Temperature dependence of the dielectric properties of the insulation system of power transformers

Due to the age of power transformers in service, diagnostic methods to monitor the dielectric properties of the insulation have become a very important subject. One of the methods to do this is the Return Voltage Method that allows the calculation of the dielectric time constants of the two insulating materials cellulose and oil. These parameters are sensitive to the contents of water in the cellulose and the amount of degradation products in the oil. Water is a by-product of the depolymerisation of the cellulose and thus a good indicator for degradation processes. As with all dielectric diagnostic methods, the measurement temperature may influence the results. To analyze this influence, measurements on two differently aged 50 kVA transformers were performed in the temperature range between 15 and 55degC. The results show that for both transformers the times t_m of the maxima U_m of the Return Voltage curves vary with temperature in accordance to a thermally activated conduction process with an activation energy between 0.5 and 0.6 eV, a finding that also supports the concept of the interpretation of Return Voltage measurements on the basis of boundary polarization processes, that can be described by a Maxwell equivalent circuit.