Temperature and Frequency dependence of dielectric parameters of Oil-paper insulation under sinusoidal and square wave excitations

Results of Frequency Domain Spectroscopy are affected by different environmental factors, among which temperature is predominant. The migration and distribution of the moisture content within the oil-paper insulation depends upon temperature. Moreover, voltages used in laboratory for FDS are sinusoidal in nature. But the insulation during onsite operation is stressed by the voltages that often deviate significantly from sinusoid. So, use of non-sinusoidal excitation for condition assessment of oil-paper insulation is desirable to obtain the proper information about the actual condition of the insulation. Considering the aforesaid facts, diagnostics of the oilpaper insulation has been performed using sinusoidal as well as non-sinusoidal excitation at different temperature. In the paper square wave excitation has been chosen as non-sinusoidal excitation. A sample with preset moisture content has been prepared in the laboratory that emulates the oil-paper insulation of the power transformer. Sinusoidal as well as square wave excitation over a wide frequency range is applied on the sample and corresponding dielectric response current is measured at different temperature. The physical condition of insulation at different temperature can be predicted more accurately using square wave excitation.