VILLY INSPECTION Share-5_Transformer Inspection Key Points – Insights from Field Cases

As the "heart" of the power system, the quality of transformers is directly related to the safe operation of the entire project. Through years of on-site third-party inspection (TPI) experience, we have found that even with perfect design and clear standards, there are still many pitfalls in actual implementation. Combined with real cases below, we share the key points and lessons learned from transformer inspection.

 

1. Winding Characteristic Test: Don’t Be Deceived by "Qualified Reports"

Case: A 110kV main transformer at an overseas substation burned out after 6 months of commissioning. A 110kV main transformer for an overseas project was burned out due to inter-turn short circuit only 6 months after commissioning. Investigation found:

- The DC resistance data in the factory report was "perfect", with a three-phase balance deviation of only 0.8%;

- However, checking the original test records revealed that the oil temperature was not recorded during the test, and the test current was only 5A (standard requirement: ≥10% of rated current);

- There was actual poor inter-turn contact, which was not detected due to improper test conditions.

 

Lessons Learned:

- During DC resistance testing, the winding temperature must be recorded and converted to the same temperature for comparison;

- The test current should be ≥10% of the rated current, otherwise poor contact cannot be detected;

- Do not only look at the report conclusion, but also verify the original test conditions and data

 

Inspection Focus:

- When witnessing the test, confirm the current value, temperature records, and instrument calibration status;

- The three-phase unbalance deviation should be ≤±2%, and there should be no obvious jump compared with historical data.

 

2. Insulation System: Partial Discharge (PD) – The "Invisible Killer"

Case: Abnormal PD Test of a box-type transformer in a wind farm before commissioning. A 35kV box-type transformer in a wind farm passed the partial discharge (PD) test during Factory Acceptance Test (FAT) (≤300pC). However, during the on-site handover test, it was found that:

- The PD value of Phase A reached 800pC;

- Disassembly found residual metal particles at the end of the high-voltage winding, which came from insufficient cleaning during assembly;

- Rework took 3 weeks, delaying the grid connection node of the entire wind farm.

 

Lessons Learned:

- A qualified factory test does not mean it is qualified on-site – transportation vibration may cause displacement of residues;

- PD test is extremely sensitive to internal insulation defects and is the key to finding the "invisible killer".

 

Inspection Focus:

- PD tests must be performed twice: during FAT and on-site handover, with comparative data analysis;

- Pay attention to discharge waveform characteristics – whether it is internal cavity discharge or surface creeping discharge;

- Ultrasonic localization can be performed on suspected parts to accurately find the discharge source.

 

3. Dissolved Gas Analysis (DGA): Early Fault Warning 6 Months in Advance

Case: Early detection of transformer fault through abnormal DGA in a refinery. Routine dissolved gas analysis (DGA) of a 110kV transformer in a refinery found:

- Total hydrocarbons increased from 15ppm to 78ppm;

- Acetylene (C₂H₂) increased from 0 to 4.5ppm;

- The three-ratio code was "102", judged as medium-temperature overheating.

The inspector resolutely recommended shutdown for inspection. Disassembly found:

- Poor contact of the on-load tap-changer (OLTC) contacts, with severe ablation;

- If it continued to operate for another 3 months, it would inevitably lead to switch explosion and transformer burnout.

 

Lessons Learned:

- DGA is the "blood test" of transformers, which can detect internal faults in advance;

- A single data point is not sensitive; trend analysis is the key – a sudden increase is an alarm;

- When a fault is suspected, shorten the monitoring interval (from semi-annual to monthly).

 

Inspection Focus:

- Pay attention to characteristic gases: rising H₂ → corona discharge; presence of C₂H₂ → arcing discharge;

- Comprehensively judge the fault type by three-ratio method and Duval Triangle method;

- Immediately recommend power-off inspection + high-frequency PD test for confirmation if abnormalities are found.

 

4. Transportation and Installation: Factory Qualified ≠ On-Site Qualified

Case: Abnormal Frequency Response Analysis (FRA) curve of a main transformer in a hydropower station after transportation. A 220kV main transformer in a hydropower station passed all factory tests. After transportation to the site, frequency response analysis (FRA) found:

- Obvious deviation of the FRA curve of the medium-voltage winding compared with the factory baseline;

- Judged as winding displacement or deformation during transportation;

- The cost of factory repair exceeded 2 million RMB, and the project period was delayed by 8 months.

Root Cause: The transport shock recorder showed that the transformer encountered an impact exceeding 3g during transportation (standard requirement: ≤1g), but the transportation company did not report it.

 

Lessons Learned:

- Transformer factory qualification is only the "starting point"; transportation is a high-risk link;

- A triaxial shock recorder must be installed for full monitoring and data export;

- FRA must be performed after arrival and compared with the factory curve.

 

Inspection Focus:

- Before transportation, confirm that the shock recorder is activated and the range is set correctly;

- Download the shock data first after arrival, and report damage immediately if it exceeds the standard;

- Alert should be raised if the main peak deviation of the FRA curve exceeds ±5%.

 

5. Practical Recommendations for Inspectors & Common Pitfall Prevention

Common Pitfalls & Countermeasures:

- Material Substitution: Aluminum windings passed off as copper windings, with load loss exceeding 30% → Spot-check conductor material with on-site spectrometer;

- Substandard Insulating Oil: Recycled oil passed off as new oil, with breakdown voltage only 25kV → Take samples for third-party testing + retain sealed samples for future reference;

- Falsified Temperature Rise Test: Test only for 1 hour instead of reaching thermal stability, with falsely low data → Witness the entire temperature rise process until stability is confirmed;

- Falsified Nameplate Parameters: Overstated capacity (630kVA marked as 800kVA) → Cross-check with design drawings + measure short-circuit impedance;

- Mixed Accessories: Domestic bushings substituted for imported ones, with insufficient creepage distance → Cross-check with procurement list + take photos of physical objects for comparison.

 

Practical Recommendations:

- Do not trust "perfect reports": All reports must have raw data, curves, and signatures, not just conclusion pages;

- Adhere to "three-way verification": Design drawings ↔ Procurement contracts ↔ Physical nameplates, all three must be consistent;

- Preserve the evidence chain: Critical tests (such as PD, lightning impulse test) must be recorded by video + data screenshots;

- Focus on details: Bushing creepage distance, tap-changer operation times, fan startup temperature, etc., are all high-frequency fault points;

- Establish a case library: Archive encountered abnormalities, photos, and cause analysis to become the team's "error notebook".

 

Transformer inspection is a "conscience-driven job" – one extra glance and one more question on site may be the key to avoiding a major accident. We hope these frontline cases can provide warnings and references for peers.

 

Discussion: Have you encountered transformer failures caused by FAT control omissions? Welcome to share and exchange in the comment area.

 

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