Elegant procedure to estimate series capacitance of a uniform transformer winding from its measured FRA: implementable on existing FRA instruments

Abstract

A simple procedure to estimate series capacitance of a uniform transformer winding from its measured frequency response analysis (FRA) and shunt capacitance is presented. Unlike previously published approaches, this method does not involve any cumbersome and time-consuming curve-fitting nor running optimisation/search algorithms, and neither does it require data of winding geometry. The procedure relies on a property that is observable in the impedance function of a lossless winding, viz., the ratio of the product of squares of open circuit natural frequencies to the product of squares of short circuit natural frequencies bears a special relation to impedance function coefficients. Its feasibility was initially verified by simulation, and then by experiments on small-sized continuous-disk and interleaved-disc windings, followed by a large-sized 33 kV, 3.5 MVA continuous-disc winding, and finally on a 315 kVA 11/13.8 kV transformer. After measuring FRA, the process involves just finding roots of a polynomial, from which the initial impulse voltage distribution constant and series capacitance can directly be determined. Given these attractive features, authors believe that this method is implementable on existing FRA instruments, so that, along with routinely measured FRA, these two important constants of a winding can be displayed.