Temperature dependent relaxation studies on oil-filled transformer

Relaxation studies in oil-filled power transformers are very important to evaluate the ageing characteristics of the oil-paper insulation. The polarization processes at the interfaces can be understood by making measurements at different operating temperatures. A laboratory study is carried out using both frequency and time domain techniques. In the frequency domain technique, a dielectrometer is used and the linear response characteristics with energizing voltage magnitude up to 200 V peak are studied. In the time domain method, the recovery voltage method (RVM) up to 300 V DC is used to study the linearity in response. The obtained response is fitted to an extended Debye model and the interrelation of the model with each measuring technique is established. Then, the temperature dependence of these parameters is studied by keeping the oil-paper insulated transformer in a humidity/temperature controller for 10 hours at a certain setting. The ambient temperature is changed from 20°C to 50°C. The responses at high temperatures differ from the measurement at room temperature. The conductance property of the transformer insulation between two windings changes significantly. Three types of analysis are done to correlate the measurements. The first analysis is to fit the frequency and time domain responses to the extended Debye model. The determined parameters are compared to evaluate the validity. of model. In the second method of analysis with RVM, RV spectra are evaluated in term of peak magnitude variation and time shift of peak occurrence with temperature. In the third method of analysis with low frequency response, it is found that the variation of admittance and loss angle has a unique bandwidth variation with temperature. It is found that the relaxation processes of oil and oil-paper are distinctive and their dependence on temperature is different.