Operating Precautions (Very Important) Baseline data is crucial: The FRA method is a "comparison method." Without factory data or historical normal data as a "baseline," it is difficult to draw definitive conclusions from a single test. Therefore, it is essential to test and archive data before a new transformer is put into operation. Connection consistency: The test connection method (end-to-end, end-to-ground), grounding position, and lead length must be exactly the same as those used in the baseline test; otherwise, errors will occur. Tap changer position: Tests must be performed at the same tap position (usually the rated tap position). External interference: During the test, all external connections on the high-voltage and low-voltage sides of the transformer (including surge arresters, neutral grounding, etc.) should be disconnected to isolate the windings and avoid external circuit influence on the frequency spectrum. Temperature effect: Although the effect is small, try to test at similar oil temperatures. 6. Relationship with Other Instruments In the equipment system you inquired about earlier, the winding deformation tester plays the role of an "orthopedic doctor": DC resistance tester: Checks whether the "blood vessels" are unobstructed (conductive circuit). Dielectric loss/withstand voltage/partial discharge tester: Checks whether the "skin and muscles" are good (insulation strength). Winding deformation tester: Checks whether the "skeleton" is distorted (mechanical structure). Microcomputer relay protection tester: Checks whether the "nervous system" is responsive (secondary protection). Only when these four types of equipment are used together can a comprehensive "physical examination" of the transformer be performed. 7. Purchase Recommendations Frequency range: It is recommended to choose a model with a range of 10 Hz - 2 MHz or even wider; the higher the high-frequency resolution, the more sensitive it is to small deformations. Anti-interference capability: The field environment is complex, so the instrument should have good shielding and digital filtering functions. Software analysis function: Whether the accompanying software is powerful, whether it can automatically calculate correlation coefficients, automatically generate comparison reports, and whether it has a rich library of typical fault patterns for reference. Portability: Field tests often require climbing to the transformer terminal box, so the instrument should be lightweight and have a long battery life.
- The geometric shape of the winding (dimensions, position, spacing) directly determines the values of its distributed inductance and distributed capacitance.
- Once the winding undergoes mechanical deformation (such as axial displacement or radial bulging), its internal L and C parameters will change.
- This change will cause the winding's impedance characteristics (transfer function) to change at different frequencies.
- Test process:
- The instrument injects a sinusoidal sweep signal (usually from 10 Hz to 1 MHz or 2 MHz) into one end of the winding.
- Measure the response signal at the other end of the winding.
- Calculate and plot the amplitude-frequency characteristic curve (magnitude vs. frequency) and phase-frequency characteristic curve.
- Compare the current test curve with the original baseline curve (factory data or previous normal test data).
- Conclusion: If the two curves have a high degree of coincidence, the winding is normal; if the curve shows obvious offset, peak shift, or waveform distortion, the winding has deformed.
(Note: There is also an older method called the "low-voltage short-circuit impedance method," which determines deformation by measuring changes in short-circuit impedance, but its sensitivity and localization ability are not as good as the FRA method, and it is now mostly used as an auxiliary means.)
2. Main Types of Deformation Detected
By analyzing the change characteristics of the curve, experienced engineers can determine the type of deformation:
- Axial displacement: The winding moves upward or downward as a whole (common after short-circuit impact).
- Radial bulging/twisting: The winding expands outward or undergoes spiral twisting.
- Inter-turn short circuit: Short circuit caused by insulation damage in some turns.
- Lead displacement: Internal leads of the winding become loose or shift.
- Poor core grounding: Although mainly for winding testing, it can sometimes reflect multiple grounding points in the core.
3. Application Scenarios
- After short-circuit impact: When a short-circuit fault occurs at the transformer outlet or in the vicinity and the protection operates to trip, a winding deformation test must be performed to confirm no damage before re-energization.
- After transportation and installation: Large transformers may experience severe vibration during long-distance transportation or hoisting; testing should be performed during commissioning.
- Periodic preventive tests: Combined with major overhauls or routine tests, establish equipment health records and monitor deformation trends.
- Fault diagnosis: When the transformer has abnormal noise or abnormal oil chromatographic analysis (acetylene increase) but the cause cannot be found, it is used to check for internal mechanical faults.
4. Key Indicators and Evaluation Criteria
- Correlation coefficient (R): Modern instruments automatically calculate the correlation coefficient between the current curve and the baseline curve.
- R > 0.95 (or 0.98): Normal.
- 0.90 < R < 0.95: Mild deformation (needs attention).
- R < 0.90: Obvious deformation (recommend lifting the cover for inspection or returning to the factory).
- (Specific thresholds refer to DL/T 911 standard and manufacturer recommendations)
- Frequency band analysis:
- Low frequency band (1 kHz - 100 kHz): Mainly reflects overall structural changes of the core and winding (such as axial displacement).
- Medium frequency band (100 kHz - 600 kHz): Mainly reflects internal twisting and bulging of the winding.
- High frequency band (>600 kHz): Mainly reflects local details such as lead displacement and inter-turn short circuits.
