Research on the Application of Plant-Based Ester Insulating Oil in Medium and High Voltage Transformers

Medium and high voltage transformers are core equipment in the power transmission and distribution system, and their safe and stable operation directly affects the reliability of the power grid and the continuity of energy supply. Insulating oil, as a core component of transformers, plays a crucial role in insulation, heat dissipation, and arc extinguishing. For a long time, mineral insulating oil has dominated the market for medium and high voltage transformer insulating oil due to its mature preparation process and stable dielectric properties. However, with the increasing global environmental awareness and the advancement of the "dual carbon" target, the defects of mineral insulating oil, such as its non-renewable nature, low biodegradability, and low ignition point which easily lead to safety accidents, have become increasingly prominent, severely limiting its application in sensitive scenarios such as urban core areas, high-rise buildings, new energy power plants, and chemical industrial parks.

Plant-based ester insulating oil, made from renewable vegetable oils, possesses natural advantages such as high ignition point, easy biodegradability, and environmental friendliness, making it an important alternative to medium and high voltage transformer insulating oil. In recent years, domestic and international academic and industrial circles have conducted extensive research and practice on the modification technology, compatibility optimization, and engineering applications of plant-based ester insulating oil. This white paper systematically reviews the current technological development status, core performance characteristics, application practices in medium and high voltage transformers, existing bottlenecks, and future trends of plant ester insulating oil. It aims to provide authoritative reference for the power industry, manufacturing enterprises, research institutions, and policy-making departments, and promote the large-scale and standardized application of plant ester insulating oil in the field of medium and high voltage transformers.

I. Industry Overview and Development Background

1.1 Current Status of the Medium and High Voltage Transformer Insulating Oil Market

Currently, the global medium and high voltage transformer insulating oil market is still dominated by mineral insulating oil, accounting for over 85% of the market. Mineral insulating oil is derived from petroleum refining, with mature technology and low cost, but it has significant shortcomings in terms of ecology and safety. According to statistics on power industry accidents, in the past five years, there have been over 100 soil and water pollution incidents caused by transformer oil leaks globally each year, with single pollution remediation costs reaching millions of yuan. At the same time, mineral insulating oil has a flash point of only 160-180℃, making it prone to overheating and fires under overload operation or equipment aging conditions, causing significant economic losses.

With the rapid development of new energy power generation, wind power, photovoltaic, and other power plants are mostly located in ecologically sensitive areas, and urban power distribution networks are developing towards high density and compactness, continuously upgrading the requirements for the environmental protection and safety of transformer insulating oil. Against this backdrop, the market demand for environmentally friendly insulating oils such as plant-based ester insulating oils and synthetic ester insulating oils has been increasing year by year. Among them, plant-based ester insulating oils have seen particularly significant growth due to their renewable raw materials and relatively controllable production costs, with an average annual growth rate exceeding 15% in the global market size from 2020 to 2024.

1.2 Policy and Technology Drivers

At the policy level, many countries have introduced environmental regulations to promote the upgrading of insulating oils. The EU's Waste Electrical and Electronic Equipment Directive and Regulation on Registration, Evaluation, Authorization and Restriction of Chemicals explicitly restrict the use of high-pollution insulating oils and require that electrical equipment prioritize the use of biodegradable insulating materials. China's "14th Five-Year Plan for Energy Conservation and Emission Reduction" and "Green and Low-Carbon Action Plan for the Power Industry" also encourage the promotion of environmentally friendly electrical equipment and supporting materials, providing policy support for the application of plant-based ester insulating oils.

At the technological level, breakthroughs in vegetable oil refining and modification technologies have laid the foundation for the industrial application of plant-based ester insulating oils. Early plant ester insulating oils were difficult to adapt to medium and high voltage transformers due to their high viscosity and poor low-temperature fluidity. However, after modification treatments such as degumming, deacidification, hydrogenation, and transesterification, their key properties have been significantly improved, gradually meeting the long-term operation requirements of medium and high voltage transformers. Simultaneously, the optimization of transformer manufacturing processes has also provided the equipment conditions for adapting plant ester insulating oils.

