transformer oil filling
The following is a detailed operation guide for **Transformer Oil Filling**, which is applicable to oil-cooled transformers
(such as 112.5 kVA oil-immersed transformers):
I. Preparation Work Before Oil Filling
1. Selection of Transformer Oil Type
- **Standards**:
- International: IEEE C57.106 (Insulating Oil Standard), ASTM D3487 (New Oil Standard)
- China: GB 2536 (Transformer Oil)
- **Common Types**:
- **Mineral Oil**: General-purpose type (such as Shell Diala S4 ZX-10), with a flash point > 140°C, suitable for ordinary
distribution transformers.
- **Synthetic Oil**: High ignition point (> 300°C), suitable for high-temperature or explosion-proof scenarios (such as Midel 7131).
- **Environmentally Friendly Oil**: Biodegradable oil (such as Envirotemp FR3), suitable for ecologically sensitive areas.
2. Oil Quality Testing
- **Required Testing Items**:
| Parameter | Qualified Standard | Testing Method |
| Breakdown Voltage | ≥ 40 kV (with a 2.5mm electrode gap) | ASTM D1816 |
| Water Content | ≤ 30 ppm (at 20°C) | ASTM D6304 |
| Acid Value | ≤ 0.03 mg KOH/g ≤ 0.03 | ASTM D974 |
| Dielectric Loss Factor (at 90°C) | ≤ 0.5% | ASTM D924 |
II. Oil Filling Operation Steps (Taking a 10kV/400V Three-phase Oil-cooled Transformer as an Example)
1. Equipment Preparation
- Oil filter (precision ≤ 10μm), vacuum oil pump (vacuum degree < 133 Pa), oil level gauge, insulation resistance tester.
2. Emptying the Old Oil (If Oil Change is Required)
- Turn off the high and low voltage side circuit breakers of the transformer and disconnect the power supply.
- Open the bottom oil drain valve and empty the waste oil into a dedicated container (the waste oil needs to be treated as
hazardous waste, such as in accordance with the US RCRA standard).
- Purge the inside of the oil tank with dry nitrogen (dew point < -40°C) to remove impurities and moisture.
3. Vacuum Oil Filling (Recommended Method)
1. **Vacuum Pumping**:
- Connect the vacuum oil pump to the air extraction port at the top of the transformer, turn on the vacuum pump, and
maintain a vacuum degree < 133 Pa for 24 hours (for small-capacity transformers, it can be shortened to 12 hours).
2. **Oil Filling**:
- Inject new oil into the transformer through the oil filter, control the flow rate ≤ 5 L/min to avoid generating foam.
- Fill the oil to the **middle scale of the oil level gauge** (refer to the temperature-oil level curve on the transformer nameplate,
for example, the oil level corresponds to 3/4 of the scale at 25°C).
3. **Pressure Maintenance**:
- After the oil filling is completed, maintain the vacuum degree for 2 hours to ensure that the oil penetrates into the windings
and insulating materials.
4. Atmospheric Pressure Oil Filling (When There is No Vacuum Equipment)
- Fill the oil by the gravity method, and the oil level should exceed the top of the winding by 10-15 cm.
- After filling the oil, it is necessary to **vent multiple times**: Open the vent valves at the top of the bushing and radiator until
pure oil without bubbles flows out.
III. Key Precautions
1. Environmental Requirements
- Ambient temperature: > 0°C (to avoid oil solidification), humidity < 60%RH.
- Outdoor operation: It is necessary to build a rain shelter to prevent rainwater from entering the oil tank.
2. Safety Specifications
- Anti-static: Use grounded metal pipes for oil filling to avoid static sparks generated by plastic hoses.
- Fire prevention: Open flames are prohibited on site, and fire extinguishers (such as CO₂ fire extinguishers) should be equipped.
- Personal protection: Wear oil-resistant gloves and goggles to avoid skin contact with the oil (it may contain polychlorinated
biphenyls PCBs and needs to be tested).
3. Venting Operation
- **Key Venting Positions**:
- Bushing lifting base, radiator, gas relay (such as QJ-80 type), oil conservator connecting pipe.
- **Judgment Standard**: The oil flowing out of the vent port has no bubbles and is continuous and stable.
IV. Testing and Acceptance After Oil Filling
1. **Insulation Resistance Testing**:
- High voltage side to low voltage side and ground: ≥ 1000 MΩ (using a 2500V megger, at 25°C).
- Low voltage side to high voltage side and ground: ≥ 500 MΩ.
2. **Oil Level Inspection**:
- After standing for 24 hours, the oil level should conform to the scale corresponding to the ambient temperature (for example,
an expansion space needs to be reserved during the high temperature in summer in Los Angeles).
3. **Withstand Voltage Test**:
- Apply an AC withstand voltage according to the standard (for example, the test voltage on the high voltage side of a 10kV
transformer is 28kV, and there should be no breakdown within 1 minute).
# V. Common Problems and Solutions
| Problem | Cause | Solution |
| The oil level drops too fast | Aging of the gasket, oil leakage at the weld seam | Replace the gasket and locate the leakage point with an ultrasonic leak detector |
| The oil breakdown voltage is unqualified | Moisture or impurities in the oil | Re-filter or replace the oil and carry out vacuum drying treatment |
| The DC resistance of the winding increases after oil filling | Insufficient oil penetration, the winding is damp | Extend the vacuum oil filling time, or use hot oil circulation (50-60°C) |
VI. Regulations and Standards
- **United States**:
- OSHA 1910.303 (Electrical Safety)
- EPA 40 CFR Part 761 (Management of Polychlorinated Biphenyls PCBs, old transformers need to be tested)
- **International**:
- IEC 60422 (Treatment of Transformer Oil)
- IEC 60076-3 (Transformer Test Standard)
**Conclusion**: Transformer oil filling needs to strictly follow the process of **vacuum oil filling → sufficient venting → insulation
testing** to ensure that the oil quality, oil level, and safety meet the standards. For distribution transformers of 112.5 kVA and below,
it is recommended to be operated by a licensed electrician (such as a California C-10 electrician in the United States) or a technical
personnel of the transformer manufacturer.
