Technical Difficulties of Biodegradable Transformer Insulating Oil as an Alternative to Mineral Oil
Mineral oil has long been the dominant insulating medium in oil-immersed transformers, thanks to its excellent insulating properties, thermal conductivity, and chemical stability, which ensure the safe and stable operation of transformers. However, mineral oil is a non-renewable fossil fuel derivative, and its poor biodegradability poses severe environmental risks—once leaked, it can persist in soil and water for a long time, causing serious pollution to the ecological environment. With the global emphasis on environmental protection and the rapid development of green power grids, the demand for biodegradable transformer insulating oils (such as vegetable-based insulating oils, synthetic ester insulating oils, and natural ester insulating oils) as alternatives to mineral oil has become increasingly urgent. Although biodegradable insulating oils have inherent advantages of environmental friendliness, renewability, and high fire resistance (with a flash point up to 300℃, much higher than the 160℃ of mineral oil), their large-scale application still faces numerous technical difficulties that restrict their popularization and replacement of mineral oil. This article systematically elaborates on the key technical difficulties of biodegradable transformer insulating oils as alternatives to mineral oil, covering stability, compatibility, operational performance, large-scale production, and cost control, combining the latest research progress and practical application experience, to provide a comprehensive reference for the research, development, and industrialization of biodegradable insulating oils.
The first and most critical technical difficulty is the poor oxidation stability of biodegradable insulating oils, which directly affects the service life and operational reliability of transformers. Unlike mineral oil, which is a saturated hydrocarbon with high chemical stability, most biodegradable insulating oils (especially vegetable-based oils) are composed of unsaturated fatty acid esters, which have reactive double bonds in their molecular structure. These double bonds are easily oxidized when exposed to oxygen, heat, light, and metal catalysts (such as copper and iron in transformers), leading to the formation of oxidation products such as acids, peroxides, and sludge. The accumulation of these oxidation products will significantly reduce the insulating performance of the oil, increase the dielectric loss, corrode the transformer’s internal metal components, and accelerate the aging of the insulation paper, ultimately shortening the service life of the transformer.
Taking vegetable-based insulating oils (such as camellia seed oil and rapeseed oil, which are widely studied as alternatives) as an example, their oxidation stability is significantly lower than that of mineral oil. In the early stage of research, the oxidation induction period of unmodified vegetable insulating oil was less than 10 hours, which is far lower than the 100 hours or more required for transformer operation. Although researchers have adopted antioxidant compounding technology (such as adding multiple phenolic antioxidants) to improve oxidation stability, there are still two key problems: first, the addition of antioxidants may affect the insulating performance and biodegradability of the oil; second, antioxidants will gradually consume during the operation of the transformer, and their long-term effectiveness is difficult to guarantee. In addition, the oxidation mechanism of biodegradable insulating oils is more complex than that of mineral oil, involving multiple reaction pathways such as free radical chain reactions and hydrolysis reactions, which makes it difficult to fundamentally solve the oxidation problem through a single technical means. This technical difficulty has become a core bottleneck restricting the long-term stable operation of biodegradable insulating oils in transformers.
The second major technical difficulty is the poor compatibility between biodegradable insulating oils and transformer materials, including insulation paper, sealing materials, and metal components. Mineral oil has been used in transformers for decades, and the design of transformer materials is fully compatible with mineral oil. However, biodegradable insulating oils have different chemical properties (such as polarity, viscosity, and solubility) from mineral oil, which often leads to incompatibility problems when used in existing transformers, affecting the operational safety of the equipment.
In terms of insulation paper compatibility, biodegradable insulating oils (especially ester-based oils) have strong polarity, which can easily cause the swelling and aging of traditional cellulose insulation paper. The swelling of the insulation paper will reduce its mechanical strength and insulation performance, while the accelerated aging will shorten its service life. For example, studies have shown that when vegetable-based insulating oil is used with traditional kraft paper, the service life of the paper is reduced by 30-50% compared with that used with mineral oil. Although modified insulation paper (such as nano-modified cellulose paper) can improve compatibility, its high cost and complex preparation process limit its large-scale application. In addition, the interaction between biodegradable insulating oils and insulation paper will produce degradation products, which further pollute the oil and reduce the overall insulation performance of the transformer.
