Polarisation based measurement provides an indirect and non-destructive alternative to determine the condition of oil and paper insulation of transformer. Polarisation based measurement includes Polarisation and Depolarisation Current (PDC) and Frequency Domain Spectroscopy (FDS) in time domain and frequency domain respectively. Both PDC and FDS measurements are sensitive to moisture, ageing condition of insulation and some of the contaminants that are responsible for the degradation of transformer insulation at elevated temperature. However, the information available from these measurements is mostly relied upon the available interpretation schemes. PDC and FDS measurements provide overall information about the condition of oil and paper insulation and it is possible to separate the condition of oil and paper insulation. However, the main difficulty still faced during the interpretation of dielectric response measurement is to separate the impact of moisture from the ageing condition of the insulation itself and to quantify amounts of contribution of different parameters from dielectric response measurements. This forms the main part of the investigation in this research study.
This thesis presents an experimental study on ageing of transformer insulation and investigates the impact of various parameters on dielectric response measurement. Ageing experiments are carried out on oil-impregnated pressboard samples with various moisture content and impact of moisture and ageing on PDC and FDS measurements are studied at different ageing time. Other measurements such as Degree of Polymerization (DP), furan analysis, Dissolved Gas Analysis (DGA) and moisture measurement by Karl Fischer Titration (KFT) are also conducted on oil/pressboard samples. Pressboard samples selected for the study are of different thickness and densities. During ageing experiment, pressboard samples are subjected to different rates of ageing mainly due to the difference in dimension, density and the amount of moisture in samples. Hence, the measurement on samples at different ageing time provides a various combination of moisture and ageing condition for oil and pressboard insulation. It allows to understand the impact of moisture and ageing condition of insulation under different situation.
The imaginary component of complex permittivity is found to be remained almost independent of frequency in the low frequency range ( < 0.01 Hz) mostly due to high DC conduction in the presence of moisture. Moreover, the real component of complex permittivity increases in the low frequency range due to so called Low Frequency Dispersion (LFD) but remained almost similar in high frequency range. Based on this observation, a relationship between imaginary permittivity and moisture in pressboard sample is proposed in this thesis. The relationship is validated with direct moisture measurement on a number of oil-impregnated pressboard samples under different ageing conditions. The amount of moisture estimated from the proposed relationship is observed to be in agreement with the direct moisture measurement by Karl Fischer Titration (KFT) for oil impregnated pressboard sample under different conditions.
Further experiments and measurements are carried out on oil/pressboard samples after 2000 h of ageing under different combinations of moisture, ageing condition of pressboard insulation, new/aged oil to separate the impact of various parameters on polarisation based measurements. The impact of ageing product on imaginary permittivity is limited to the mid-frequency range (0.1 Hz - 100 Hz). The impact of ageing (DP) of pressboard sample is smaller compared to the influence of moisture on dielectric response and it could be compared as an equivalent to that of unaged pressboard sample with less than 0.5% moisture content. Frequency response is mostly dominated by moisture and the contribution of ageing products is smaller than that of moisture. Dielectric response measurements are sensitive to the ageing condition of insulation, but the presence of moisture masks their capability to provide useful information about the ageing condition of insulation.
In this thesis, an airgap electrode arrangement is proposed to determine moisture diffusion profile in oil-impregnated pressboard sample through continuous dielectric response measurement. A number of moisture diffusion experiments are carried out on oil-impregnated pressboard samples through airgap electrode arrangement at different humidity and temperature. A mathematical model for moisture diffusion in oil/paper insulation is given based on Fick’s second law and simulations are performed based on the finite volume method. Dielectric response measurements are very sensitive to the smallest change in moisture contents and moisture diffusion profile of oil impregnated pressboard sample can be accurately determined from continuous dielectric response measurement. Overall, the rate of change of moisture in pressboard sample estimated from dielectric response measurement is in agreement with direct moisture measurement by Karl-Fischer Titration at the end of the experiment and moisture diffusion profile calculated from simulation results.
Moreover, fractal nature of polarisation and depolarisation current is identified in this thesis and it is demonstrated using sandbox method and wavelet transform modulus maxima (WTMM) method. Scaling properties of polarisation and depolarisation current of pressboard sample and transformer insulation under different condition are studied using multifractal spectrum. Finally, dielectric response measurements and chemical based measurements are performed on the field transformers and results obtained from the measurements are correlated and pros and cons of various insulation diagnostic techniques are discussed.