Ultra-high voltage (UHV) transformers are the core equipment of UHV power transmission and distribution systems, undertaking the key task of long-distance and large-capacity power transmission, and their safe and stable operation is the foundation of the UHV power grid’s reliability. Transformer insulating oil, as the core medium inside UHV transformers, undertakes dual functions of insulation and heat dissipation under extreme working conditions of high electric field, high temperature and large load, and its performance directly determines the operational life, insulation reliability and overload capacity of UHV transformers. In the past decade, with the rapid construction of UHV AC/DC power grids worldwide, the performance requirements for UHV transformer insulating oil have been continuously upgraded, and the research, development and application of insulating oil adapted to UHV working conditions have achieved remarkable results. UHV-grade mineral insulating oil has become the mainstream application product through process upgrading and performance optimization; environmentally friendly ester insulating oil has broken through key technical bottlenecks and realized pilot engineering application in UHV transformers; functional modified insulating oils represented by nano-modified insulating oil have made important progress in laboratory research and small-scale testing. This paper systematically expounds the core performance requirements of insulating oil for UHV transformers based on the extreme working characteristics of UHV transformers, analyzes the application status, technical adaptation schemes and typical engineering practices of mainstream insulating oil types (mineral oil, environmentally friendly ester oil) in UHV transformers, deeply discusses the key technical challenges faced by insulating oil in UHV engineering application, and puts forward targeted optimization strategies and improvement measures. Finally, the future development trends of UHV transformer insulating oil in the direction of green low-carbonization, high performance, functionalization and digital intelligence are prospected, aiming to provide a comprehensive reference for the type selection, engineering application and operation maintenance of UHV transformer insulating oil in the power industry.
Keywords
UHV Transformer Insulating Oil; Transformer Insulating Oil; UHV Power Transformer; Mineral Insulating Oil; Environmentally Friendly Ester Oil; Oil-Paper Insulation Synergy; Insulating Oil Performance Matching; UHV Transformer Operation and Maintenance
1 Introduction
UHV power transmission technology is the core solution to realize long-distance, large-capacity and low-loss power transmission, and is an important pillar of the construction of modern power grids and energy interconnection systems. UHV transformers, including UHV AC transformers (1000 kV and above) and UHV DC converter transformers (±800 kV, ±1100 kV and above), are the core key equipment of UHV power grids. Different from conventional medium and high-voltage transformers, UHV transformers operate under extreme working conditions such as ultra-high electric field strength, high thermal load density, long-term continuous operation and complex environmental changes, which put forward extremely strict and personalized requirements for all core components and auxiliary materials of the transformer.
Transformer insulating oil is an indispensable core medium inside UHV transformers, and its role is far more important than that in conventional transformers: on the one hand, it needs to isolate the high-voltage live parts with huge potential differences inside the transformer, prevent electrical breakdown and corona discharge under ultra-high electric field, and ensure the absolute insulation reliability of the equipment; on the other hand, it needs to efficiently transfer the large amount of heat generated by the core and winding under full load operation, control the temperature rise of the transformer within the rated range, and avoid insulation aging caused by overheating. In addition, UHV transformers have a design service life of more than 40 years, and the insulating oil must maintain stable physical, chemical and electrical properties for a long time, and form a synergistic anti-aging system with insulating paper, insulating cardboard and other materials to ensure the long-term operation stability of the whole transformer insulation system.
In the past two decades, with the large-scale construction of UHV power grids in China, Europe, Brazil and other countries and regions, the research and development of UHV transformer insulating oil has been pushed to a new height. From the initial adaptation of conventional high-grade mineral oil to the targeted development of UHV special grade insulating oil through process upgrading and additive innovation, and then to the research and pilot application of environmentally friendly ester oil and functional modified oil in UHV transformers, the application technology of UHV transformer insulating oil has realized continuous iteration and upgrading. At present, UHV-grade fully hydrogenated mineral insulating oil is the mainstream application product in global UHV engineering due to its excellent comprehensive performance, mature application technology and stable engineering practice; environmentally friendly ester insulating oil is gradually entering the pilot application stage in UHV transformers with its advantages of fire resistance, environmental protection and long life; functional modified insulating oils such as nano-modified oil are still in the stage of laboratory research and small-scale testing, and are the key research directions for the future upgrading of UHV insulating oil technology.
This paper takes the application of transformer insulating oil in UHV transformers as the research core, combs the core performance requirements of insulating oil adapted to UHV working conditions, analyzes the application status and technical characteristics of different types of insulating oil, summarizes the typical engineering application practices, and discusses the technical challenges and optimization strategies in the application process, so as to provide a reference for the technological progress and engineering application of UHV transformer insulating oil.
2 Core Performance Requirements of Insulating Oil for UHV Transformers
The extreme working characteristics of UHV transformers (ultra-high electric field, high thermal load, long service life, complex environment) determine that the insulating oil used must have ultra-high comprehensive performance that is far higher than that of conventional medium and high-voltage transformer insulating oil. The core performance requirements cover four aspects: electrical insulation performance, thermal performance, physical and chemical performance, and engineering application performance, and all indicators must meet the strict standards of UHV power grid construction (such as GB/T 2536-2024, IEC 60296, DL/T 1811 and other UHV special standards).
