Diagnosing Specific Winding Deformation Patterns

Different types of winding movement create characteristic fingerprints on the FRA trace. Radial Deformation, an expansion or contraction of the winding cylinder, primarily causes significant shifts in the mid-frequency range (typically 10 kHz to 500 kHz), altering the amplitude of resonant peaks. Axial Displacement, where windings shift vertically, heavily impacts the low to mid-frequency response (often 1 kHz to 20 kHz) due to changes in inductive coupling between discs and coils. Local Buckling or a Partial Collapse of winding sections introduces new, smaller resonant peaks or dips at high frequencies, superimposed on the main trace. Recognizing these patterns allows engineers to understand the nature of the damage sustained during a through-fault or transport incident.

Identifying Core-Related Issues via FRA

While FRA is most famous for winding analysis, it also provides clues about core integrity. Problems such as core lamination shorting or loss of core grounding manifest as clear deviations in the very low-frequency region (below 2 kHz). This range relates to the magnetizing impedance. A change in the slope or magnitude here can indicate issues with the core's magnetic circuit. Similarly, faulty core clamping can affect the mechanical stability of the entire active part, leading to broad-based shifts across multiple frequency bands when compared to a stable baseline. Disentangling core effects from winding effects requires careful analysis of all three phase responses.

The Role of Numerical and Statistical Analysis

Visual trace comparison, while essential, can be subjective. Advanced diagnosis employs numerical correlation methods to objectively quantify differences. Common indices include the Correlation Coefficient (CC) and the Root Mean Square Deviation (RMSD), calculated for separate frequency bands. For example, a low CC value in the high-frequency band alone might point to a minor bushing or tap changer connection issue, while a low CC across all bands suggests major global damage. Statistical analysis of results from a transformer fleet can also establish population norms, helping to identify outliers that require immediate attention, even in the absence of a pristine baseline.

Integrating FRA with a Comprehensive Diagnostic Portfolio

No single diagnostic test provides the complete picture. Advanced FRA diagnosis is most powerful when conclusions are cross-verified with other techniques. For instance, suspected winding deformation from FRA should be correlated with Short-Circuit Impedance measurements, which are also sensitive to winding movement. Potential core faults suggested by low-frequency FRA deviations must be reviewed alongside Exciting Current test results. Evidence of internal discharge from Dissolved Gas Analysis (DGA), combined with a specific FRA signature, can confirm active arcing due to displaced windings. This multi-parameter approach transforms FRA from a standalone tool into the central component of a definitive condition assessment strategy.

Mastering advanced FRA interpretation transforms raw data into actionable intelligence. It enables asset managers to move from knowing a transformer is "faulty" to understanding exactly what is wrong, assessing its operational risk, and making informed, economic decisions on repair strategies. This depth of analysis is key to extending service life and ensuring the reliability of critical high-voltage transmission networks.