In terms of sealing material compatibility, biodegradable insulating oils have strong solubility, which can dissolve or swell traditional rubber sealing materials (such as nitrile rubber and neoprene) used in transformers, leading to seal failure and oil leakage. This not only causes environmental pollution but also affects the safe operation of the transformer. Although some special sealing materials (such as fluororubber) have good compatibility with biodegradable insulating oils, their high cost and poor processing performance make it difficult to replace traditional sealing materials on a large scale. In addition, the compatibility between biodegradable insulating oils and transformer metal components (such as copper, iron, and aluminum) is also a problem—some biodegradable oils will accelerate the corrosion of metal components under high temperature and oxygen conditions, leading to the generation of metal ions, which further catalyze the oxidation of the oil and form a vicious cycle.
The third technical difficulty is that the operational performance of biodegradable insulating oils is difficult to fully match the requirements of high-voltage and large-capacity transformers. Mineral oil has excellent dielectric properties, thermal conductivity, and low-temperature fluidity, which can meet the operational requirements of transformers under various working conditions. However, most biodegradable insulating oils have inherent defects in these performance indicators, making it difficult to adapt to the harsh working environment of high-voltage, large-capacity transformers.
In terms of dielectric properties, although the breakdown voltage of most biodegradable insulating oils is higher than that of mineral oil, their dielectric loss factor is significantly higher, especially under high temperature and high frequency conditions. The high dielectric loss will lead to increased heat generation of the transformer, which not only increases energy consumption but also accelerates the oxidation and aging of the oil and insulation paper. In addition, the partial discharge resistance of biodegradable insulating oils is also lower than that of mineral oil, which makes them more prone to partial discharge under high voltage conditions, further damaging the insulation system of the transformer.
In terms of thermal conductivity, the thermal conductivity of biodegradable insulating oils (especially vegetable-based oils) is lower than that of mineral oil, which affects the heat dissipation effect of the transformer. Transformers generate a lot of heat during operation, and the insulating oil plays a key role in heat transfer. The low thermal conductivity of biodegradable oils will lead to the accumulation of heat inside the transformer, increasing the operating temperature of the transformer and accelerating the aging of the insulation system. Although adding nano-particles (such as graphene, carbon nanotubes) can improve the thermal conductivity of biodegradable oils, the dispersion stability of nano-particles in the oil is poor, and they are easy to agglomerate, which not only affects the insulating performance but also may block the transformer’s oil circuit.
In terms of low-temperature fluidity, most biodegradable insulating oils (especially vegetable-based oils) have a high pour point (usually above -10℃), which is much higher than that of mineral oil (below -25℃). In cold regions, the viscosity of biodegradable oils will increase significantly at low temperatures, even solidify, which affects the heat dissipation and insulation performance of the transformer, and may even lead to transformer failure. Although researchers have adopted methods such as hydrogenation modification and additive addition to reduce the pour point of biodegradable oils, these methods will either reduce the biodegradability of the oil or increase the cost, making it difficult to achieve a balance between low-temperature performance and environmental protection.
The fourth technical difficulty is the lack of mature large-scale production technology and standard system. Unlike mineral oil, which has a mature refining and production process, the production of biodegradable insulating oils (especially vegetable-based and natural ester-based oils) still faces many technical challenges in large-scale industrialization. Taking vegetable-based insulating oil as an example, the production process involves oil extraction, degumming, deacidification, decolorization, and other steps, and the quality of the final product is greatly affected by the raw material quality and production process parameters. In the early stage of research, the production of vegetable insulating oil was mainly limited to laboratory scale, and the batch production had problems such as unstable product quality, high acid value, and high dielectric loss. Although some teams have developed large-scale production equipment (such as 300-ton/year and 1500-ton/year production lines), the production process is still complex, the energy consumption is high, and the production cost is difficult to control.
In addition, the lack of a unified standard system is another important factor restricting the large-scale application of biodegradable insulating oils. At present, most countries and regions still adopt the standard system formulated for mineral oil, which cannot fully adapt to the performance characteristics of biodegradable insulating oils. For example, the indicators such as oxidation stability, dielectric loss, and low-temperature fluidity in the existing standards are not suitable for evaluating biodegradable oils, leading to inconsistent evaluation results of different products and difficulties in quality control. Although some countries have begun to formulate special standards for biodegradable insulating oils, the standards are not unified, which increases the difficulty of cross-border promotion and application. In addition, the lack of standard test methods for biodegradability, compatibility, and long-term performance also restricts the research and development and application of biodegradable insulating oils.