2.1 Ultra-high Electrical Insulation Performance
UHV transformers have an internal electric field strength of up to 20-30 kV/mm, which is much higher than that of conventional 220 kV and 500 kV transformers. The insulating oil must have ultra-high electrical insulation performance to resist the breakdown of ultra-high electric field and prevent local discharge and corona discharge. The key indicators include:
Breakdown voltage: The AC breakdown voltage (GB/T 507) must be ≥75 kV (2.5 mm electrode gap), and the breakdown voltage after long-term operation must remain above 60 kV to ensure insulation reliability under ultra-high electric field.
Dielectric loss factor (tanδ): Ultra-low dielectric loss is the core indicator to reduce heat generation under high electric field. The tanδ at 90℃ must be ≤0.0005, and the tanδ at 20℃ must be ≤0.0002, which is an order of magnitude lower than that of conventional high-grade mineral insulating oil.
Water content: Moisture is the main factor leading to the decline of insulation performance of insulating oil. The water content of UHV transformer insulating oil must be strictly controlled at ≤5 ppm (mass fraction) during filling, and the water content during long-term operation must not exceed 10 ppm to avoid water-induced electrical breakdown.
Volume resistivity: It must have ultra-high volume resistivity to reduce leakage current under ultra-high electric field. The volume resistivity at 90℃ is ≥1×10¹⁴ Ω·m, and at 20℃ is ≥1×10¹⁶ Ω·m.
In addition, the insulating oil must have good anti-corona performance and partial discharge resistance, and can capture free electrons generated by partial discharge, inhibit the development of electron avalanche, and prevent the expansion of local discharge defects into large-scale insulation breakdown.
2.2 Excellent Thermal Performance
UHV transformers have a rated capacity of up to several million kVA, and the heat generated by the core and winding under full load operation is huge. The insulating oil must have excellent thermal performance to realize efficient heat dissipation, control the temperature rise of the transformer (the top oil temperature rise is generally ≤45 K), and avoid insulation aging caused by high temperature. The key indicators include:
Thermal conductivity: Higher thermal conductivity can improve the heat transfer efficiency of the oil. The thermal conductivity at 25℃ is ≥0.14 W/(m·K), and the thermal conductivity remains stable in the temperature range of -40℃ to 120℃.
Oxidation stability: The insulating oil will undergo oxidative decomposition under long-term high-temperature operation (the top oil temperature can reach 90-100℃), producing acids, sludge and other impurities, which will reduce the insulation performance and block the heat dissipation channel. The oxidation induction period (GB/T 17623) of UHV insulating oil must be ≥1000 h (110℃), and the fully hydrogenated mineral oil can reach more than 2000 h.
Temperature adaptability: UHV transformers are applied in different climatic regions (high cold, high temperature, high altitude), and the insulating oil must have a wide temperature adaptation range, with a pour point of ≤-45℃ and a flash point of ≥140℃, to ensure normal fluidity and heat dissipation performance in low-temperature environments and avoid fire hazards in high-temperature environments.
2.3 Stable Physical and Chemical Performance
Under the long-term operation of UHV transformers, the insulating oil is in a complex environment of high temperature, high electric field, metal ion catalysis and oxygen contact, and must maintain stable physical and chemical properties to avoid performance attenuation and ensure the long-term operation of the transformer. The key requirements include:
Low viscosity and good fluidity: Low viscosity can improve the heat transfer speed and ensure uniform oil circulation inside the large-volume UHV transformer. The kinematic viscosity at 40℃ is ≤9.6 mm²/s, and at 100℃ is ≤2.0 mm²/s.
Good material compatibility: It must have excellent compatibility with all insulating materials and structural materials of UHV transformers, including insulating paper/cardboard (kraft paper, aramid paper), rubber sealing parts (nitrile rubber, fluororubber), metal parts (copper, iron, aluminum), etc., without causing corrosion, swelling or aging of materials, and forming a stable oil-paper-metal composite system.
Anti-aging and anti-sludge performance: It can inhibit the generation of acidic substances and sludge under the catalysis of high temperature and metal ions, the acid value after long-term operation is ≤0.03 mgKOH/g, and the sludge content is ≤0.01% (mass fraction), to avoid blocking the oil tank and radiator.
Low volatility: The evaporation loss (GB/T 7325) is ≤0.5% (mass fraction) at 99℃ for 22 h, to avoid the loss of light components of the insulating oil under high temperature and the change of oil composition.
2.4 Reliable Engineering Application Performance
UHV transformers are large in volume (the oil tank volume can reach thousands of cubic meters), difficult in on-site construction and maintenance, and high in replacement cost of insulating oil. Therefore, the insulating oil must have reliable engineering application performance to adapt to the construction, operation and maintenance characteristics of UHV engineering. The key requirements include:
Long service life: It is consistent with the design service life of UHV transformers (≥40 years), and the performance attenuation rate is low during operation, without the need for large-scale oil change or oil regeneration.
