Evaluation of Distribution Class Transformers Using Narrow Band Dielectric Frequency Response Measurements

The analysis and diagnostics of power and distribution transformers has been performed primarily by time and frequency domain methods. In the frequency domain, power factor or dissipation factor at line frequency is a method widely used but one single measurement at line frequency encounters limitations to analytical and qualitative interpretation of the insulation of a variety of electrical equipment. With the benefits found by wide band dielectric frequency response, the need to incorporate a method capable of better condition assessment in very short time is required in the field. Narrow Band Dielectric Frequency Response (NB DFR) is a series of dielectric loss measurements performed in a narrow frequency band typically between 1 and 500 Hz or even up to 1 kHz, at a low voltage. In 2015, Southern California Edison began utilizing NB DFR for dielectric loss measurements in the interwinding insulation in condition assessment of used distribution class transformers. The scope of the work included thermal accelerated aging subjecting the transformers different levels of overload while using NB DFR to document changes in the dielectric response as the oil and paper throughout the process. Based on the experimental results obtained, authors provide throughout this document an analytical procedure to evaluate the NB DFR described as a plot of PF/DF as a function of frequency as well as capacitance (C) as a function of frequency. Most important is to consider the factors influencing the changes in the dielectric response after scheduled thermal accelerated aging. On the basis of specific characteristics observed within the plot, definite assertions may be made in regards to the condition of the cellulose and oil. This work reflects the benefits of NB DFR to evaluate the aging process of oil-paper insulation in comparison to conventional line frequency power factor testing.