Winding Deformation Tester (Transformer Winding Deformation Tester), typically based on Frequency Response Analysis (FRA) or Low Voltage Short Circuit Impedance Method, is a specialized diagnostic instrument used to detect the mechanical structural integrity of power transformer windings.
It is the "gold standard" tool for determining whether the internal windings have suffered mechanical faults such as displacement, distortion, bulging, or inter-turn loosening after the transformer has experienced short-circuit current surges, transportation collisions, or long-term operational vibration.
1. Core Function
The transformer winding is the "heart" of the transformer, and its mechanical stability directly determines the equipment's lifespan. The main functions of this instrument are:
Detect mechanical deformation: Identify axial displacement (compression/stretching), radial deformation (bulging/concavity), inter-turn short circuits, lead loosening, etc.
Post-accident assessment: After a transformer experiences an outlet short circuit, lightning strike, or external impact, quickly determine whether internal damage has occurred, and decide whether to perform a tank-removal inspection or scrap the transformer.
Condition monitoring: As a preventive test item, establish a "fingerprint" of the winding, and monitor the gradual trend of winding condition through historical data comparison.
Acceptance testing: Before a new transformer is put into operation or after a major overhaul, verify the winding installation quality and whether damage occurred during transportation.
2. Working Principle
Currently, there are two mainstream testing methods, and instruments typically incorporate or emphasize one of them:
A. Frequency Response Analysis (FRA) - Most commonly used and most sensitive
Principle: The transformer winding can be considered a complex distributed parameter network composed of resistance, inductance, and capacitance. When the winding undergoes mechanical deformation, the distributed parameters of its internal inductance (L) and capacitance (C) change slightly, causing its frequency response characteristic curve to change.
Process:
The instrument injects a series of swept sine wave signals (typically ranging from 10 Hz to 1 MHz or higher) into one end of the winding.
Measure the amplitude (dB) and phase of the response signal at the other end (or the same end).
Plot the amplitude-frequency characteristic curve (Bode plot).
Judgment: Compare the measured curve with the original factory curve (fingerprint), historical curves of the same phase, or curves of similar windings among the three phases. If the curve shows significant peak/valley shifts, amplitude changes, or shape distortion in specific frequency bands, it indicates winding deformation.
B. Low Voltage Short Circuit Impedance Method
Principle: Based on the principle that the transformer short-circuit impedance is closely related to the winding geometry (especially the distance between windings).
Process: Apply low voltage on the low-voltage side, short-circuit the high-voltage side (or vice versa), and measure the short-circuit impedance value.
Judgment: If the measured short-circuit impedance value changes by more than a certain threshold (typically ±1% to ±2%) compared to the factory value or historical value, it indicates possible severe winding deformation. This method is sensitive to overall deformation but less sensitive to local minor deformation than FRA.
3. Typical Deformation Types and Spectrum Characteristics
Based on the response of the FRA curve in different frequency bands, the type of deformation can be preliminarily identified:
Low frequency band (1 kHz - 100 kHz): Mainly reflects changes in the overall geometric dimensions of the winding.
Characteristics: Overall shift or large offset of the curve.
Corresponding faults: Axial displacement of winding, overall compression, core shift.
Mid frequency band (100 kHz - 600 kHz): Mainly reflects local structural changes of the winding.
Characteristics: Changes in the number or position of peaks/valleys.
Corresponding faults: Winding distortion, bulging, inter-turn short circuit, spacer shift.
High frequency band (600 kHz - 1 MHz+): Mainly reflects the leads and end structure of the winding.
Characteristics: High-frequency resonance points become chaotic or disappear.
Corresponding faults: Lead loosening, poor contact of tap changer, end support failure.
4. Main Functional Features
Broadband scanning: Supports high-resolution sweep from 10 Hz to 1 MHz (or even 2 MHz), with thousands of sampling points to ensure capture of minor changes.
Multiple wiring modes:
End-to-end measurement: Input at one end, output at the other end (most commonly used, highest sensitivity).
End-to-ground measurement: Input at one end, output at the ground terminal (used when there is no neutral point lead).
Inter-phase measurement: Used for delta-connected windings.
Intelligent analysis software:
Built-in correlation coefficient algorithms (such as R factor, ASLE index, etc.) automatically calculate the similarity (0-1) between the measured curve and the reference curve, providing a quantitative score.
Supports overlay comparison of three-phase curves, visually displaying differences.
Has powerful database management functions to store historical "fingerprints".
Anti-interference capability: Uses digital filtering technology to extract effective signals in the complex electromagnetic environment of substations.
Portability: The main unit is lightweight, equipped with multiple sets of dedicated shielded test leads, suitable for on-site work in high or confined spaces.
5. Operation Procedure
Power off and discharge: Ensure the transformer is completely de-energized, fully discharge all windings, and remove external connections.
Record information: Record transformer nameplate parameters, oil temperature, and tap changer position (usually set to the rated tap).
Wiring:
Securely connect the instrument's signal leads to the bushing test tap or directly to the winding terminals.
Key: Ensure good grounding, keep test leads as short and fixed as possible, avoid dangling to introduce noise.
Set parameters: Select sweep frequency range, number of sweep points, and wiring mode.
Execute sweep: Start the test; the instrument automatically completes the sweep and generates a curve. Typically, all windings (high, medium, low) of all three phases need to be tested separately.
Data analysis:
Longitudinal comparison: Compare with the factory report or previous test data (most reliable).
Transverse comparison: Compare similar windings among the three phases (if no historical data is available, this method can be used as a reference, but note that the three phases themselves may have asymmetry).
Generate report: Based on the correlation coefficient and waveform distortion degree, provide a conclusion of "normal", "mild deformation", "moderate deformation", or "severe deformation".
6. Result Criteria (Reference Standards)
According to standards such as "DL/T 911 Frequency Response Analysis for Power Transformer Winding Deformation":
Correlation coefficient (R):
R > 0.95 (or 0.98): No significant winding deformation.
0.90 < R < 0.95: Possible slight deformation, need to combine with other tests for comprehensive judgment.
R < 0.90: Likely significant winding deformation, recommend tank-removal inspection.
Waveform observation:
If the peak/valley position shifts significantly (>2 frequency points) or amplitude changes exceed ±6 dB, it is usually considered abnormal.
Changes in the low-frequency band are usually more destructive than those in the high-frequency band.
7. Precautions
Reference data is crucial: The core of the FRA method is "comparison". Without factory or initial "fingerprint" data, it is difficult to make accurate judgments based solely on three-phase comparison (because the three-phase design may be inherently asymmetrical). Therefore, it is essential to perform and archive tests before a new transformer is put into operation.
Wiring consistency: The wiring position, lead length, and grounding method must be strictly consistent for each test; otherwise, introduced errors may be misinterpreted as deformation.
Tap changer position: Tests must be performed at the same tap position; different tap positions have different inductance values, making curves incomparable.
Residual charge: Ensure thorough discharge before testing; otherwise, residual charge may damage the instrument or affect test results.
Environmental factors: Although FRA is less affected by temperature, extreme temperatures or drastic changes in oil level may still have minor effects on the high-frequency band.
Summary
The Winding Deformation Tester is the "CT scanner" for transformer condition-based maintenance. It can non-invasively see through the mechanical health of the internal windings without removing the tank or draining oil. In the context of frequent grid short-circuit faults, it is a key decision-making basis for assessing whether a transformer can continue to operate safely and avoid catastrophic accidents (such as main transformer explosion due to winding collapse). Establishing and maintaining a "frequency response fingerprint library" for each transformer is a prerequisite for maximizing the value of this instrument.
