Interpreting Loss of Clamping Pressure and Axial Displacement in 1-$\Phi$, 3-$\Phi$ Windings Using Effective Air-Core Inductance: An Experimental Study

Recently authors proposed an indirect method based on measured FRA to estimate an important electrical parameter of the winding, viz., effective air-core inductance (
$L_{eff}$
), which together with winding capacitances creates the resonant frequencies. Since,
$L_{eff}$
is directly linked to producing resonances, it is imperative that it possibly might be the right quantity to interpret frequency shifts observable in peaks and/or troughs when comparing two FRAs. The feasibility of such a possibility is investigated here. In the first part, a detailed study of uniformly distributed axial displacement (AD) viz., loss in clamping pressure on an actual single isolated winding is done to examine suitability of
$L_{eff}$
to not only detect it but also quantify it. Results revealed that
$L_{eff}$
has exceptional sensitivity, bears a physical relation to AD, and most importantly is monotonic with respect to physical change
$-$
a unique feature. Thus, emerging as an ideal candidate to link frequency deviations in two FRAs to a measurable quantity. Encouraged by this success, the concept was next extended to interpret a localized AD of 5 mm introduced in one phase of a 3-
$\Phi$
winding (Y or
$\Delta$
) rated for 33 kV, 3.5 MVA. During these experiments, the phase position and extent of AD could be varied at will and
$L_{eff}$
successfully identified the phase containing AD, and its extent qualitatively. The method requires only measured FRA data, is applicable to any winding and does not need design data.