A dielectric liquid is a material used in a transformer to enhance cooling efficiency of active parts and provide electrical insulation. Together, the dielectric liquid and the solid insulation (i.e. paper and pressboards), provide enhanced dielectric withstand strength of the transformer structure. The dielectric liquid also acts as an information carrier on issues related to internal faults within the tank and active parts. Currently, hydrocarbon based mineral oils represent the highest volume of dielectric liquids used in transformer applications. These oils are derived and refined from crude oil and achieve the desired properties that meet most of the relevant standards. However, despite mineral oil being a good dielectric liquid, it is neither a K class liquid (fire point greater than 300°C), nor is it readily biodegradable (environmentally friendly). As such, over the past several decades, natural and synthetic esters have emerged to meet the K class requirements, are environmentally friendly and improve the life of the insulation system inside the transformer. This article provides information on differences between the various liquids, similarities between natural and synthetic esters and guidance on selection of the most appropriate liquid based on the application.
Natural ester dielectric liquids are produced from vegetable oils and their properties are affected by the base vegetable oil from which they are made.
Differences Between the Dielectric Liquids
To define the differences between mineral oil and ester based dielectric liquids, it is worth looking at their chemical structures.
Mineral Oil
Figure 1 shows the basic chemical structure of mineral oil. The hydrocarbon compounds in mineral oil vary and can be categorized into paraffinic, iso-paraffinic, naphthenic, aromatic, and poly-aromatic. These compounds exist in mineral oil with different ratios and thus contribute to slight differences in properties such as flash and fire points, pour point, viscosity, and oxidation stability to name a few.
Ester Liquids
The basic building block of an ester is shown in Figure 2. It is comprised of carbon (C), oxygen (O), and hydrocarbon chains presented by R and R’. The single and double lines represent a single bond and a double bond respectively. Note that C=O double bonds behave differently from the C=C double bonds found in the R chains of natural esters.
Natural esters should only be used in sealed systems where there would be no direct exposure to air for elongated periods of time.
Natural Ester
Natural ester dielectric liquids are produced from vegetable oils, which are manufactured from plant crops (soybean, canola/rapeseed, sunflower, etc.). The structure of natural esters is based on a glycerol backbone, to which are bonded 3 naturally occurring fatty acids as shown in Figure 3. Plants produce these esters as part of their natural growth cycle and contain different amounts of double bonds in the fatty acid chain (i.e. R’).
Single and double bonds, saturation and oxidation stability
For the natural ester where all bonds in the ‘R chain are single, the liquid is saturated and highly resistant to oxidation. Where double bonds are present, these can be broken as the liquid reacts with oxygen. Liquids which contain these double bonds are therefore referred to as unsaturated and need some degree of chemical stabilization to remain high performing in the presence of oxygen. Just as mineral oil dielectrics require inhibitors, natural ester dielectrics require antioxidant stabilizers for this reason and should only be used in sealed systems where there is no direct exposure to air for elongated periods of time.
Natural esters fall on a spectrum with respect to saturation. The spectrum is formally classified into mono, di- and tri- unsaturated (poly unsaturated) components (Table 1) which also carry the names oleic, linoleic and linolenic respectively [2]. While canola oil has a higher proportion of oleic content, this mono-unsaturated content is less susceptible to oxidation than the di-unsaturated linoleics and tri-unsaturated linolenics. Canola-derived esters therefore contain less of the more oxidation-prone fatty acids.
Figure 4 shows several vegetable based oils with different ratios of poly-unsaturated, mono-unsaturated and saturated fatty acids. The higher the saturated fatty acid content, the higher the pour point of the liquid, which can limit the operational temperature of the transformer. By using specially formulated pour point suppressants, the pour point of the natural ester liquids can be improved.
