Combined Spectral Analysis of Thermal and Vibration Time Fields for Assessing the Condition of Oil-Immersed Power Transformers with SF6 Cooling

   Objective: To develop a methodology for the comprehensive diagnostics of the cooling system and mechanical integrity of windings based on the joint spectral analysis of synchronized time series of the vertical temperature profile in SF6 gas and winding vibration.

   Methods: The primary method is multifrequency spectral analysis. For thermal processes, an adapted model of advective-diffusive heat transfer is used, allowing the estimation of the SF6 flow velocity and effective thermal diffusivity coefficient from the relationship between the amplitude ratio and phase shift of vertically spaced sensors. For analyzing mechanical condition, the use of a miniature fiber-optic vibration sensor installed in the inter-winding space is proposed.

   Results: Based on the conducted experiments, diagnostic features were identified. It was established that a decrease in the SF6 flow velocity leads to an increase in the amplitude ratio of the low-frequency (1.1×10^-4 Hz) temperature component. Weakening of the winding clamping manifests as an almost twofold increase in the normalized vibration amplitude at 100 Hz, while local deformation manifests in the appearance of new resonant peaks. High coherence between slow temperature oscillations and vibration was revealed.

   Conclusions: A comprehensive approach is proposed, which enhances diagnostic reliability through the synergy of thermal and mechanical process analysis and the formation of digital condition "fingerprints". The method is intended for integration into predictive maintenance systems.

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