(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. It is now mostly used as an auxiliary method.) 2. Main Types of Deformation Detected By analyzing the characteristics of the curve changes, 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: Partial insulation damage between turns causes a short circuit. Lead displacement: Internal leads of the winding become loose or shift. Poor core grounding: Although mainly measuring the winding, it can sometimes reflect multiple grounding points of the core. 3. Application Scenarios After short-circuit impact: When a short-circuit fault occurs at the transformer outlet or in the near zone and the protection operates to trip, winding deformation testing 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 tests. Periodic preventive tests: Combined with major overhauls or routine tests, establish equipment health archives and monitor deformation trends. Fault diagnosis: When the transformer has abnormal sounds or abnormal oil chromatography analysis (acetylene increase) but the cause cannot be found, it is used to troubleshoot 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 reference curve. R > 0.95 (or 0.98): Normal. 0.90 < R < 0.95: Mild deformation (needs attention). R < 0.90: Significant 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. 5. Operation Precautions (Very Important) Reference data is crucial: The FRA method is a "comparison method." Without factory data or historical normal data as a "reference," it is difficult to draw a definite conclusion from a single test result. Therefore, be sure to test and archive data before commissioning a new transformer. Wiring consistency: The wiring method (end-to-end, end-to-ground), grounding position, and lead length during testing must be exactly the same as those during the reference 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 testing, all external connections on the high and low voltage sides of the transformer (including surge arresters, neutral grounding, etc.) should be disconnected to isolate the winding and avoid external circuits affecting 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 deformed (mechanical structure). Microcomputer relay protection tester: Checks whether the "nervous system" is functioning (secondary protection). Only when these four types of equipment are used together can a comprehensive "physical examination" of the transformer be performed. 7. Purchasing Recommendations Frequency range: It is recommended to choose a model with a range of 10 Hz - 2 MHz or wider; the higher the high-frequency resolution, the more sensitive it is to small deformations. Anti-interference capability: The field environment is complex; the instrument should have good shielding and digital filtering functions. Software analysis functions: Whether the accompanying software is powerful, can automatically calculate correlation coefficients, automatically generate comparison reports, and has a rich library of typical fault patterns for reference. Portability: Field testing often requires climbing onto the transformer terminal box; the instrument should be lightweight and have strong battery life.
