Comparative study on the effect of temperature on frequency domain spectroscopy results under sinusoidal and triangular excitation

Condition assessment of in-service power transformers are becoming increasingly important for maintaining reliable and uninterruptable power supply. Among non-invasive, modern condition assessment tools, frequency domain spectroscopy is predominant in determining the aging of oil-paper insulation in power transformers. However, the results of frequency domain spectroscopy are severely influenced by other parameters, such as temperature. Also, the voltages used for studying frequency response in the laboratory are usually sinusoidal in nature. But the insulation system in operation on site is stressed by voltage that may deviate significantly from a sinusoid. Thus the use of non-sinusoidal waveform in obtaining the dielectric response is desirable for advanced insulation diagnostic technique. Considering the aforesaid facts, in this paper frequency response analysis has been performed by applying sinusoidal as well as non sinusoidal waveforms at different temperatures. A laboratory prepared oil-paper insulation sample is used that emulated the composite insulation system of a transformer. The results from sinusoidal response show a good agreement with the conventional frequency response analysis thereby validating the proposed methodology. From non-sinusoidal response, added information is obtained for closer interpretation of the actual physical condition of the sample under test.