Easy to degas and dehydrate: UHV transformers require vacuum filling of insulating oil, and the oil must have good degassing and dehydrating performance, which can quickly remove dissolved gas and moisture under high vacuum (≤1 Pa), to avoid gas bubbles and moisture remaining in the transformer.
Stable batch performance: The production batch of UHV insulating oil is large, and the performance indicators of different batches must be highly consistent to avoid performance differences caused by batch changes affecting the operation of the transformer.
Easy to regenerate and maintain: When the performance of the insulating oil attenuates slightly during operation, it can be restored to the standard index through on-site regeneration technology (such as vacuum filtration, adsorption purification), without complex off-site treatment.
3 Application Status of Main Transformer Insulating Oils in UHV Transformers
At present, the insulating oils applied or researched in UHV transformers are mainly divided into three categories: UHV-grade mineral insulating oil, environmentally friendly ester insulating oil (natural ester/synthetic ester), and functional modified insulating oil (nano-modified/high fire point). Among them, UHV-grade mineral insulating oil has become the mainstream application product due to its mature technology, excellent comprehensive performance and rich engineering practice; environmentally friendly ester insulating oil is in the pilot application stage; functional modified insulating oil is still in the laboratory research and small-scale testing stage. The application status, technical characteristics and adaptability of the three types of insulating oil in UHV transformers are analyzed as follows.
3.1 UHV-Grade Mineral Insulating Oil: Mainstream Application Product
Mineral insulating oil is refined from crude oil through physical and chemical processes such as distillation, solvent refining, hydrogenation and clay refining. After targeted upgrading of the refining process and additive system, it forms UHV-grade special mineral insulating oil, which is the most widely used insulating oil in global UHV transformer engineering at present, accounting for more than 95% of the UHV insulating oil market share.
3.1.1 Core Technical Upgrades
The conventional high-grade mineral insulating oil cannot meet the ultra-high performance requirements of UHV transformers, and the UHV-grade mineral insulating oil has realized a leap in performance through two core technical upgrades:
Refining process upgrading: Adopt the fully hydrogenated refining + molecular distillation deep refining combined process. The fully hydrogenated process can remove more than 99% of sulfur, nitrogen, oxygen and other heteroatoms in the oil, as well as aromatic hydrocarbons, colloid and other impurities that are easy to cause aging, and the saturation of the oil is close to 100%; the molecular distillation technology further removes the low-boiling point volatile components and high-boiling point heavy components in the oil, improving the flash point, pour point and thermal stability of the oil. The oxidation induction period of the oil produced by this process can reach more than 2000 h, and the dielectric loss tanδ (90℃) is ≤0.0003, which fully meets the UHV performance requirements.
Additive system innovation: Adopt the high-efficiency composite additive system with ultra-low dosage (total adding amount ≤0.5%), including hindered phenol high-temperature antioxidants, benzotriazole metal deactivators, polyacrylate pour point depressants and anti-foaming agents. The composite additive has a synergistic effect, which can inhibit oil oxidation, metal ion catalysis and low-temperature viscosity increase, and does not affect the electrical insulation performance of the oil (such as breakdown voltage, volume resistivity).
3.1.2 Application Advantages and Engineering Practice
UHV-grade mineral insulating oil has become the mainstream of UHV application due to its excellent comprehensive performance, mature application technology, low cost and perfect industrial chain. It has been widely applied in a large number of UHV AC/DC engineering projects in China, Europe and other countries, such as China’s Changji-Guquan ±1100 kV UHV DC Project, Jindongnan-Nanyang-Jingmen 1000 kV UHV AC Project, and Europe’s 1000 kV UHV AC Interconnection Project. The operation practice of more than 10 years shows that the UHV-grade mineral insulating oil can maintain stable electrical, thermal and physical-chemical properties under the long-term operation of UHV transformers, and the oil-paper insulation system has no obvious aging phenomenon, which effectively guarantees the safe operation of UHV transformers.
3.1.3 Limitations
Although UHV-grade mineral insulating oil has excellent performance, it still has inherent limitations of mineral oil: poor fire resistance (flash point ~140℃) and poor environmental compatibility (biodegradability rate <30%). In UHV transformer substations in fire-prone areas (such as high-rise buildings, chemical parks, forest areas), mineral insulating oil has potential fire safety hazards; in case of oil leakage, it will cause soil and water source pollution, which is inconsistent with the green low-carbon development concept of the power industry.
3.2 Environmentally Friendly Ester Insulating Oil: Pilot Application Product
Environmentally friendly ester insulating oil is divided into natural ester insulating oil (refined from renewable vegetable oils such as rapeseed oil and palm oil) and synthetic ester insulating oil (synthesized by esterification of polyol and fatty acid). It has the inherent advantages of high fire resistance (flash point ≥260℃), excellent environmental compatibility (biodegradability rate ≥80%) and good anti-aging performance, which makes it an ideal alternative to mineral insulating oil for UHV transformers. In the past decade, with the optimization of ester oil production process and performance, it has realized pilot engineering application in UHV transformers.
