Solutions for removing water from transformer oil

Dehydration of transformer oil is a key step to ensure the safe operation of power equipment (such as transformers, mutual inductors, etc.). Water will reduce the insulation strength of the oil, accelerate oil aging, and may cause discharge failures. The following are common methods for dehydrating transformer oil and their principles, advantages and disadvantages, and applicable scenarios:

Vacuum dehydration method (most commonly used)

Principle

- Heat the oil (usually 50~65℃) in a vacuum environment (above -0.095 MPa) to reduce the boiling point of water (such as the boiling point of water at -0.095 MPa is ≈40℃), and free water and dissolved water evaporate rapidly, and then condense and discharge through the condenser.

- Often combined with atomization and thin film evaporation technology to increase the surface area of the oil and improve the dehydration efficiency.

Equipment

- Vacuum oil filter (with heating system, vacuum separation tank, condenser, precision filtration).

Advantages

- Can remove free water and dissolved water (deep dehydration to ≤10 ppm).

- Remove gases (such as oxygen and hydrogen) at the same time to improve insulation strength (breakdown voltage ≥ 60 kV).

- High degree of automation, suitable for large-scale processing.

Disadvantages

- High equipment cost.

- High temperature may accelerate oil oxidation (temperature needs to be strictly controlled).

Applicable scenarios

- Pre-treatment of new oil injection.

- Regeneration and purification of transformer oil in operation.

Adsorption filtration method

Principle

- Use adsorbents (such as silica gel, molecular sieves, activated alumina) or water-absorbing filter elements (cellulose, polypropylene materials) to physically absorb water in the oil.

- Some equipment uses circulating filtration to make the oil pass through the adsorption layer multiple times.

Equipment

- Portable oil filter (with adsorption filter element).

- Transformer built-in adsorption respirator (such as silica gel desiccant).

Advantages

- Simple equipment, suitable for on-site maintenance.

- No heating is required to avoid the risk of oil oxidation.

Disadvantages

- Only free water can be removed, and the effect on dissolved water is limited (usually dehydrated to 20~50 ppm).

- The adsorbent needs to be replaced regularly.

Applicable scenarios

- Small transformers or trace moisture treatment.

- As an auxiliary means of vacuum dehydration.

Centrifugal separation method

Principle

- Use the centrifuge to rotate at high speed (usually 4000~6000 rpm) to separate water (density greater than oil) from oil through centrifugal force.

Advantages

- A large amount of free water can be quickly removed.

- It does not rely on heating or vacuum, and is suitable for high-water content oil (such as post-treatment of water ingress accidents).

Disadvantages

- Ineffective for dissolved water.

- The equipment is large and the maintenance is complicated.

Applicable scenarios

- Preliminary treatment after a large amount of water ingress in the transformer oil.

- Used in conjunction with other methods (such as vacuum dehydration).

Natural sedimentation method

Principle

- Utilize the density difference between water and oil, let the oil tank stand and let the water settle naturally to the bottom before discharging.

Advantages

- Extremely low cost, no equipment required.

Disadvantages

- Only applicable to free water, and extremely time-consuming (several days to weeks).

- Unable to handle dissolved water or emulsified water.

Applicable scenarios

- Temporary measures in emergency situations.

- Used in conjunction with other methods.

Thermal drying method (drying oil)

Principle

- Heat the transformer oil to 80~100℃ (nitrogen protection is required to prevent oxidation), remove moisture by evaporation, and then condense and recover.

Advantages

- Deep dehydration (≤5 ppm) is possible.

Disadvantages

- High temperature can easily lead to oil oxidation, and strict oxygen control is required.

- High energy consumption and complex operation.

Applicable scenarios

- Deep dehydration in laboratories or special occasions.

Notes

1. Moisture standard:

- New transformer oil: Moisture ≤15 ppm (GB/T 7600).

- Operating oil: ≤35 ppm (≤25 ppm for transformers above 330kV).

2. Safe operation:

- Control the temperature (≤65℃) during vacuum dehydration to prevent oil oxidation.

- The adsorbent needs to be replaced regularly (such as regenerating or discarding silica gel after discoloration).

3. Comprehensive treatment:

- For oil with severe water ingress, free water can be separated by centrifugation first, and then vacuum dehydration is performed.

- Vacuum dehydration is preferred (efficient and thorough, suitable for most scenarios).

- A small amount of water is filtered by adsorption (convenient and low-cost).

- A large amount of water ingress is first centrifuged (quickly remove free water).

- After treatment, the moisture content (coulometric or Karl Fischer method) and breakdown voltage (≥40 kV) should be tested.