and shift factors of the real and imaginary susceptibility components were obtained by the temperature normalization method. According to the horizontal shift factor, the complex susceptibility of the insulation increased exponentially with temperature. Regarding the vertical shift factor, the characteristic peak of the imaginary component of the complex susceptibility, representing the interface polarization of the epoxy-mica insulation, increased exponentially with temperature, and its movement can be expressed by the Arrhenius equation. The master curves of the temperature from 20 to 100°C and the frequency from 1 to 103 Hz can be fitted to the six-degree Debye relaxation model and Havriliak-Negami function. Through the shift factors and master curves, FDS data from 20 to 100°C can be estimated without temperature control.
Power cable diagnostics by dielectric spectroscopy
Dielectric spectroscopy, a method for nondestructive material characterization, is now available for practical diagnostic measurements in the field. The method is, due to new technology, easy to use and shows significant advantages in the interpretation of the results, especially compared to simplified methods like measurement of dissipation factor at a fixed frequency. The method is briefly explained with application on polymer cables. Additionally, the application on paper insulated power cables is introduced.