Accurate temperature correction of dissipation factor data for oil-impregnated paper insulation bushings: Field experience

Typically, dissipation factor (DF) or power factor (PF) test is carried out in the field following well known procedures. It is not necessary to emphasize the importance of dielectric test for power system operators. Accurate recording of insulation temperature values during the test is critical but not always feasible in the field. DF measured values are later normalized to a 20°C base for future comparison and trending. Nevertheless, as stated in several international publications, accuracy of temperature correction is still under investigation because temperature correction factors (TCF) from reference tables do not consider the percentage moisture concentration of the insulation system. The existing Temperature Correction Tables correspond to a variety of insulation materials and construction of different high voltage electrical equipment and components. Therefore, the application of state-of-the-art technologies to determine “specific” temperature correction factors for DF, is essential to provide reliable interpretation of results and proper equipment condition assessment. Frequency Domain Spectroscopy (FDS) in conjunction with DF Analysis are a powerful tool to determine the percentage moisture concentration in solid insulation capable to estimate Individual Temperature Compensation (ITC) of DF measured data of power transformers. Throughout this document, field experience is summarized when a three-winding transformer is removed from service and put to a series of testing procedures including DF, FDS and tip-up test on the high voltage bushings. Results of the analysis, experimental data and conclusions made based on the obtained results are presented herein.