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Time：2025-06-21 13:44:40 Reading volume：
To determine whether transformer oil needs to be filtered, a comprehensive evaluation of appearance inspection, equipment operation status monitoring and laboratory oil sample testing is required. The specific standards are as follows:
Abnormal color
Normal oil is a light yellow transparent liquid. If it turns dark brown or black (such as carbonized particles), it indicates that the oil is aging or internal faults need to be filtered.
Turbidity or suspended matter
The oil is turbid and white (water emulsified) or contains visible impurities (fibers, metal chips), which need to be filtered immediately.
Odor change
The appearance of a burnt smell indicates internal overheating or arc discharge. It is necessary to combine testing to confirm the necessity of filtration.
2. Equipment operation status monitoring
Degradation of electrical performance
The noise during transformer operation increases abnormally (may be mixed with air or impurities).
An increase in partial discharge or a decrease in insulation resistance indicates that the insulation performance of the oil has deteriorated.
Abnormal temperature rise
The oil temperature continues to be higher than the historical average, which may cause increased friction or reduced heat dissipation efficiency due to increased impurities.
Pressure fluctuation
The hydraulic system pressure is unstable, indicating that pollutants in the oil block valves or affect the sealing.
| Test items | Filter threshold | Reasons for exceeding the standard |
| Breakdown voltage | <35kV (new oil)/<20kV (operating oil) | Excessive moisture or particle contamination |
| Water content | >30ppm | Seal failure or moisture absorption |
| Acid value | >0.1mgKOH/g (operating oil) | Oxidation and aging to generate acidic substances |
| Dielectric loss factor | >0.04 (90,℃) | Increased polar impurities or colloid contamination |
| Flash point | <130℃ or="">5℃ lower than the previous time | Local overheating failure or mixing with light oil |
| Particle contamination | >NAS level 6 | Mechanical wear or external contamination |
Before the first injection of new oil (which may be contaminated during transportation);
After transformer overhaul or oil pump burnout;
Operating oil that has not been filtered or tested for more than 3 years.
A[Take an oil sample to observe the appearance] --> B{Is it turbid/discolored/impurities? }
B -->|Yes| E[Filter immediately]
B -->|No| C[Laboratory test of 6 core indicators]
C --> D{Any indicator exceeds the threshold? }
D -->|Yes| E[Filter + troubleshooting]
D -->|No| F[Continue monitoring, re-inspect within 3 years]
Operation tips:
Oil samples of conventional transformers should be tested at least once every 3 years.
When a fault is suspected, the breakdown voltage and water content should be tested first (the fastest reflection of insulation degradation);
After filtering, verification is required: breakdown voltage ≥ 50kV, water content ≤ 5ppm, and no free carbon.
