The transformer is the most important part of the power system. Therefore, the insulating fluid is an integral and important part of the whole system as well. Since power equipment cannot operate without energy losses, which lead to rises in temperature, it is essential to dissipate the generated heat. One of the significant purposes of the dielectric fluid is just that – reducing the heat within the transformer. These insulating fluids prevent short circuits, protect the transformer from adverse chemical reactions in the unit, and act as a cooling agent by actively removing heat by means of flow through the unit and the cooling fins. We will look at the development of transformer fluids over the last century and how they evolved to meet the needs of industry more efficiently.

History of transformer fluids

Mineral oil has been the fluid of choice for power transformers for more than 100 years. The first operational power transformer was built in 1884. In 1890, the first three-phase transformer was built and in the same year the first oil-cooled, oil-insulated transformer was constructed –demonstrating the value of transformer insulating fluids for over a century.

Petroleum based oils have been used as liquid dielectrics since before 1887. These low viscosity paraffinic petroleum oils provide superior insulation when impregnated into paper or other solid dielectrics, and
they also provide excellent heat transfer for the removal of heat generated due to electric losses. However, the paraffinic crudes contain large quantities of wax and therefore have a high pour point which renders them unacceptable for use in electrical apparatus exposed to low temperatures. Namely, in sub-zero climates a large amount of insoluble sludge is formed, which reduces the heat transfer capacity due to lower viscosity.

Later the paraffinic variant was replaced with naphthenic oil, which remained in liquid form at very low temperatures, but had a higher flammability than the paraffinic oils, posing a new problem. The oxidation products of naphthenic oils are higher in quantity, but these products are soluble in the oil and therefore do not create any adverse problems. The aromatic compounds of naphthenic oils remain in liquid form even up to -40°C.

Modern petroleum refining has changed quite extensively over the last 30 to 40 years. Improved technology, specifically catalytic hydro-processing, has brought many benefits and efficiencies. Refined oils are extremely complex blends and may consist of more than 3,000 different hydrocarbons, principally paraffinic (40-60%), naphthenic (30-50%), and aromatic (5-20%). Aromatic hydrocarbons come under many different names, e.g. polynuclear aromatics, polycyclic aromatics and polyaromatic hydrocarbons (five to fifty years in the development of insulating liquids). The aromatic content determines the water solubility of different oils. Polyaromatic hydrocarbons may present a health concern, with recent studies suggesting that naphthenic oils with more than 2% polyaromatic hydrocarbon content are potentially carcinogenic.

Naphthenic oils are manufactured by typical solvent refining processes with hydroprocessing or hydrotreating, or mild hydrofinishing. These processes tend to leave residual substances in the oil, including sulfur compounds and aromatic nitrogen. When a process of hydrocracking or hydroisomerization is used during the refining process, almost all contaminants are removed, including sulfur. Sulfur content in oils has degradation consequences in transformers, with high failure probability. The failure due to corrosive sulfur content has been studied, and extensive data mining results have been published in Engineering Failure Analysis, Vol. 92.

Isoparaffinic oils have better heat transfer capabilities than naphthenic oils. In 1930 Polychlorinated Biphenyls (PCBs) or Askarels were produced to overcome the high flammability problem in naphthenic oils. PCBs had a very low flammability, low acid formation and better oxidation stability than naphthenic oils, making them a perfect solution to the naphthenic oil problem. The PCB oil is a synthetic liquid, a chlorinated aromatic hydrocarbon used in high risk areas where a fire could cause loss of life or extreme disaster. The use of this insulating fluid continued up to the 1970s when it was determined that they were not environmentally acceptable, and they were found to be a carcinogenic substance. The use of PCBs has since been banned due to their effect on the environment and organisms.

Transformer insulating fuilds prevent short circuits, protect the unit from adverse chamical reactions, and act as a cooling agent by actively removing heat by means of flow thrugh the unit and the cooling fins.