3.2.1 Technical Adaptation for UHV Application
The early ester insulating oil has the problems of high viscosity, poor low-temperature fluidity and high hydrolysis tendency, which cannot meet the engineering application requirements of UHV transformers. Through targeted technological innovation, the performance of ester insulating oil has been significantly improved, and it has realized the technical adaptation to UHV working conditions:
Viscosity optimization: Select high-oleic acid vegetable oil as the raw material (natural ester) and optimize the molecular structure of synthetic ester (synthetic ester), reducing the kinematic viscosity of ester oil at 40℃ to ≤20 mm²/s, which is close to that of mineral oil, and improving the fluidity and heat transfer efficiency of the oil inside the UHV transformer.
Hydrolysis resistance improvement: Add high-efficiency anti-hydrolysis additives and optimize the esterification process to reduce the free acid content of ester oil to ≤0.03 mgKOH/g, and the hydrolysis tendency is significantly reduced, which can maintain stable performance in the humid environment inside the transformer.
Oil-paper synergy optimization: Ester oil has a good compatibility with aramid insulating paper (the main insulating paper of UHV transformers), and can form a oil-paper synergistic anti-aging system—ester oil can slow down the aging of aramid paper, and aramid paper can inhibit the oxidative decomposition of ester oil, which significantly prolongs the service life of the insulation system (up to 60 years).
3.2.2 Pilot Application Status and Engineering Practice
At present, environmentally friendly ester insulating oil has realized pilot application in UHV auxiliary transformers and low-capacity UHV converter transformers in China, Brazil and other countries. For example, in some ±800 kV UHV DC converter stations in China, synthetic ester insulating oil is used in auxiliary transformers, and the operation data of more than 5 years shows that the ester oil maintains stable performance, the oil-paper insulation system operates normally, and the fire resistance and environmental protection advantages are prominent. In addition, natural ester insulating oil has been applied in 35 kV/110 kV distribution transformers in large quantities, and the technical research of its application in large-capacity UHV transformers is in progress.
3.2.3 Key Restrictions for Large-Scale Application
Although ester insulating oil has obvious advantages, its large-scale application in UHV transformers is still restricted by three key factors:
High production cost: The cost of ester insulating oil is 3-5 times that of UHV-grade mineral insulating oil, which significantly increases the manufacturing cost of UHV transformers.
Large volume effect: UHV transformers have a huge oil tank volume, and the high viscosity of ester oil may lead to uneven oil circulation in the transformer, affecting heat dissipation performance (the problem to be solved for large-capacity application).
Lack of long-term operation data: There is a lack of long-term (≥20 years) operation data of ester oil in large-capacity UHV transformers, and the long-term stability of its performance under ultra-high electric field and high thermal load needs to be further verified by engineering practice.
3.3 Functional Modified Insulating Oil: R&D and Testing Stage
Functional modified insulating oil is a new type of insulating oil developed by modifying the base oil (mineral oil/ester oil) through nanomaterial modification, composite blending and functional group introduction, including nano-modified insulating oil, high fire point flame-retardant insulating oil, anti-corona insulating oil, etc. It has personalized excellent performance and is the key research direction for the future upgrading of UHV transformer insulating oil technology, but it is currently in the laboratory research and small-scale testing stage, and has not yet entered engineering application.
The most representative is nano-modified insulating oil, which is prepared by adding nanomaterials with special properties (TiO₂, Al₂O₃, graphene oxide, etc.) into the base oil through a special dispersion process. Laboratory tests show that nano-modified insulating oil has ultra-high electrical insulation performance and thermal performance—the AC breakdown voltage is increased by 20%-50% compared with the base oil, the thermal conductivity is improved by more than 30%, and the partial discharge resistance is significantly enhanced, which is very suitable for the extreme working conditions of UHV transformers. However, the nano-modified insulating oil still faces technical bottlenecks such as long-term dispersion stability of nanomaterials (easy agglomeration in the oil), unclear interaction mechanism with oil-paper-metal materials, and high production cost, which restrict its engineering application. At present, domestic and foreign research institutions are carrying out in-depth research on these key technologies, and it is expected to realize small-scale pilot application in UHV transformers in the next 5-10 years.
4 Technical Adaptation and Engineering Practice of Insulating Oil in UHV Transformers
The application of insulating oil in UHV transformers is not only a simple "product selection", but also a systematic technical adaptation project involving oil-paper insulation synergy, on-site filling process, operation and maintenance technology and other aspects. Combined with the engineering practice of UHV power grids in China and other countries, the key technical adaptation schemes and typical engineering application practices of insulating oil in UHV transformers are summarized as follows.
