Impact of Inter-Winding Magnetic and Capacitive Coupling on SFRA Signature

Abstract

Sweep Frequency Response Analysis (SFRA) serves as a highly sensitive diagnostic technique for identifying mechanical deformations within power transformer windings. The inherent magnetic and capacitive inter-winding couplings in transformers mean that a fault localized to a single winding can propagate detectable anomalies across the frequency responses observed at the terminals of all windings. This study systematically explores this interdependency through detailed simulations and case studies. A comprehensive six-winding equivalent circuit model of a 40 MVA, 132/11 kV power transformer, incorporating both magnetic and capacitive coupling effects is simulated to validate the underlying hypothesis. Complementing this, faults comprising an open-circuit condition and radial buckling were deliberately induced in the low-voltage (LV) winding of a 20 MVA, 66/11 kV oil-immersed transformer for experimental validation. The resultant frequency response traces reveal pronounced deviations in both LV and high-voltage (HV) signatures attributable to faults within a single phase winding. The study concludes that, due to unavoidable coupling, analyzing a single winding signature is sufficient to ensure the presence of mechanical deformation, whereas comparison of multiple signatures retains value for fault localization and severity assessment.