Estimation of Series Capacitance of an Actual, Uniform, Single, Isolated Transformer Winding from Measured Driving Point Impedance Data

The authors’ research group established in 2011 that series capacitance of a single, uniform, isolated transformer winding was indirectly measurable via the frequency response measurement. The present work is a sequel and proposes a relatively simpler estimation method. It is based on exploiting a unique property of the first few coefficients (viz., s0, s1, and s2 terms) of the numerator and denominator polynomial of the driving point impedance (DPI) function, which is determined by curve-fitting the measured frequency response data. Essentially, the method solves a set of simultaneous equations which are formulated by equating pertinent coefficients of the fitted DPI(s) to the corresponding coefficients of the generalized expressions computed for an N-section ladder network. Going further, it is shown how the measured DPI data corresponding to the neutral-open and neutral-short cases can be usefully employed to cross-validate the estimated series capacitance. Experimental results on three different windings (viz., 2.2 kV/70 kVA continuous-disc, 33 kV/3.5 MVA continuous-disc, and a fully interleaved-disc) are presented to demonstrate the method’s feasibility.