4.1 Oil-Paper Insulation Synergy: Core of UHV Insulation System
The insulation system of UHV transformers is a oil-paper composite insulation system composed of insulating oil and insulating paper/cardboard, and the synergy between oil and paper is the core to ensure the long-term insulation reliability of the transformer. For UHV-grade mineral insulating oil and environmentally friendly ester insulating oil, the targeted oil-paper matching and synergy optimization schemes are adopted respectively:
UHV-grade mineral oil + aramid insulating paper: Aramid insulating paper has the advantages of high temperature resistance, high mechanical strength and good anti-aging performance, which is the main insulating paper for UHV transformers. Mineral oil with ultra-low dielectric loss and high oxidation stability is matched with aramid paper, which can reduce the interface polarization between oil and paper, improve the overall insulation performance of the system, and the oil and paper form a mutual protective system to inhibit each other's aging.
Ester oil + modified aramid insulating paper: For ester insulating oil, modified aramid paper with surface hydrophilic modification is adopted to improve the wettability of ester oil on the paper surface, reduce the oil-paper interface gap, and improve the overall breakdown voltage of the system. At the same time, the ester oil can penetrate into the aramid paper to form a stable oil film, which slows down the aging of the paper caused by high temperature and oxygen.
In engineering practice, the oil-paper insulation system of UHV transformers must undergo strict type tests (including thermal aging test, partial discharge test, breakdown voltage test, etc.) to verify the synergy and long-term stability of the system, and only the qualified systems can be applied in engineering.
4.2 On-Site Filling and Dehydration Degassing Technology: Guarantee of Initial Performance
UHV transformers have a huge oil volume (thousands of cubic meters), and the on-site filling process of insulating oil directly determines the initial performance of the oil and the safe operation of the transformer. The core technical requirements of on-site filling are ultra-high vacuum, deep dehydration and degassing, and clean filling:
Ultra-high vacuum filling: The transformer tank and oil pipeline are evacuated to an ultra-high vacuum of ≤1 Pa and maintained for more than 72 hours to remove the gas adsorbed on the inner surface of the tank and the insulating material. Then the insulating oil is filled into the transformer under vacuum to avoid air bubbles remaining in the transformer.
Deep dehydration and degassing of oil: Before filling, the insulating oil is treated by multi-stage vacuum filtration and adsorption purification to reduce the water content to ≤5 ppm and the dissolved gas content (total hydrocarbon) to ≤5 μL/L, ensuring that the oil meets the UHV filling standard.
Clean filling: The filling pipeline adopts a closed stainless steel pipeline, and the oil is filtered through a high-precision filter (precision ≤1 μm) during filling to remove solid impurities in the oil, and the particle count of the oil meets the requirement of NAS 1638 Class 3 or above.
4.3 Typical Engineering Application Cases
China is the country with the largest scale of UHV power grid construction and the richest engineering practice in the world. Taking two typical UHV projects in China as examples, the application of insulating oil is summarized:
Case 1: Changji-Guquan ±1100 kV UHV DC Project: The world's highest voltage level UHV DC project, the main converter transformers adopt UHV-grade fully hydrogenated mineral insulating oil (produced by domestic enterprises), with an oxidation induction period of 2200 h, tanδ (90℃) of 0.00025, and water content of ≤3 ppm. The oil-paper insulation system is mineral oil + aramid insulating paper, and the on-site filling adopts ultra-high vacuum filling technology. Since the project was put into operation in 2019, the insulating oil has maintained stable performance, and the transformers have operated safely without any insulation faults.
Case 2: Zhangbei-Shengli 1000 kV UHV AC Project (new energy supporting project): The auxiliary transformers of the project adopt synthetic ester insulating oil (pilot application), with a flash point of 280℃, biodegradability rate of 95%, and kinematic viscosity (40℃) of 18 mm²/s. The oil-paper insulation system is synthetic ester oil + modified aramid paper. Since the project was put into operation in 2021, the ester oil has operated stably, and the fire resistance and environmental protection advantages have been fully exerted, providing valuable engineering data for the large-scale application of ester oil in UHV transformers.
5 Key Technical Challenges in the Application of UHV Transformer Insulating Oil
Although the application technology of UHV transformer insulating oil has achieved remarkable results, with the continuous upgrading of UHV power grid voltage level (such as ±1200 kV DC, 1100 kV AC) and the increase of transformer capacity, the insulating oil is facing more severe technical challenges in engineering application. Combined with the current application status and engineering practice, the key technical challenges are summarized into four aspects:
5.1 Long-Term Operation Stability Under Extreme UHV Conditions
UHV transformers operate under the extreme conditions of ultra-high electric field, high thermal load and long-term continuous operation for more than 40 years, and the insulating oil is facing severe tests of performance attenuation and aging:
Ultra-high electric field-induced partial discharge: Under the ultra-high electric field of 20-30 kV/mm, local micro-discharge is easy to occur in the oil, which will cause the oxidative decomposition of the oil and the generation of acidic substances, and the discharge by-products will further reduce the insulation performance of the oil, forming a "discharge-aging" vicious circle.
