Identifying the Phase Undergoing a Discrete and Localized Radial Displacement in 3-$\Phi$, Y and $\Delta$ Windings Using FRA: An Experimental Study

Mechanical winding damages in transformers are quite common. Inherently these damages at an advanced stage are very complex in appearance and are a challenge to represent accurately or model by any known means. Alternatively, as a first approximation, one might treat an incipient damage as a combination of one or more localized and discrete radial displacements and/or axial displacements. When 3-
$\Phi$
transformers suffer a minor and discrete radial displacement, the measured FRA magnitude reveals minor deviations (compared to reference-case). However, it is impossible to infer the nature of damage and which phase has actually suffered the damage. For diagnostics, it is crucial to detect such instances at the earliest. This paper describes a simple method based on measured FRA and winding shunt capacitance
$-$
leading to identification of phase undergoing radial displacement. The method is based on authors' recent analytical expressions, which connects winding's inductances and capacitances to open-circuit and short-circuit natural frequencies. Mathematical formulation to identify which phase contains radial displacement for both Y- and
$\Delta$
- winding is presented first, followed by experiments on a 3.5 MVA, 33 kV setup. The method is extended to identify a simultaneous and discrete radial+axial displacement, occurring in one phase of a 3-
$\Phi$
winding.