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.
Condition assessment of distribution PILC cables
The condition assessment of paper insulated lead covered cables (PILC) cables is a crucial factor for many utilities and this paper is devoted to the evaluation of some new diagnostic techniques enabling users to effectively manage their PILC cable assets. We used electrical, metallurgical and chemical techniques to measure the electrical, chemical, dielectric and metallurgical properties of paper-impregnated insulation. The nondestructive electrical tests performed on three full-length field-aged PILC cables were: the isothermal relaxation current (IRC), the LIpATEST leakage current test and the return voltage method (RVM). Chemical tests were performed on small samples of paper tapes and oil taken from the same samples. They were: dielectric analysis, Fourier transform infra red (FTIR) spectroscopy and moisture content analysis. The electrical techniques ranked the cables consistently, that is one cable aged 23 years seemed to be more severely aged than the older (34 years) and younger (4 years) cables. This could possibly be explained by the acids detected in the oil of the 23 year old cable using FTIR spectroscopy. Although more data on more cables would be needed it already appears that the tested techniques could assess the condition of the insulation of PILC cables. Water ingress is often associated with cracks in the lead sheaths due to fatigue and creep failure. Metallurgical tests were performed on the lead sheaths of four PILC cables and they revealed that artificial cracks of different depths introduced on the surface of the lead sheaths were easily detected by the visual and fluorescent dye penetrant inspections. An eddy current inspection technique was successful in detecting the artificial surface flaws on the lead sheaths. The hardness of the older lead sheath, as measured by the Brinell test, tends to be lower than that of the lead sheath of the younger vintage cables.