High-temperature metal ion catalysis aging: The high temperature inside the transformer (90-100℃) and the metal ions (Cu²+, Fe³+) released by the metal parts will catalyze the oxidative decomposition of the insulating oil, producing a large amount of sludge and acidic substances, which block the heat dissipation channel and corrode the metal parts.
Performance attenuation of oil-paper insulation system: The long-term interaction between oil and paper will lead to the mutual aging of the two materials. For example, the small molecular substances produced by oil aging will accelerate the degradation of paper fiber, and the aging products of paper will pollute the oil, leading to the overall performance attenuation of the oil-paper system.
At present, the long-term stability of insulating oil under extreme UHV conditions is still lack of sufficient experimental data and theoretical support, and it is the core technical challenge to be solved.
5.2 Compatibility of New Insulating Oils with UHV Transformer Materials
With the research and pilot application of new insulating oils (ester oil, nano-modified oil) in UHV transformers, the long-term compatibility between new insulating oils and UHV transformer materials has become a key technical problem:
Ester oil and sealing material compatibility: Ester oil has a certain swelling effect on some rubber sealing parts (such as ordinary nitrile rubber), which may lead to the aging and leakage of sealing parts after long-term contact, affecting the air tightness of the transformer.
Nano-modified oil and oil-paper-metal interaction: The interaction mechanism between nanomaterials in nano-modified oil and insulating oil, insulating paper and metal parts is still unclear. Nanomaterials may be adsorbed on the surface of paper or metal parts, leading to the decline of material performance, and the agglomeration of nanomaterials will form insulation defects in the oil.
New oil and auxiliary material compatibility: New insulating oils may have incompatibility with transformer auxiliary materials (such as paint, adhesive), leading to the falling off of paint and the failure of adhesive, which affects the safe operation of the transformer.
5.3 High Cost and Imperfect Industrial Chain of High-Grade UHV Insulating Oil
The high-grade UHV insulating oil (ester oil, nano-modified oil) has the problems of high production cost and imperfect industrial chain, which restrict its large-scale application:
High production cost: The raw material cost and production process cost of ester oil and nano-modified oil are much higher than that of mineral oil, and the market scale is small, forming a "high cost-low scale" vicious circle.
Imperfect domestic industrial chain: Although China has realized the localization of UHV-grade mineral insulating oil, the key raw materials (such as high-purity polyol for synthetic ester oil, high-performance nanomaterials for nano-modified oil) and production equipment of new insulating oils are still dependent on imports, and the industrial chain is imperfect, which restricts the technological innovation and cost reduction of new insulating oils.
Lack of unified product standards: At present, there is no unified national/international product standard for UHV-grade ester oil and nano-modified oil, and the performance indicators and test methods of different enterprises are not uniform, which affects the engineering selection and application of new insulating oils.
5.4 Low Precision of Intelligent Monitoring and Fault Early Warning Technology
The operation and maintenance of UHV transformer insulating oil is based on real-time monitoring and fault early warning, but the current monitoring technology still has the problems of low sensor precision, poor environmental adaptability and imperfect early warning model:
Low precision of on-line monitoring sensors: The on-line monitoring sensors (water content, dielectric loss, dissolved gas) of UHV insulating oil have low precision in harsh environments (high temperature, high humidity, strong electromagnetic interference), and the monitoring error is large, which cannot reflect the real performance of the oil.
Single monitoring parameter: Most of the current on-line monitoring systems only monitor a small number of parameters such as dissolved gas and water content, and lack the comprehensive monitoring of key parameters such as oil viscosity, acid value and sludge content, which cannot realize the comprehensive evaluation of oil performance.
Imperfect AI fault early warning model: The current fault early warning model is mostly based on threshold judgment, and the AI model based on big data and machine learning is still in the research stage. It is difficult to realize the accurate prediction of oil aging and insulation faults, and the early warning accuracy is low.
6 Optimization Strategies and Application Improvement Measures
Aiming at the key technical challenges faced by the application of UHV transformer insulating oil, combined with the development trend of power grid technology and material science, the targeted optimization strategies and application improvement measures are put forward from four aspects: insulating oil performance optimization, oil-paper insulation system improvement, intelligent operation and maintenance upgrading, and industrial chain improvement.
6.1 Performance Optimization of UHV Insulating Oil: Targeted R&D and Innovation
Aiming at the performance attenuation and aging problems of insulating oil under extreme UHV conditions, carry out targeted R&D and innovation of insulating oil performance, and improve the long-term stability and adaptability of the oil:
Develop high-performance anti-aging additives: R&D of ultra-high temperature resistant composite antioxidants (such as hindered phenol + organic phosphorus composite antioxidants) with low dosage and high efficiency, which can inhibit the oxidative decomposition of oil under the catalysis of high temperature and metal ions, and extend the oxidation induction period of the oil to more than 3000 h.
