Overview of Transformer Oil Types and Maintenance Strategies

Introduction

Transformer oil can be classified by its composition, purpose, and performance. The main types include:

Mineral Oil Transformer Oil

Mineral oil transformer oil is a specialized insulating oil designed for transformers and other electrical equipment. It primarily serves as an electrical insulator, cooling medium, and protection against oxidation and corrosion.

2.1.1 Basic Composition

Mineral oil transformer oil is derived from mineral oil (usually petroleum) and refined through various processes. It mainly consists of:

• Alkanes: Saturated hydrocarbons with straight or branched chains, offering high chemical stability.
• Cycloalkanes: Provide excellent oxidation resistance and good low-temperature fluidity.
• Aromatic hydrocarbons: Present in small amounts to enhance impurity dissolution, though excessive amounts can reduce oxidation stability.

Advanced refining processes (e.g., solvent extraction, hydrocracking) remove impurities such as sulfur compounds, nitrogen compounds, and oxides, ensuring high purity and superior performance.

2.1.2 Key Properties

The performance of mineral oil transformer oil directly affects the efficiency and lifespan of transformers. Its main characteristics include:

Insulating Properties

• Transformer oil is an excellent electrical insulator with high dielectric strength (usually above 40 kV/mm).
• It fills the gaps between windings and components, preventing arcs and breakdowns.

Cooling Properties

• Mineral oil has excellent thermal conductivity and fluidity, allowing it to circulate within the transformer and dissipate heat generated during operation.
• It ensures efficient heat transfer, maintaining a safe temperature range inside the transformer.

Oxidation Resistance

• With the addition of antioxidants, mineral oil slows down oxidation when exposed to air, extending its service life.
• Oxidation resistance affects aging speed and prevents the formation of sludge and acidic substances.

Low-Temperature Fluidity

• Maintains good fluidity in low-temperature environments, ensuring normal startup and operation under cold conditions.

Chemical Stability

• Resists the effects of heat, oxygen, moisture, and impurities during long-term operation, maintaining stable performance.

2.1.3 Classification

Mineral oil transformer oil can be categorized based on refining processes and performance:

1. Uninhibited Transformer Oil: Free of antioxidant additives.

• Advantages: Cost-effective, suitable for environments with low operating temperatures or limited oxygen exposure.
• Disadvantages: Lower oxidation stability, prone to sludge and acid formation.

2. Inhibited Transformer Oil: Contains antioxidants (e.g., 2,6-ditert-butyl-p-cresol).

• Advantages: High oxidation stability and longer service life, suitable for high-temperature or prolonged operation.
• Disadvantages: Slightly more expensive.

2.1.4 Performance Indicators

Below are some critical performance indicators of transformer oil:

Synthetic Oil Transformer Oil

Synthetic Transformer Oil is a high-performance insulating and cooling oil specifically designed for power transformers and other electrical equipment. Its main components are artificially synthesized base oils combined with high-performance additives. This type of oil is primarily used in high-voltage, ultra-high-voltage, or harsh environmental conditions, and due to its superior performance, it is often chosen as a replacement for traditional mineral transformer oils.

2.2.1 Main Components

Synthetic Base Oil:

• Typically composed of synthetic esters, synthetic hydrocarbons, or polyalphaolefins.
• The molecular structure of synthetic base oils is uniform, eliminating impurities commonly found in mineral oils, resulting in higher chemical stability and electrical insulation performance.

Additives:

• Antioxidants: Enhance oxidation resistance and extend service life.
• Antistatic agents: Reduce discharge phenomena.
• Antifoaming agents: Minimize foam formation, maintaining oil purity.
• Anti-aging agents: Inhibit oil degradation caused by high temperatures or oxidation.

2.2.2 Performance Characteristics

Electrical Insulation:

• Synthetic transformer oil has a high breakdown voltage and low dielectric loss, providing excellent insulation properties to ensure reliable equipment operation.

Outstanding Thermal Stability:

• Synthetic oil maintains its physical and chemical properties in high-temperature environments, resisting decomposition and deposit formation.

Low-Temperature Performance:

• It typically has a low pour point, allowing it to flow and operate normally in cold regions, making it suitable for extreme climatic conditions.

Oxidation Resistance:

• Due to its uniform molecular structure and the role of additives, synthetic oil is less prone to oxidation, offering a longer service life and reducing the frequency of oil replacements.