II. Preparation and Core Characteristics of Plant Ester Insulating Oils

2.1 Raw Materials and Preparation Process

2.1.1 Core Raw Materials
The raw materials for plant ester insulating oils are mainly renewable vegetable oils, with mainstream varieties including soybean oil, rapeseed oil, palm oil, and sunflower oil. Different raw materials have different properties and applicable scenarios. Rapeseed oil has a wide range of sources, a stable supply in Northwest and Southwest my country, and relatively low cost. Palm oil has a high saturated fatty acid content and outstanding thermal stability, but weak low-temperature performance, making it suitable for tropical and subtropical regions. Soybean oil has balanced dielectric properties and is one of the most widely used raw materials in commercial applications. In addition, non-edible vegetable oils such as jatropha oil and tung oil are gradually entering the research and development field, which can avoid competing with food crops for land and further improve the sustainability of raw materials.

2.1.2 Preparation and Modification Processes The basic preparation process of vegetable ester insulating oil includes raw material pretreatment, refining, modification, and finished product blending. Raw material pretreatment mainly removes impurities, moisture, and colloids from the oil; the refining process reduces the content of free fatty acids and harmful substances in the oil through steps such as deacidification, decolorization, and deodorization; the core modification process optimizes the performance of vegetable oils by addressing their inherent defects. Mainstream technologies include:

Hydrogenation modification: Increasing the saturation of fatty acid chains through hydrogenation reactions improves oxidation stability, but the degree of hydrogenation must be controlled to avoid excessive hydrogenation leading to increased viscosity;

Transesterification modification: Utilizing alcohols such as methanol and ethanol to undergo transesterification reactions with vegetable oils adjusts the molecular structure, reduces viscosity, and improves low-temperature fluidity;

Composite modification: Combining hydrogenation and transesterification technologies to simultaneously optimize oxidation stability and low-temperature performance is currently the mainstream industrial modification solution.

III. Application Practice of Vegetable Ester Insulating Oil in Medium and High Voltage Transformers

3.1 Application Scenario Adaptability Analysis
The different application scenarios of medium and high voltage transformers impose different performance requirements on insulating oils. Vegetable ester insulating oil, with its safety and environmental protection advantages, demonstrates significant adaptability in the following core scenarios:
Urban core areas and high-rise buildings: These scenarios are characterized by dense populations, concentrated equipment, and high risks and costs associated with fire and pollution. The high flash point of plant-based ester insulating oil eliminates the need for complex fireproofing and isolation facilities in transformers, reducing floor space and adapting to the compact layout of urban power distribution networks.

New Energy Power Stations: Wind and photovoltaic power stations are often located in ecologically sensitive areas such as grasslands and mountains. The high biodegradability of plant-based ester insulating oil prevents oil leaks from damaging the ecological environment and is suitable for the frequent start-stop and large load fluctuations of new energy power stations.

Chemical Industrial Parks and Mines: Chemical industrial parks contain flammable and explosive media, and mining environments are complex. The high safety of plant-based ester insulating oil reduces equipment operation risks and its strong resistance to pollution makes it suitable for harsh operating environments.

Transmission in Sub-sea and Remote Areas: Transformers in sub-sea areas and distribution transformers in remote areas are difficult to maintain. The stability and environmental friendliness of plant-based ester insulating oil can reduce maintenance costs after leaks and improve equipment operation and maintenance efficiency.

3.2 Typical Application Cases at Home and Abroad

3.2.1 Domestic Case
A 220kV smart substation in a provincial power grid: Two transformers using soybean-based plant-based ester insulating oil were put into operation in 2022 and have been operating stably for over two years. Monitoring data shows that the transformer oil temperature was on average 3-5℃ lower than that of mineral oil transformers of the same capacity, the aging rate of the insulation paper was slowed down, and no abnormalities such as partial discharge were observed, making it suitable for the high-load operation requirements of substations. A 35kV box-type transformer at a large photovoltaic power station: This power station is located in a grassland ecological protection area. In 2023, a batch of transformers with palm-based modified vegetable ester insulating oil were replaced. During this period, one minor oil leak occurred. After natural degradation, no ecological abnormalities were observed in the soil of the leaked area, verifying its environmental advantages.