Synthetic Esters
A synthetic ester dielectric liquid is designed using carefully selected acids and alcohols. A chemical reaction is created through esterification with heat to produce an ester and then any excess water is removed. The process is illustrated in Figure 5. Synthetic ester dielectric liquids are derived from chemicals and are usually the product of polyol alcohols (a molecule with more than one alcohol functional group) with synthetic or natural carboxylic acids as shown in Figure 6. Chemically, synthetic esters consist of four ester groups where there is no double bond in the R chains between carbons (saturated bonds), which increases its oxidation and thermal stability.
Synthetic ester dielectric liquids are derived from carefully selected acids and alcohols through the chemical reaction of esterification.
Difference Between Natural and Synthetic Esters
The two fundamental differences in the physical properties of natural esters compared to synthetic ester liquids are oxidation stability and pour point.
The two fundamental differences in the physical properties of natural ester and syntheric ester liquids are oxidation stability and pour point.
Oxidation Stability
Figure 7 shows test results for the different dielectric liquids following … the ASTM D2112 pressurized vessel oxidation test method. The test is conducted in the presence of copper catalyst, pressurized at 90 psi with pure oxygen and heated to 140°C. The time is measured till the pressure drops to 65 psi due to pure oxygen consumption from the oxidation process. The longer the time, the better the oxidation stability.
Oxidation stability is an important characteristic in the manufacturing, operation and maintenance of a transformer. If an insulating liquid oxidizes, the physical and chemical properties of the liquid change. In natural ester liquids, the viscosity of the liquid increases, the acid value increases, and the liquid starts to polymerize over time. The oxidation process in a transformer is a slow process that can be accelerated by an increase in temperature. In order to improve the oxidation stability of liquids, oxidation inhibitors are added which can prolong these changes to the chemical and physical properties of the liquid.
Pour Point
Another key difference between synthetic and natural esters is their pour point temperature. Pour point is defined as the temperature below which the liquid loses its ability to flow and pour down from a beaker. Table 2 shows the pour points of synthetic ester, canola based natural ester, and soybean based natural ester.
A lower pour point indicates better cold temperature performance of the transformer.
Comparison of ester Liquids to mineral Oil in Transformers
The chemical structures of ester liquids are more polar than mineral oil. This polarity has an effect on the moisture saturation limit, power factor and insulation resistance in a transformer. The gases generated (hydrogen, methane, ethane, ethylene, acetylene, carbon monoxide and carbon dioxide) in ester liquids are the same as mineral oil, however the gassing behavior is different [3]. IEEE C57.155 has guidance on the gassing behavior of ester liquids. There are differences in the dielectric and thermal behavior of ester liquids compared to mineral oil. Leading transformer manufacturers are aware of these differences and address them when designing transformers. End users should be cautious in using the same limits set for mineral oil filled transformers as for an ester filled transformer in tests such as power factor, insulation resistance, and DGA.
Benefits of Using Esters in Transformers
As outlined earlier, the main reasons for adopting an ester as a transformer dielectric liquid include increased fire safety and better environmental protection. Additionally, esters have high moisture tolerance that can improve the life of the insulation system. These features are discussed as follows.
Fire Protection
Dielectric liquids are classified according to their Fire Point and Net Calorific Value, following the IEC 61039:2008 Standard “Classification of insulating liquids” [4]. Table 3 summarizes the classification.
K class liquids are recognized in the industry as “Less Flammable Liquids”, which allows reduced separation distances between a transformer and its surroundings when it is installed indoor or outdoor. This is specifically mentioned in the IEC 61936 – 1, “Power installations exceeding 1 kV a.c. – Part 1: Common rules” [5] and FM Global Property Loss Prevention Data Sheets 5-4 [6].
As an example, the distances between a less flammable liquid insulated transformer with enhanced protection and an adjacent transformer or building can be reduced from 15.2 m in case of mineral oil down to 1.5 m in case of less flammable liquid in a transformer with enhanced protection, see table 3 in the IEC 61936-1 for more details [4].
Figure 8 shows a visual demonstration of the benefits of using a less flammable liquid in a transformer. Savings can be achieved on space, concrete, and construction times following the FM Global guidelines [6].