Mineral oil as transformer insulating liquid

The chemical structure of mineral oils is extremely complex, including both hydrocarbons and non-hydrocarbons. The hydrocarbon chains constitute the main part of mineral oil, containing only carbon and hydrogen in the chain. The aromatic (ring) structure increases stability of the oil, but if too many aromatic rings are present, it will decrease the dielectric property of the oil by increasing its solvency for the solid particles immersed in the oil.

The sulfur compound concentration is dependent on the source of the crude oil. In high concentrations it will cause corrosion of the copper plate and wire that are in direct contact with the oil in the transformer. Similarly, with the presence of nitrogen compounds the stability of the oil will be reduced.

The dielectric fluid should ideally have a high impulse strength, high electric strength, high volume resistivity, high thermal conductivity, high specific heat, high flash point, low viscosity, low volatility, and low dielectric dissipation factor. It must also have a high resistance to chemical deterioration, be non-flammable, inexpensive and easily obtainable.

The dielectic fluid should ideally have a high impulse strenght, high electric strenght, high volume resistivity, high termal conductivity, high specific heat, high flash point, low viscosity, low volatility, and low dielectric dissipation factor. It must also have a high resistance to chemical deterioration, be non-flammable, inexpensive and easily obtainable.

The transformer industry had to find alternatives to PBCs thus exploring new fluids that would meet all the required criteria needed to ensure safe and sustainable operation of the transformers.

Alternatives to mineral oil: Synthetic and natural ester fluids

Various oils and fluids have been tested to use as an alternative to mineral oil.

Silicone fluids (polydimethylsiloxane or PDMS), for example, have excellent insulating properties, higher fire point than mineral oil, they are less flammable and have excellent anti-oxidative properties and thermal stability due to higher bonding energy (more energy-heat is required to have them bond with other molecules). This oil serves as a replacement for PCBs. Silicone oils are ideally used with Aramid paper, but not Kraft paper [1].

HTH – High temperature hydrocarbons (high molecular weight hydrocarbons) were used as the second alternative to PCB oils.

C2C14 – Tetrachloroethylene is a non-flammable fluid that was introduced in 1980 under the tradename “WECOSOL” [4]. Good electrical properties and low viscosity of this fluid provides excellent heat transfer. It can be mixed with mineral oil or used unmixed, which will provide non-flammability and improved lubrication properties at a lower cost.

Isopropyl biphenyl is a hydrocarbon composed of propylated biphenyl isomers used in capacitors as dielectric fluid. It was introduced under the tradename “WEMCOL” as a replacement for PCBs in 1978. It is flammable but has excellent properties for capacitor use.

The crude oil selection is the most important step in the manufacturing of mineral oils. The degree of refining and the kind of processes and treatments may greatly change the characteristics of the final product. The end-product should have a structure with the perfect balance of naphthenes, paraffins and aromatic rings. There is no failsafe way to produce the perfect mineral oil. Regardless of the method the oil should have high electrical performance, gas absorbing properties, chemical stability and oxidation resistance. Aromatic hydrocarbons, sulfur and nitrogen content are extremely important factors when buying mineral oil.

Mineral oil is poorly biodegradable and therefore poses a threat to the environment which forced industry to look at alternatives.

In the mid-1990s the industry seriously started considering biodegradable, readily available sources as a successful replacement of mineral oil. Vegetable oil was regarded as the most viable option considering it is readily available, a good insulator and fully biodegradable. However, a major concern with vegetable oils was long term stability, which is vitally important, as vegetable oils contain components that degrade in a relatively short time. The degree of unsaturation is an indication of thermal instability, becoming more unstable as the degree of unsaturation progresses from mono- to triunsaturation. These oils have high oxidation instability.