3.2.2 International Cases

A 110kV distribution network in a German city: Starting in 2020, transformers in the city's core area were gradually replaced with rapeseed oil-based insulating oil. By 2024, over 50 units had been put into operation. The fire risk rate decreased by 80% compared to mineral oil transformers, and maintenance costs were reduced by 15%.

A 66kV transformer in a US offshore wind power project: Utilizing composite modified vegetable ester insulating oil, suitable for the high humidity and high salt spray environment at sea, its dielectric performance remained stable over three years of operation, with no insulation degradation issues observed.

3.3 Equipment Adaptation and Adjustment in Application

Vegetable ester insulating oil has a higher viscosity than mineral insulating oil. When used in medium and high voltage transformers, targeted adaptation and adjustment of the equipment are required to ensure operational efficiency:

Heat dissipation system optimization: Increase the radiator area or upgrade the forced air cooling device to improve heat dissipation efficiency and avoid poor heat dissipation due to high viscosity;

Sealing material adaptation: Vegetable esters can cause swelling of some rubber sealing materials, requiring replacement with ester-resistant materials such as fluororubber and silicone rubber to prevent oil leakage;

Insulation structure adjustment: Optimize the winding insulation spacing design, taking advantage of the better match between the dielectric constant of vegetable esters and insulating paper, to further improve the reliability of the insulation system.

IV. Existing Technical Bottlenecks and Challenges

4.1 Shortcomings in Core Technologies

Insufficient Low-Temperature Performance: Most plant-based ester insulating oils crystallize or experience a sharp increase in viscosity below -20℃, affecting the low-temperature start-up and operation of transformers. This limits their promotion in high-latitude, cold regions.

Oxidative Stability Needs Improvement: Unsaturated fatty acids in plant esters are prone to oxidation, generating acids, colloids, and other products that accelerate the aging of insulating paper and shorten transformer lifespan. While additives can mitigate this, long-term stability still needs verification.

Large-Scale Production Process Needs Improvement: Consistency control in the modification process is difficult, resulting in significant performance fluctuations between different batches compared to mineral oils. Furthermore, the supply of high-purity raw materials is affected by agricultural production cycles, leading to insufficient stability.

4.2 Market and Cost Constraints

Currently, the production cost of plant-based ester insulating oil is approximately 2-3 times that of mineral insulating oil. This higher cost slows its penetration rate in the medium- and high-voltage transformer market. In addition, while the supply chain for mineral insulating oil is mature, the supply chain systems for plant-based ester insulating oil, including raw material procurement, modification processing, warehousing, and transportation, are not yet fully mature, further hindering its large-scale promotion. 4.3 Lagging Standards and Specifications
Standards for vegetable ester insulating oils, both domestically and internationally, remain incomplete. Current Chinese standards largely reference mineral oil standards, failing to fully reflect the characteristics of vegetable esters. While international standards include specific specifications, significant regional differences lead to insufficient product compatibility and mutual recognition, hindering cross-border applications and technical exchanges. Furthermore, operation and maintenance standards and aging assessment methods for vegetable ester insulating oil transformers are still in the exploratory stage, lacking unified guidance.

V. Technological Optimization Directions and Solutions

5.1 Performance Optimization Technology Development

Breakthroughs in novel modification technologies: Develop new technologies such as catalytic isomerization and genetic modification to adjust the molecular structure of vegetable esters, improving both oxidative stability and low-temperature performance. For example, isomerization reactions can convert unsaturated fatty acids into branched structures, lowering the freezing point to below -30℃.

