Comprehensive Guide to Winding Deformation Detection Using Transformer Frequency Response Analyzers

Introduction to Winding Deformation in Power Transformers

Power transformers are subjected to various mechanical and electrical stresses throughout their operational life. Short-circuit currents, improper transportation, seismic events, and aging can all contribute to physical deformation of transformer windings. Unlike insulation degradation, which can often be detected through oil analysis, mechanical deformations remain hidden until they cause catastrophic failure. The Transformer Frequency Response Analyzer (SFRA) has become the gold standard for detecting these mechanical changes, offering sensitivity unmatched by any other diagnostic technique .

Understanding the Physics Behind Frequency Response Analysis

The fundamental principle of FRA testing lies in the distributed nature of transformer windings as electrical networks. Each winding consists of numerous turns and discs arranged in specific geometric configurations. These physical structures create natural capacitances between turns, between discs, and between windings and ground . When an alternating voltage is applied across a range of frequencies, the winding responds differently depending on its internal geometry:

• At low frequencies (10 Hz - 2 kHz): The response is dominated by the magnetizing inductance of the core. Changes in this region typically indicate core-related issues such as residual magnetism, core grounding problems, or open/shorted laminations .

• At medium frequencies (2 kHz - 200 kHz): This region reflects interactions between the winding inductance and the capacitances between winding sections. It is the primary zone for detecting major winding deformations, axial displacement, and radial buckling .

• At high frequencies (200 kHz - 10 MHz+): Here, the response is governed by the internal capacitances within the winding structure. Changes in this region often indicate problems with the internal insulation system, turn-to-turn faults, or movement of internal leads .

By analyzing deviations across these three distinct frequency regions, trained technicians can isolate the type and approximate location of mechanical damage without ever opening the transformer tank .

Standardized SFRA Test Procedures

Test Setup and Connections

Proper test setup is critical for obtaining repeatable and meaningful FRA results. The SFRA test procedure typically follows these steps:

Preparation: Before testing, ensure the transformer is de-energized, isolated, and properly grounded. All bushings should be clean and dry to ensure good electrical contact. Remove any external connections that could create parallel paths affecting the measurement .

Lead Configuration: Modern FRA analyzers use coaxial cables to maintain signal integrity. The standard test configuration involves connecting the signal source to one terminal of a winding while measuring the response at the other terminal. The remaining terminals are typically left open or grounded depending on the specific test type .

Standard Test Types: A comprehensive FRA diagnostic includes multiple test configurations:

• End-to-End Open Circuit: Measures the transfer function from one end of the winding to the other with all other terminals floating. This is the most common test for detecting winding deformation .

• End-to-End Short Circuit: Similar to open circuit but with the non-tested windings shorted. This configuration is more sensitive to changes in leakage inductance and can help differentiate between core and winding issues .

• Capacitive Inter-winding: Measures the capacitive coupling between different windings, providing information about the insulation condition between them .

Interpretation of FRA Signatures

Comparative Analysis Methods

Interpreting FRA results relies on comparison with reference signatures. Three primary comparison methods are used in practice:

Time-Based Comparison: Comparing current measurements with previous measurements on the same transformer. This is the most reliable method as it accounts for the unique characteristics of each unit. Baseline measurements should ideally be taken at factory commissioning or immediately after installation .

Type-Based Comparison: Comparing results from identical transformers from the same manufacturer. This approach is useful when baseline data is unavailable but requires careful consideration of manufacturing tolerances .

Phase-to-Phase Comparison: Comparing the three phases of the same transformer. For core-type transformers, the outer phases typically show similar responses while the center phase may differ slightly due to magnetic circuit asymmetry .

Quantitative Interpretation Metrics

To reduce subjectivity in interpretation, several numerical indices have been developed:

• Correlation Coefficient (CC): Measures the linear relationship between two FRA traces. Values above 0.98 typically indicate no significant deformation .

• Standard Deviation (SD): Calculates the spread of differences between traces. Higher SD values indicate greater deviation .

• Absolute Sum of Logarithmic Error (ASLE): Provides frequency-dependent weighting to emphasize deviations in critical frequency bands .

Modern analyzers automatically calculate these indices and provide preliminary fault classification based on established databases and machine learning algorithms .

Common Fault Signatures and Their Identification

Axial Displacement

Axial displacement occurs when windings shift vertically along the core leg. This type of fault primarily affects the capacitive coupling between high and low voltage windings. In FRA traces, axial displacement typically causes significant deviations in the medium frequency range (10 kHz - 100 kHz) with characteristic resonant frequency shifts .

Radial Buckling

Radial buckling results from excessive compressive forces during short-circuit events. The outer windings may develop permanent deformations inward between spacers. This deformation alters the series capacitance and inductance of the affected sections, producing distinctive changes in the high-frequency region (above 100 kHz) with reduced resonant amplitudes .

Disc Space Variation

Uneven spacing between winding discs can occur due to improper clamping pressure or mechanical shocks. This variation affects the inter-disc capacitances, leading to multiple small deviations across the medium and high-frequency spectrum rather than a single dominant change .

Best Practices for Accurate FRA Measurements

Environmental Considerations

Temperature and humidity can affect FRA measurements. Ideally, tests should be conducted at similar temperatures to ensure valid comparisons. When this is impossible, temperature correction factors can be applied, though their accuracy varies with transformer design .

Cable Management

The quality of test cables significantly impacts measurement accuracy. High-quality, shielded coaxial cables with consistent impedance characteristics are essential. Cables should be kept as short as practical and routed away from power lines and other sources of electromagnetic interference .

Measurement Verification

Always perform verification measurements to ensure data quality. Repeat measurements on at least one winding per test session to confirm repeatability. Additionally, check for proper grounding and absence of external influences that could corrupt the frequency response signature .

Conclusion

The Transformer Frequency Response Analyzer represents the most advanced and reliable technology for detecting mechanical deformations in power transformer windings. By understanding the physical principles governing frequency response, following standardized test procedures, and applying systematic interpretation methods, maintenance professionals can identify developing faults before they lead to catastrophic failures. As the electrical grid continues to age and demand for reliability increases, SFRA testing will remain an essential component of comprehensive transformer asset management programs, extending equipment life and preventing costly unplanned outages .

For utilities and industrial facilities seeking to implement or enhance their transformer diagnostic capabilities, investing in modern FRA equipment with automated interpretation features provides the best return on investment by combining technical excellence with practical usability. Regular SFRA testing, integrated with other diagnostic techniques such as DGA and partial discharge measurement, creates a complete picture of transformer health that enables truly predictive maintenance strategies .