The main reasons for adopting an ester as a transformer dielectric liquid include increased fire safety and better environmental protection. Additionally, esters have high moisture tolerance that can improve the life of the insulation system.
According to test method OECD 301B (test conducted over a 28-day period), in order for a product to be considered “readily biodegradable”, a sample must achieve at least 60% degradation by day 28. Once a sample exceeds 10% biodegradation it must reach 60% within the next 10 days. For test materials that are not soluble in water, such as transformer liquids, measurements are taken of the oxygen consumed, or carbon dioxide produced, to determine the biodegradation percentage.
Figure 9 demonstrates that synthetic ester achieved 10% degradation by day 3, and by day 10 it achieved 61% degradation. On the 28th day, it reached 89% degradation, putting it comfortably in the “readily biodegradable” category. Similarly, soybean and canola based natural esters are superior in their biodegradation behavior; by day 10 they biodegrade to 85%.
Note that ester liquids will not biodegrade in a transformer. This is due to the fact that the conditions within the transformer are too hot and dry to sustain microbial life.
Ester liquids are recognized as environmentally friendly because they are “readily biodegradable”. However, they will not biodegrade in a transformer since the conditirions within the transformer are too hot and dry to sustain mictobial life.
Moisture Tolerance
Due to the chemical structure, esters are able to tolerate more moisture while retaining higher breakdown voltage in comparison to mineral oil. This is well indicated with the plots shown in Figure 10. This provides advantages in locations of high humidity and remote transformers that are costly to check and monitor for leaks on the distribution network and thus save money in the field and generate better Total Cost of Ownership.
Esters have higher moisture saturation levels due to the free oxygen in the ester linkage which can attract and retain water molecules. In the ageing mechanism of the transformer, moisture is produced during the degradation of the cellulose material. The moisture generated attacks the cellulose chains further, increasing the rate of degradation. Because ester liquids hold more moisture than mineral oil, less becomes available to attack the cellulose chains and thus reduce the rate of degradation. The slower aging rate of cellulose in ester liquids is well documented in aging tests [7].
Application of Ester Liquids in Transformers
Natural ester and synthetic ester filled transformers have been used in distribution, power, and special transformers since the 1970s. As of today, the largest natural ester filled transformer is rated at 420 kV and the largest synthetic ester filled transformer is rated at 433 kV [8]. As these transformers are used more and more, it is important to understand what the strengths and limitations are of using the ester liquids based on application.
- Breathing and non-breathing transformers
Natural esters are not recommended for use in breathing transformers. Natural ester liquids are prone to oxidation and can solidify over time if used in breathing transformers causing cooling and dielectric issues. Synthetic esters on the other hand are perfectly suited for use both in breathing and non breathing transformers. There have been instances where breathing transformers have been retrofitted for use with natural ester liquids. However, the long-term maintenance requirements need to be taken into consideration when retrofitting a breathing transformer. - Distribution transformer vs. power transformer
Natural esters are perfectly suited for sealed distribution transformers. They are a cost-effective solution that makes the transformers safer, provide an increased overload capability and are environmentally safer. Synthetic esters also work well for distribution transformers but are even more suited for power transformers than natural ester liquids. This is because they have the same benefits as natural ester in improving fire safety, environmental performance and insulation system life, but they can also be managed in a similar way to mineral oil. Because power transformers are more expensive, more attention is given to monitoring of the transformers to make sure that they operate under all circumstances. From a risk mitigation point of view, a synthetic ester can provide a better overall solution. - Operating conditions
Transformers filled with ester liquids can work at a higher average winding rise (greater than 65°C rise). This is possible because ester liquids can operate at 90°C top liquid temperature rise compared to 65°C for mineral oil as per the IEEE C57.154 standard as the ageing rate of cellulose material in ester liquids is slower compared to mineral oil at similar operating temperatures. One thing to be mindful about at the higher operating temperatures is that in the event of a leak, the oxidation rate of insulating liquids increases as well. This would have to be taken into consideration with natural ester liquids as they have higher oxidation rate compared to mineral oil or synthetic ester liquids. In the case studies below, the 400 kV Highbury substation transformer is an excellent example of an innovative project that was designed to operate at high temperatures and the heat generated was captured and repurposed for another application. - For cold temperature applications, transformer location, transformer loading profile, and what mechanical operations the transformer is subject to, need to be considered. Due to the difference from seed to seed for natural ester liquids, the proper selection needs to be made based on the conditions listed above. Soybean natural ester has a pour point of -18°C while canola natural ester has a pour point of -31°C. Synthetic esters, due to the pour point being below -50°C, are well suited in all cold temperature applications. If a transformer is operating at full load, the pour point of the transformer is not important, however if the transformer is lightly loaded, the cooling characteristics of the transformer would have to be analyzed to determine the effect on the hotspots of the transformer. The selection of a tap changer needs to be analyzed as a heater would have to be installed if the operating temperatures are below -18°C for synthetic ester and -10°C for natural ester liquids [9].