Environmental Friendliness:

• Synthetic oils are more biodegradable, and some synthetic ester-based oils are classified as environmentally friendly insulating oils.

Low Volatility:

• Synthetic oil has low volatility, reducing loss caused by oil evaporation.

2.2.3 Advantages Compared to Mineral Oil

2.2.4 Performance Indicators

Synthetic transformer oil must meet strict technical standards and performance requirements. Below are the key performance indicators:

2.3 Vegetable Oil Transformer Oil

Vegetable oil transformer oil, also known as natural ester transformer oil or plant-based insulating oil, is an environmentally friendly insulating oil developed in recent years. It is derived from renewable plant oils and serves as a substitute for traditional mineral oil in transformers. Vegetable oil transformer oil is mainly made from plant-based oils, such as rapeseed oil, soybean oil, sunflower oil, or palm oil, which are chemically modified or physically treated. This type of insulating oil has excellent environmental characteristics and good biodegradability under high-temperature conditions.

 2.3.1 Properties and Advantages

Environmental Performance

• Renewable Source: Vegetable oils are renewable resources, reducing dependence on limited fossil resources compared to mineral oil.
• Biodegradability: The biodegradability of vegetable oil transformer oil exceeds 90%, making it far less harmful to the environment than traditional mineral oils.
• Low Toxicity: Vegetable oil is non-toxic and poses minimal risk to water and soil pollution in case of leakage.

Thermal Performance

• High Flash Point: Vegetable oil has a higher flash and fire point compared to mineral oil (typically above 300°C), reducing the risk of fire due to overheating.
• Excellent Thermal Conductivity: It efficiently transfers heat generated during transformer operation, maintaining the transformer’s operating temperature.

Electrical Performance

• Superior Insulating Properties: Vegetable oil has a high breakdown voltage, meeting the insulation needs of transformers.
• Moisture Absorption: Compared to mineral oil, vegetable oil can absorb more moisture, thereby extending the lifespan of transformer insulation paper.
• Chemical Stability
• Oxidation Resistance: After appropriate modification, vegetable oil exhibits improved oxidative stability, prolonging its service life.
• Low Corrosiveness: Vegetable oil has low corrosive effects on metals and insulation materials in transformers, reducing maintenance costs.

2.3.2 Technical Specifications

The typical technical specifications of vegetable oil transformer oil (may vary slightly across brands) are as follows:

2.3.3 Limitations and Challenges

Despite its many advantages, vegetable oil transformer oil has some limitations:

Higher Cost: The raw material and processing costs of vegetable oil are higher, leading to a higher market price compared to mineral oil.

Poor Low-temperature Performance: Vegetable oil has lower fluidity at low temperatures, requiring formulation improvements or the addition of pour-point depressants for use in colder regions.

Oxidation Stability: While improved through modification, its oxidative stability is still lower than that of mineral oil.

Technical Acceptance: Since mineral oil has been widely used for many years, it will take time for vegetable oil transformer oil to gain broader acceptance.

2.4 Silicone Transformer Oil

Silicone Transformer Oil is a special type of insulating oil primarily based on silicone (siloxane). Silicone oil is a synthetic polymer composed of silicon, oxygen, and hydrocarbon compounds, with polydimethylsiloxane being the most common type. Due to its unique chemical properties and exceptional performance, silicone transformer oil is suitable for transformers, instrument transformers, or other electrical equipment in special applications.

2.4.1 Main Components of Silicone Transformer Oil

• Siloxane Molecular Chain: The chemical formula of silicone oil typically involves a repeating chain structure of  , offering high stability.
• Additives: Depending on the application, antioxidants, corrosion inhibitors, and other additives may be included to enhance performance.

2.4.2 Key Performance Features

High Insulation Properties

• Silicone oil exhibits excellent dielectric strength, making it an ideal insulating medium for transformers.
• Its insulation performance remains stable under high-voltage conditions.

Exceptional High and Low-Temperature Resistance

• The operating temperature range of silicone transformer oil is typically from -50°C to 200°C, far exceeding that of mineral oil.
• It maintains good fluidity in low-temperature environments, avoiding freezing issues.
• At high temperatures, it demonstrates superior resistance to thermal oxidation and degradation.

