Estimating the Equivalent Air-Cored Inductance of Transformer Winding From Measured FRA

A novel indirect measurement method to estimate the equivalent air-core inductance of an iron-cored transformer winding is proposed. The method utilizes frequency response analysis (FRA) magnitude data to estimate this quantity. Its implementation involves just two FRA measurements: one with the neutral end open circuited, whereas the other one is with a known external lumped capacitance connected between the neutral and ground. This equivalent inductance is straightforward to estimate from the short-circuit natural frequencies extracted from the two FRA magnitude plots. The recently derived analytical relationship connecting short-circuit natural frequencies to winding inductances and capacitances is employed for this purpose. Experimental demonstration is reported initially for actual windings with core removed to prove the feasibility of the method, and later, for an actual transformer. Finally, experiments on an air-cored winding are also reported to highlight one possible use of this quantity in discriminating radial and axial displacements.