- Retrofilling of transformers
The use of ester liquids in retrofill applications is growing due to insurance, fire safety concerns and construction costs to existing transformer sites. Both natural and synthetic ester liquids have proven to be effective in retrofilling of existing transformers to 69 kV. Above this size, retrofilling decisions become design dependant due to the dielectric property differences between esters and mineral oils. Selection of using natural ester versus synthetic ester is dependent on the end users’ needs and capability of handling maintenance requirements of the transformer with respect to oxidation.
K class liquids are recognized in the industry as “Less Flammable Liquids”, which allows reduced separation distances between a transformer and its surrounding when it is installed indoor or outdoor (further reducing costs).
Case Studies and Future Projects
The use of ester based dielectric liquids in transformer applications extend to over 40 years; starting when the synthetic ester MIDEL 7131 was first invented and used to retrofill existing furnace transformers to replace PCB oil at British Steel, in the U.K.
Some recent cases include:
First canola natural ester filled transformer in North America rated at 138 kV
A 28 MVA transformer filled with canola natural ester was commissioned for use in a mine in Canada (Figure 11). The end user selected canola over soybean natural ester because the transformer was to be operational in ambient temperature below -25°C.
Highbury Substation, London, U.K.
Three 400 kV power transformers, filled with synthetic ester, were used to reduce the substation footprint in a highly confined location in London, U.K. The original substation had three old transformers filled with mineral oil and once the substation was due for refurbishment, the utility was restricted in using a limited space for the substation. Synthetic ester allowed reduced clearances between the transformers and adjacent objects/building and this saved considerable amounts on estate, concrete, and construction times without compromising on fire safety or on environmental protection. On top of achieving the primary objectives from synthetic ester use, the utility desired to upgrade the project to extract the heat generated from the transformers from electric losses and heat a nearby school and residential building, in effect significantly reducing the carbon footprint of the substation. Figure 12 shows a picture of one of the transformers used in that substation enclosed with thermal jacket to control heat dissipation from the transformer to water heat exchanger of the substation.
Some of the upcoming projects with ester filled transformers include:
- 400 kV natural and synthetic ester filled transformers in India
- 420 kV canola natural ester filled transformer in Germany
- Largest wind turbines rated at 12 MW will include transformers filled with synthetic ester
- Several 230 – 345 kV syntheticester filled power transformers for utilities in North America
- 230 kV canola natural ester filled transformers in China
- Several mines worldwide are retrofilling mineral oil filled transformers and purchasing new transformers filled with natural and synthetic ester liquids. The esters are also being specified instead of silicone liquid for new transformers because ester filled transformers are cheaper to manufacture and operate, easy to source and provide the benefits of less flammability as silicone liquid.
- Larger use of natural and synthetic esters in indoor transformers, renewables, and industrial application
- Research in use of synthetic ester liquid in transformer rated at 500 kVA and up
- Research in use of synthetic esters in HVDC transformers
Conclusion
From their initial use and through the decades, there have been no reported fires due to the use of ester liquids in transformer fleets. Ester liquids allow for innovations to take place in transformers to make them – fit in smaller footprint, have better return on investment, environmentally friendly and most of all SAFER. Regarding the types of natural or synthetic esters to use in transformers, an end user can make a selection and decision based on the requirements of the project as the various liquids offer different options in each application.