Optimize the molecular structure of base oil: For mineral oil, further improve the degree of hydrogenation and deep refining process to remove trace impurities that cause aging; for ester oil, optimize the molecular structure of synthetic ester to reduce the viscosity of the oil and improve the heat transfer efficiency; for nano-modified oil, develop a nano-material surface modification technology with long-term dispersion stability to solve the agglomeration problem of nanomaterials.
Develop anti-corona and anti-partial discharge insulating oil: Introduce functional groups with electron capture ability into the base oil molecular structure, or add nano-materials with anti-corona performance, to improve the anti-partial discharge and anti-corona performance of the oil, and inhibit the oil aging caused by partial discharge.
6.2 Improvement of Oil-Paper Insulation System: Synergy and Integration
The oil-paper insulation system is the core of UHV transformer insulation, and the improvement of the system's synergy and integration is the key to ensure the long-term operation stability:
Develop customized oil-paper matching materials: According to the characteristics of different insulating oils (mineral oil, ester oil), develop customized insulating paper/cardboard materials, such as aramid paper modified by oil-compatible functional groups, to improve the wettability and interface bonding force between oil and paper, reduce interface polarization, and improve the overall insulation performance of the system.
Research oil-paper synergy anti-aging technology: Carry out in-depth research on the interaction mechanism between oil and paper under UHV conditions, develop oil-paper synergy anti-aging additives, and form a stable oil-paper protective system to inhibit the mutual aging of oil and paper and extend the service life of the system to more than 60 years.
Optimize the structural design of oil-paper insulation: Combine the characteristics of insulating oil with the structural design of UHV transformers, optimize the oil gap and paper insulation thickness distribution inside the transformer, reduce the local electric field strength, and avoid partial discharge caused by electric field concentration.
6.3 Upgrading of Intelligent Operation and Maintenance System: Digital and Intelligent
Aiming at the problems of low precision of monitoring and imperfect early warning model, build a digital and intelligent operation and maintenance system of UHV transformer insulating oil based on IoT, big data and AI technology, and realize real-time monitoring, accurate evaluation and predictive early warning of oil performance:
Develop high-precision on-line monitoring sensors: R&D of high-precision, anti-interference and long-life on-line monitoring sensors adapted to UHV harsh environments, including multi-parameter integrated sensors (water content, dielectric loss, acid value, viscosity, dissolved gas), to realize comprehensive real-time monitoring of insulating oil performance parameters.
Build a big data platform for insulating oil monitoring: Integrate the monitoring data of insulating oil, the operation data of transformers and the environmental data of substations, build a unified big data platform, and realize the centralized management and analysis of data of UHV transformers across the power grid.
Develop AI-based fault prediction and early warning model: Based on machine learning and deep learning algorithms, develop an AI fault prediction and early warning model for insulating oil aging and insulation faults, take the multi-parameter monitoring data as the input, and realize the accurate prediction of oil performance attenuation and potential insulation faults, with the early warning accuracy reaching more than 95%.
Realize full-life cycle management of insulating oil: Based on the big data platform and AI model, build a full-life cycle management system of UHV insulating oil, covering oil production, filling, operation, maintenance, regeneration and replacement, and realize the refined and intelligent management of insulating oil throughout the life cycle.
6.4 Improvement of Industrial Chain and Standard System: Localization and Standardization
Aiming at the problems of high cost and imperfect industrial chain of new insulating oils, accelerate the localization, industrialization and standardization of high-grade UHV insulating oil, and reduce the application cost:
Accelerate the localization of key raw materials and equipment: Increase R&D investment, realize the localization of key raw materials (high-purity polyol, high-performance nanomaterials) and production equipment (molecular distillation equipment, nano-dispersion equipment) of new insulating oils, break the monopoly of foreign enterprises, and reduce the production cost.
Expand the market scale of new insulating oils: Through policy support and engineering demonstration, expand the pilot application scale of new insulating oils (ester oil, nano-modified oil) in UHV transformers, form an economy of scale, and further reduce the production and application cost.
Formulate unified national/international product standards: Accelerate the formulation of unified product standards, test methods and engineering application specifications for UHV-grade ester oil and nano-modified oil, unify the technical indicators of enterprises, and provide a standard basis for the engineering selection and application of new insulating oils.
Build a collaborative innovation system: Establish a collaborative innovation system of "enterprise-university-research institute", integrate the advantages of enterprises, research institutions and universities, carry out in-depth research on key core technologies of UHV insulating oil, and accelerate the transformation of scientific research achievements into engineering applications.
7 Future Development Trends of UHV Transformer Insulating Oil
Driven by the global energy transformation, the construction of green low-carbon power grids and the progress of material science and digital technology, the application of UHV transformer insulating oil will show four core development trends in the future: green low-carbonization, high performance functionalization, digital intelligent application and integration of oil and transformer design.