High-efficiency additive development: Develop specialized composite antioxidants and pour point depressants that can inhibit oxidation reactions and reduce negative impacts on insulating paper. Currently, nitrogen-containing heterocyclic antioxidants have demonstrated excellent synergistic antioxidant effects.
Non-edible raw material development: Increase R&D efforts in non-edible plant oils such as hemp seed oil and Chinese pistache oil to reduce dependence on edible oils. Simultaneously, cultivate high-yield, high-purity specialty raw material crops through gene breeding technology.

5.2 Cost Control Path
Process cost reduction: Optimize and modify processes, simplify production flows, for example, by adopting continuous transesterification equipment to improve production efficiency; recycle by-products from the production process to reduce raw material loss.
Supply chain integration: Establish an integrated supply chain encompassing raw material planting, processing, and production; sign long-term cooperation agreements with agricultural bases to stabilize raw material prices; promote regionalized production to reduce raw material transportation costs.
Large-scale effect release: As market penetration increases, expand production scale to amortize R&D and equipment depreciation costs, gradually narrowing the price gap with mineral insulating oil.

5.3 Recommendations for Improving the Standards System

Develop Specialized Standards: Based on the characteristics of plant-based ester insulating oils, develop specialized national standards covering raw materials, modification processes, core performance, and testing methods, clearly defining key indicators such as oxidation stability and low-temperature performance.

Unify Operation and Maintenance Standards: Establish operation and maintenance standards for plant-based ester insulating oil transformers, including operation monitoring, aging assessment, and oil change cycles, to guide standardized operation and maintenance in the industry.

Promote International Standard Recognition: Strengthen cooperation with organizations such as the International Electrotechnical Commission (IEC) to promote the coordination of domestic and international standards and enhance the international competitiveness of my country's plant-based ester insulating oil products.

VI. Future Development Outlook

6.1 Technological Development Trends
In the future, plant-based ester insulating oils will develop towards high performance, multifunctionality, and low cost. On the one hand, the integration of genetic engineering and novel modification technologies will achieve breakthrough improvements in the low-temperature performance and oxidation stability of plant esters, making them suitable for all regions and operating conditions. On the other hand, multifunctional composite plant-based ester insulating oils will become a research hotspot, such as products with insulation, thermal conductivity, and antibacterial functions, further expanding application scenarios. Furthermore, the combination of plant esters and nanomaterials is expected to achieve synergistic optimization of dielectric and heat dissipation performance.

6.2 Market Promotion Prospects

With the continued tightening of environmental policies and the rapid development of new energy power, the market penetration rate of plant-based ester insulating oil in medium and high voltage transformers is expected to exceed 30% by 2030. Sub-sectors such as low-temperature products for high-latitude regions and customized products for new energy power plants will experience rapid growth. Simultaneously, as costs decrease, its application will gradually expand from high-end scenarios to ordinary power distribution networks, forming a large-scale promotion trend.

6.3 Industry Collaborative Development Recommendations

Deep Industry-University-Research Collaboration: Encourage universities, research institutions, and enterprises to jointly tackle core technologies, establish pilot-scale production bases, and accelerate the transformation of technological achievements;

Precise Policy Support: Recommend the introduction of subsidy policies to support the research and development and demonstration application of plant-based ester insulating oil, while including it in the green power equipment procurement list to guide market demand;

Industry Exchange and Popularization: Strengthen technical exchange and promotion through industry exhibitions, technical seminars, and other forms to enhance the industry's understanding of plant-based ester insulating oil and promote the collaborative development of the entire industry chain.

In conclusion, plant-based ester insulating oil, as an environmentally friendly and safe new type of insulating material, aligns with the green and low-carbon transformation of the power industry and has enormous application potential in medium and high-voltage transformers. Currently, although facing multiple challenges in technology, cost, and standards, with breakthroughs in modification technologies, improvements in the supply chain, and a sound policy framework, plant-based ester insulating oil will inevitably gradually replace mineral insulating oil and become the mainstream choice for insulating oil in medium and high-voltage transformers. The entire industry needs to work together to overcome technical difficulties, improve the industrial ecosystem, and jointly promote the power industry towards a safer, more environmentally friendly, and more sustainable development.