References
[1] M. Eklund, “Mineral insulating oils; functional requirements, specifications and production,” Conference Record of the 2006 IEEE International Symposium on Electrical Insulation, Toronto, Ont., 2006, pp. 68-72
[2] Hanson, D., et al., “Understanding dissolved gas analysis of ester liquids: An updated review of gas generated in ester liquid by stray gassing, thermal decomposition and electrical discharge”, IEEE Electrical Insulation Conference (EIC), 2016
[3] M. Lashbrook and R. Martin, “Fluid maintenance and dissolved gas analysis for ester-based transformer liquids”, M&I Materials Ltd, Manchester, UK. [Online]
[4] IEC 61039, Classification of insulating liquids, 2008
[5] IEC 61936-1:2010, Power installation exceeding 1 kV a.c. – Part 1: Common rules
[6] FM Global, Property Loss Prevention Datasheets 5-4, 2019
[7] H. P. Gasser, C. Krause, M. Lashbrook and R. Martin, “Aging of pressboard in different insulating liquids,” IEEE International Conference on Dielectric Liquids, Trondheim, 2011, pp. 1-5
[8] M. Lashbrook, A. Gyore, M. Ramakrishna and R. Martin, “The Development of 400 kV Transformers with Ester-based Dielectric Liquids,” ARWtr2016, La Toja Island, Spain, 2016
[9] T. Smolka, T. Macarthur, R. Frotscher, and R. Mattos, “Natural/synthetic esters usage from an OLTC Perspective,” in Proc. TechCon Aus-NZ, 2018, pp. 1-20
Jinesh Malde has worked for M&I Materials Inc. since 2016 as Applications Engineer providing technical support to customers on use of natural and synthetic esters in transformers. He received a Bachelor of Science degree in Electrical Engineering from Lawrence Technological University, Southfield MI. Forthree years he worked as Transformer Design Engineer at Marcie Electric Inc. followed by six years at Weidmann Electrical Technology Inc. as Distribution Transformer Engineer. He is a member of the IEEE Transformers Committee, participating in several working groups, task forces, and as Secretary of Insulation Life Subcommittee. He is a member of the ASTM Committee D27 – Electrical Insulating Liquid and Gases.
Muhammad Daghrah finished his PhD (2017) in electrical and electronic engineering from The University of Manchester with transformer thermal modelling as the researched subject. He also holds MSc (2013) in Electrical Power Systems Engineering from the University of Manchester and BEng (2010) in Electrical Engineering from Birzeit University in Palestine. Muhammad worked from 2010 till 2012 as a lecturer at Birzeit University at which he was responsible for lab developments and students training on diverse topics including electrical machines, machine drives, wireless communications, and engineering drawings through which he developed strong interpersonal, teaching, and communication skills. In 2018, he joined MIDEL technical team as Applications Engineer with responsibilities involving technical support for customers, leading research in understanding transformer thermal behaviour with alternative liquids, attending exhibitions and technical conferences.
Attila Gyore has a PhD from the Budapest University of Technology and Economics (BUTE), with a focus on superconducting transformers. For 10 years he was an Assistant Professor at the Department of Electric Power Engineering at BUTE, and from 2011 he worked with CG Hungary as an active part design engineer of power transformers. Dr. Attila joined M&I Materials in 2012 as a senior application engineer, where he is an expert in the applications of natural and synthetic ester liquids and retrofilling of transformers. Attila is an active participant in conferences worldwide and is a highly engaged member of IEEE, CIGRE D1 as well as IEC TC10 and TC14.
This article was originally published in the March 2020 issue of the Transformer Oils & Fluids: Development of Transformer Fluids Over the Last Century magazine.
View Magazine
