Frequency domain dielectric response of oil gap in time-varying temperature conditions

Abstract

The frequency domain dielectric response method has been one of the most effective approaches for the condition assessment of transformers. Prior to conducting the test, the transformer should be taken offline and given adequate time to reach internal temperature equilibrium in order to diminish any temperature-related effects. This can greatly increase the maintenance time; moreover, there is not enough time for some special transformers to reach this internal temperature equilibrium; for example the traction transformers used in the railway industry. The oil-paper insulation in transformers was composed of oil gap, oil-impregnated insulation paper (board). The study related to the dielectric response of the oil gap under time-varying temperature conditions is the basis for studies regarding oil-paper insulation. In this work, the frequency domain dielectric spectrums of oil gap were tested in three cases with different temperature change rates, and the currents in the three cases were tested when the oil gap was applied with low frequency excitations. A current model of the oil gap has been presented here, which is applicable to the time-varying temperature conditions. The current model was verified by comparing the simulated and measured currents. During the verification process, the parameters used in the current model were obtained on the basis of experimental tests. When combined with the established model, a method was developed to obtain the modified results in time-varying temperature conditions; this method was verified by comparing the results calculated from the proposed model with experimentally measured values. In the modification process, the parameters used in the current model were obtained by directly analyzing the tested frequency domain dielectric spectrum of oil gap, rather than through experimental tests, which is more suitable for the maintenance of field transformers. Lastly, the frequency domain dielectric spectrum of oil gap tested in time-varying conditions can be modified based on the proposed model and method.