7.1 Green Low-Carbonization: Main Direction of Development
Under the background of the "double carbon" goal and the green development of the power industry, green low-carbon insulating oil will become the main development direction of UHV transformer insulating oil. Environmentally friendly ester insulating oil (natural ester/synthetic ester) will gradually replace mineral insulating oil and become the mainstream product of UHV insulating oil with the advantages of high fire resistance, good environmental compatibility and long service life. At the same time, the research and development of bio-based insulating oil (refined from waste kitchen oil, microalgae oil and other renewable resources) will be accelerated, which further improves the environmental friendliness and resource utilization of insulating oil, and realizes the green low-carbon of UHV insulating oil from raw materials to application.
7.2 High Performance Functionalization: Core of Technological Upgrading
With the continuous upgrading of UHV power grid voltage level (±1200 kV DC, 1100 kV AC) and the increase of transformer capacity, the performance requirements for UHV insulating oil will be further upgraded, and high performance and functionalization will become the core of technological upgrading. Nano-modified insulating oil with ultra-high insulation performance and thermal performance will realize large-scale engineering application in UHV transformers with the breakthrough of key technologies (nanomaterial dispersion stability, material compatibility); high fire point flame-retardant insulating oil with a flash point of more than 350℃ will be widely applied in UHV transformer substations in fire-prone areas; anti-corona, anti-partial discharge and long-life functional insulating oils will be developed and applied according to the personalized working conditions of UHV transformers, forming a diversified product system of UHV insulating oil.
7.3 Digital Intelligent Application: Key to Operation and Maintenance Upgrading
Digital and intelligent technology will be deeply integrated with the application of UHV transformer insulating oil, and digital intelligent application will become the key to the upgrading of insulating oil operation and maintenance. The high-precision multi-parameter on-line monitoring system will be popularized and applied in all UHV transformers, realizing the real-time and comprehensive monitoring of insulating oil performance; the AI fault prediction and early warning model based on big data and digital twin will realize the accurate prediction of oil aging and insulation faults; the digital twin system of UHV transformer insulating oil will be built, which can simulate the performance change of insulating oil under different working conditions, and provide a scientific basis for the optimal operation and maintenance of the transformer. The digital and intelligent application will realize the transformation of UHV insulating oil operation and maintenance from "post-maintenance" to "predictive maintenance" and "condition-based maintenance".
7.4 Integration of Oil and Transformer Design: New Trend of Engineering Application
In the future, the application of UHV transformer insulating oil will break the traditional mode of "first transformer design, then oil selection", and realize the integration of insulating oil and UHV transformer design. In the initial stage of UHV transformer design, the characteristics of insulating oil (viscosity, thermal conductivity, insulation performance) are fully considered, and the structural design of the transformer (oil tank structure, oil circulation mode, insulation structure) is optimized according to the characteristics of the oil, so as to realize the optimal matching between oil and transformer. At the same time, customized insulating oil will be developed according to the personalized working conditions of different UHV transformers (such as high cold, high temperature, high altitude), and the customized design of insulating oil and transformer will be realized, which further improves the operational reliability and adaptability of UHV transformers.
8 Conclusion
UHV transformers are the core equipment of UHV power grids, and transformer insulating oil is the key medium to ensure the safe and stable operation of UHV transformers. Under the extreme working conditions of UHV transformers (ultra-high electric field, high thermal load, long service life), the insulating oil must have ultra-high comprehensive performance in electrical insulation, thermal performance, physical and chemical performance and engineering application performance. In the past decade, with the rapid construction of UHV power grids worldwide, the application technology of UHV transformer insulating oil has achieved remarkable results: UHV-grade fully hydrogenated mineral insulating oil has become the mainstream application product through process upgrading and additive innovation, and has rich engineering practice; environmentally friendly ester insulating oil has realized technical adaptation and pilot engineering application in UHV transformers with its green low-carbon and fire resistance advantages; functional modified insulating oils such as nano-modified oil have made important progress in laboratory research and are the key research directions for future upgrading.
At the same time, the application of UHV transformer insulating oil still faces key technical challenges such as long-term operation stability under extreme conditions, material compatibility of new insulating oils, high cost and imperfect industrial chain, and low precision of intelligent monitoring. Aiming at these challenges, targeted optimization strategies must be adopted: accelerate the performance optimization and innovation of UHV insulating oil, improve the synergy of the oil-paper insulation system, build a digital and intelligent operation and maintenance system, and perfect the industrial chain and standard system of high-grade UHV insulating oil.
In the future, driven by the global energy transformation and the progress of material science and digital technology, UHV transformer insulating oil will develop in the direction of green low-carbonization, high performance functionalization, digital intelligent application and integration of oil and transformer design. Environmentally friendly ester insulating oil will gradually replace mineral oil to become the mainstream; nano-modified insulating oil and other functional oils will realize large-scale engineering application; digital and intelligent technology will be deeply integrated with the application of insulating oil; the integration of oil and transformer design will become a new trend of engineering application. The technological progress and engineering application of UHV transformer insulating oil will provide a strong technical support for the safe, stable and green operation of the global UHV power grid, and promote the construction and development of the global energy interconnection system.
