Experimental Results on Estimation of Effective Inductance Associated With Winding Resonances of 1-Φ and 3-Φ Transformers Using Measured FRA

An indirect measurement method based on an analytical formula is presented to estimate the effective inductance which is associated with the winding resonances of an iron core transformer winding. It is applicable to any 1-Φ or 3-Φ (Y, Δ) winding, with any condition of the neutral. A new analytical method is developed using authors' recent work (which linked the harmonic sum of squares of natural frequencies of a winding to its inductances and capacitances). Data about the trough (SCNFs) or peak (OCNFs) frequencies corresponding to two FRAs is the only required input; one FRA corresponding to the nominal condition and another measured after connecting a known lumped capacitor across the winding. Details of derivations for each 1-Φ and 3-Φ configuration is initially presented. Thereafter, experimental results are discussed on 1-Φ and 3-Φ, 33 kV, 3.5 MVA winding setup, where any desired winding connection can be configured and examined. Then, results on two actual transformers are discussed to prove method's practical implementability. Finally, some preliminary thoughts on using this newly defined quantity for assessing mechanical integrity of winding is briefly examined.