Flame Resistance

• Silicone transformer oil is non-flammable or flame-resistant, with a high flash point (typically above 300°C), offering excellent safety.
• It is particularly suitable for transformers requiring flame retardancy, such as those in urban areas or nuclear power plants.

Aging Resistance

• Silicone oil is highly resistant to heat and oxygen, resulting in minimal degradation over time and a longer service life.
• It generates fewer acidic substances or deposits, reducing corrosion of equipment.

Environmental Friendliness

• Silicone oil is low in toxicity and has minimal environmental impact in case of leakage, making it easier to manage and dispose of.

High Chemical Stability

• Silicone oil is chemically inert and resistant to reactions with water, oxygen, or impurities.
• Its performance remains stable even in humid environments, and it is less likely to emulsify.

Prevent Oil Degradation:

• Good Sealing: Ensure the transformer oil tank and circulation system are well-sealed to prevent air and moisture ingress.
• Avoid High Temperatures: Prolonged high temperatures accelerate oil oxidation, so keep oil temperatures within a reasonable range.

Regular Oil Filtration:

• Use filtration equipment to remove impurities, moisture, and oxidation by-products (such as sludge) to maintain oil cleanliness.
• The frequency of oil filtration depends on the transformer’s operational status, typically every six months to a year.

Replenishment or Replacement:

• If tests indicate significant performance decline, consider replenishing or replacing part or all of the transformer oil.
• New oil should be degassed, dehydrated, and purified before addition to meet insulation and cooling requirements.

Maintain Oil Levels:

• Regularly check the oil level, especially with seasonal changes or load fluctuations.

Prevent Contamination:

• Avoid introducing impurities, dust, or moisture during maintenance.
• Use appropriate storage containers and transport tools.

1. Choose a good material seal

Transformer maintenance and treatment of leakage, should choose high temperature resistance, good oil resistance seals. The most commonly used sealing material in the domestic transformer industry is nitrile rubber; its oil resistance mainly depends on the acrylonitrile content in nitrile rubber, the higher the acrylonitrile content, the better the oil resistance, the greater the hardness, the more difficult to deformation. In general, nitrile rubber with Shaw hardness between 70 and 80 should be selected. When identifying the oil resistance of the gasket, it is generally necessary to do the aging test of the gasket and the compatibility test with the transformer oil, soak it in the hot oil at 120 ° C for 168h, and then measure the change rate of its weight, volume and hardness, and select the sealing parts with little deformation and in line with the standard.

2. Choose high quality butterfly valve

Butterfly valve Choose ZF80 vacuum eccentric butterfly valve. Compared with the ordinary butterfly valve, the vacuum eccentric butterfly valve has been greatly improved in mechanical strength and surface finish, and the biggest advantage of the product is that the double-layer seal is used at the transformer flange interface, so as to eliminate the leakage of oil at the transformer interface.

3. The use of electric welding leakage

For the porosity, sand hole left by the casting of the transformer, welding, welding joints, welding, crack, can be used for welding plugging.

The leakage point should be identified before plugging welding. If the leakage point is small, the leakage point can be killed directly by electric welding. If the leak point is large, it should be filled with asbestos rope or metal filler first, and then surfacing around it, and then using small electrode and high current fast arc welding.

4. Standardize the sealing parts replacement process

For different types and different capacities of transformers, whether using flange connection or threaded connection, the dust and rust on the connecting surface must be removed before replacing the seals. After cleaning the seals, apply sealant on both sides of the seals (generally 609 polymer liquid sealant). After the sealant dries for a period of time, the solvent evaporates. Secure the flange and screw connection.

5. Improve the installation process level, eliminate leakage caused by improper installation methods

If the flange interface is uneven or deformed and misaligned, correct the interface first. If the misalignment is serious and cannot be corrected, cut off the flange and weld it again. Ensure that the interface is parallel. When installing the sealing gasket compression is about 1/3 of its thickness is appropriate.

6. Quick sealing and plugging glue stick

This method can be used for small leakage and drip leakage of transformer, and can be used to plug the leakage of transformer radiator tube wall is thin, and the leakage point is not suitable for welding plugging. When using the plugging stick to plug the leak, it is necessary to completely remove the oil, patent leather and oxide in the plugging part to make the metal show its natural color. Then adjust the plugging glue according to the ratio, and plug the leakage part until it does not leak.