The fifth technical difficulty is the high cost of biodegradable insulating oils, which makes it difficult to compete with mineral oil in the market. The production cost of biodegradable insulating oils is usually 2-5 times that of mineral oil, mainly due to three reasons: first, the raw material cost is high—vegetable-based oils are mainly derived from crops such as camellia seeds and rapeseeds, and the price of raw materials is affected by factors such as climate and market supply and demand; second, the production process is complex, requiring multiple purification and modification steps, which increases the production cost and energy consumption; third, the production scale is small, and the economies of scale cannot be formed, leading to high unit production cost.
Although biodegradable insulating oils have the advantage of long service life, which can reduce the maintenance cost of transformers in the long run, the high initial investment cost still makes most power grid companies and transformer manufacturers hesitant to adopt them. In addition, the lack of policy support and subsidies also limits the promotion and application of biodegradable insulating oils. For example, in some regions, there is no preferential policy for the use of biodegradable insulating oils, which makes it difficult for them to compete with mineral oil in terms of cost.
Another technical difficulty that cannot be ignored is the lack of mature maintenance and fault handling technology. Mineral oil has a complete set of maintenance and fault handling methods (such as oil filtration, oil regeneration, and dissolved gas analysis) formed after decades of application, which can effectively ensure the safe operation of transformers. However, biodegradable insulating oils have different physical and chemical properties from mineral oil, and the traditional maintenance methods are not applicable. For example, the dissolved gas components generated by the fault of biodegradable insulating oils are different from those of mineral oil, and the traditional dissolved gas analysis method has a high misjudgment rate, making it difficult to accurately diagnose transformer faults. In addition, there is a lack of on-site treatment equipment for biodegradable insulating oils, and once a fault occurs, it is difficult to handle it in a timely manner, which affects the operational reliability of the transformer.
In addition, the research on the long-term operational performance of biodegradable insulating oils is insufficient. Most of the existing research is limited to short-term laboratory tests, and there is a lack of long-term practical application data. Transformers have a service life of 20-30 years, and the long-term stability, aging mechanism, and performance degradation law of biodegradable insulating oils in actual operation are still unclear. For example, the oxidation products of biodegradable oils in long-term operation may have unknown effects on the transformer’s insulation system and metal components, and the long-term effectiveness of antioxidants and additives also needs to be verified by practical application. The lack of long-term performance data makes it difficult for power grid companies and transformer manufacturers to fully trust the reliability of biodegradable insulating oils, which further restricts their large-scale application.
It should be noted that although biodegradable insulating oils face many technical difficulties, significant progress has been made in related research in recent years. For example, researchers have developed high-stability vegetable insulating oils through antioxidant compounding and raw material optimization, which have an oxidation induction period of more than 100 hours, approaching the level of mineral oil. In terms of compatibility, modified insulation paper and special sealing materials have been developed to improve the compatibility between biodegradable oils and transformer materials. In addition, the large-scale production technology of biodegradable insulating oils has also made breakthroughs, and some production lines have been put into use, reducing the production cost to a certain extent. However, these technologies still need to be further optimized and improved to achieve large-scale application.
In conclusion, the replacement of mineral oil with biodegradable transformer insulating oil is an inevitable trend in the development of green power grids, but it still faces numerous technical difficulties, including poor oxidation stability, poor compatibility with transformer materials, insufficient operational performance, immature large-scale production technology, high cost, lack of standard system, and insufficient maintenance technology. These technical difficulties are interrelated and restrict each other, requiring joint efforts from researchers, manufacturers, and policy makers to solve. In the future, it is necessary to further strengthen the research on the oxidation mechanism and stability improvement technology of biodegradable insulating oils, develop high-performance compatible materials, optimize the production process to reduce costs, establish a unified standard system, and accumulate long-term practical application data. Only by overcoming these technical difficulties can biodegradable transformer insulating oils truly replace mineral oil, realize the green and sustainable development of the power grid, and reduce environmental pollution caused by oil leakage.
