What are the methods for purifying transformer oil?

Purifying transformer oil is essential to maintain the performance, insulation, and longevity of transformers. Over time, transformer oil becomes contaminated with moisture, gases, particulates, and oxidation products, all of which degrade the oil’s insulating properties and may lead to transformer failure. Several methods are used to purify transformer oil, depending on the type and extent of contamination. Below are the most common methods for purifying transformer oil:

1. Vacuum Dehydration and Degassing

• Purpose: To remove moisture and dissolved gases from the transformer oil.
• How it works: The oil is heated and placed in a vacuum chamber. The vacuum lowers the boiling point of water, allowing moisture to evaporate and be removed from the oil. The lower pressure also allows dissolved gases (such as oxygen, nitrogen, carbon dioxide) to be released and removed.
• Benefits:
  • Efficiently removes moisture and gases that degrade the dielectric strength of oil.
  • Restores oil's insulating properties.
• Applications: Common in large transformers, power plants, and substations.

Typical Equipment: Vacuum dehydration units, vacuum degassing systems.

2. Oil Filtration (Full-Flow and Bypass Filters)

• Purpose: To remove solid contaminants like dirt, dust, and particulates from transformer oil.
• How it works: The oil passes through a filter media (such as cellulose or mesh), which traps particulate matter. Some systems may also use activated carbon or clay to absorb contaminants from the oil.
• Benefits:
  • Removes solids that cause wear and tear on transformer components.
  • Improves the clarity and cleanliness of the oil.
• Applications: Routine maintenance, for both new and used transformer oil.

Typical Equipment: Pressure filters, coalescing filters, inline filtration units.

3. Oil Regeneration (Chemical and Physical Treatment)

• Purpose: To restore the oil’s chemical properties and remove acids, oxidation products, and other dissolved contaminants.
• How it works: Oil regeneration typically involves filtering the oil through a series of adsorbents (like activated clay or molecular sieves) and/or chemical treatments. The adsorbents capture acids, dissolved gases, and oxidation products, thereby restoring the oil’s dielectric strength and chemical stability.
• Benefits:
  • Restores the oil’s dielectric properties by removing acids and oxidation by-products.
  • Extends the life of the transformer oil and its insulating capabilities.
• Applications: For highly contaminated oil, or when the oil has been aged or oxidized.

Typical Equipment: Molecular sieve units, activated clay regeneration systems, dual-stage oil regeneration units.

4. Vacuum Oil Filtration

• Purpose: To remove both moisture and particulate contaminants from transformer oil.
• How it works: In this method, the oil is passed through a vacuum chamber where it is subjected to low pressure. This removes moisture through evaporation, while the oil is also filtered to remove particulate matter.
• Benefits:
  • Removes both water and particulate contamination.
  • Often used as a preliminary step before other purification methods.
• Applications: Used for transformers with significant water or solid contamination.

Typical Equipment: Vacuum filtration units, mobile filtration units.

5. Desiccant Dehydration

• Purpose: To remove moisture from transformer oil, especially in cases where vacuum dehydration is not sufficient.
• How it works: A desiccant (a drying agent, often silica gel or molecular sieve) is used to absorb moisture from the oil. The oil passes through a bed of desiccant material, which binds the water molecules, removing them from the oil.
• Benefits:
  • Effective for removing trace amounts of moisture.
  • Often used in conjunction with vacuum or thermal treatments.
• Applications: Can be used for both routine maintenance or emergency treatments where moisture levels are high.

Typical Equipment: Desiccant dehydration units, molecular sieve systems.

6. Thermal Treatment (Heat Treatment)

• Purpose: To remove dissolved gases and moisture from transformer oil.
• How it works: The oil is heated to a high temperature under a vacuum. The heat helps evaporate dissolved gases and moisture, which are then extracted by the vacuum system. This method can also help break down chemical impurities.
• Benefits:
  • Removes dissolved gases and moisture effectively.
  • Can rejuvenate oil that has been exposed to moisture or gas accumulation.
• Applications: Used for oil with high moisture or dissolved gas content.

Typical Equipment: Vacuum thermal units, oil dryers.

7. Coalescing Filtration

• Purpose: To remove water and oil-soluble contaminants by aggregating water droplets into larger droplets that can be easily removed.
• How it works: Coalescing filters use a special filtration medium that encourages small water droplets suspended in the oil to combine into larger droplets, which then separate from the oil. The filtered oil is then passed through a final polishing filter to remove any remaining fine particles.
• Benefits:
  • Highly effective for separating water from transformer oil.
  • Ideal for treating emulsified or contaminated oils.
• Applications: Primarily used when water contamination is a major concern (e.g., after transformer failures or when oil absorbs moisture).

Typical Equipment: Coalescing filter systems, water removal filters.

8. Gas Stripping (For Dissolved Gases)

• Purpose: To remove dissolved gases (such as hydrogen, methane, ethane) from transformer oil.
• How it works: This method involves passing the oil through a stripping column or using a vacuum to expel dissolved gases. Often, the oil is heated to help release the gases more effectively.
• Benefits:
  • Helps restore the dielectric properties of the oil by removing harmful gases.
  • Reduces the risk of transformer malfunction caused by dissolved gases.
• Applications: Used when the oil has been exposed to partial discharge or electrical arcing inside the transformer, causing gas buildup.

Typical Equipment: Gas stripping units, vacuum degassing units.

9. Acid Removal and Neutralization

• Purpose: To remove acids and neutralize the oil’s acidity.
• How it works: Some purification systems use specific adsorbent materials (like activated clay or fuller’s earth) to remove dissolved acids and neutralize the oil. These materials can adsorb acidic compounds formed during the aging of the oil.
• Benefits:
  • Helps restore the oil’s chemical balance, improving its dielectric properties.
  • Prevents further degradation of the oil.
• Applications: Used for oil that has been aged or exposed to high temperatures, resulting in acid formation.

Typical Equipment: Activated clay treatment systems, neutralizing oil purifiers.

10. Electrostatic Separation

• Purpose: To remove contaminants, particularly fine particles, from transformer oil.
• How it works: Electrostatic separators use an electric field to charge and attract particulate contaminants to electrodes, removing them from the oil. This method is often used in combination with other purification techniques.
• Benefits:
  • Highly effective at removing very fine particulates and micro-particles.
  • Provides additional polishing after filtration.
• Applications: Used when oil needs to be purified to a very high standard (e.g., in high-voltage transformers).

Typical Equipment: Electrostatic oil separators.

Summary of Transformer Oil Purification Methods:

Each of these methods is selected based on the type of contamination and the extent of oil degradation, and often multiple methods are combined in a multi-stage purification process for optimal results.