The presence of copper in transformers enhances oxidation. So, powerful oxidation inhibitors are required to ensure stability in the vegetable oil structure. The purity of the oil is also very important as the presence of ionic impurities can act as conductors. The first commercially available vegetable oil was BIOTEMP®, patented by ABB in September 1999. The base fluid consists of high oleic oil sunflower or amended canola oil, with the oleic content over 80%. Another oil was registered in March 2000 by Cooper Industries; it was Envirotemp™ FR3™.

Vegetable oils are hygroscopic, absorbing water in vast quantities at room temperature, up to about 1,200 ppm. The moisture value should be decreased to 100 ppm. Antioxidants are added to stabilize the oil. Commonly used antioxidants are DBPC (2,6-ditertbutyl-paracresol). A special antioxidant package is used in BIOTEMP® consisting of complex amine and phenols. The FR3 fluid does not pass the ASTM oxidation test because of its lower mono-unsaturated content, which should be over 80% for long term transformer use [5].

Synthetic esters are a broad class of synthesized compounds from organic acids and alcohols. They do not generate dioxins or other toxic products in the presence of fire, and they have good biodegradability. During biodegrading they will only form carbon dioxide and water. These liquids have been developed to resist oxidation and they can absorb considerably more moisture than mineral oils before insulation performance deteriorates. Esters can also be used to retro-fill units previously filled with mineral oil, since up to 3% of mineral concentration in ester fluids does not affect the electrical or dielectric properties of the insulating system. Several types of esters are used in electro-technology, e.g. tetra-esters and phosphoric esters.

Silicon fluids

These fluids are chemically known as polydimethylsiloxanes or PDMS. They were introduced in the 1970s as substitutes for PCB oils and have proven to be very popular. Their use has been restricted to those units that pose risk to personnel and/or property. They are environmentally friendly, flame retardant, age well, and they strongly resist oxidation and sludge formation. However, their higher viscosity leads to problems with efficient cooling of the units, so their use requires de-rating of up to 10% for safe operation and to avoid overheating. Also, they have very bad biodegradability.

Vegetable oils

Vegetable oils are readily available natural products, and therefore should be considered as ideal raw material for fully biodegradable insulating liquids. There have been ongoing trials since the 1900s to use these liquids as dielectrics. However, their poor dissipation factor and oxidation stability, higher pour point, relative permittivity and viscosity have been their main disadvantages as dielectric. Since the 1990s they have been in the spotlight again due to environmental concerns about mineral oils, especially in the coastal regions. The development of chemical additive packages reduces the pour point and enhances oxidation stability, but sometimes these packages also contain an antimicrobial copper deactivator.

Nano liquids

Liquids that contain added nanosized particles have better thermal and dielectric properties, thus extending transformer lifetime and increasing loading/cooling capacity. The most common nano-additives include metals and metal oxides. Mineral oil infused with TiO2 particles enhances dielectric properties. It has also been suggested that the addition of magnetic nanoparticles may increase the dielectric strength of transformer oil. However, although the dielectric strength of magnetic nano liquids may be up to 13% higher, their increased loss factor may cause thermal problems under operational conditions. Recently, nano liquids containing a new type of semi-conductive nanoparticle was found to improve insulating and anti-aging properties of mineral oil but have little effect on other electrical parameters e.g. conductivity/resistivity and dissipation factor.

Cryogenic liquids and liquefied gas

The feasibility of substituting CF4 gas for SF6 gas as an insulant for bushings of high-temperature superconductivity materials was recently explored. Mixtures such as liquid oxygen/nitrogen may exhibit better cooling performance than liquid nitrogen, although highly chemically reactive oxygen may have serious risks associated when used in units with partial discharge or arcing. Superconductors cooled by cryogenic liquid have considerable industrial and research potential because they facilitate high current densities without Joule heating. However, there is still much research needed in this area.

Performance analysis of different insulating fluids

What follows is an overview of an overall performance analysis of different insulating fluids [6], and while it may seem that the analysis slightly favors vegetable and synthetic ester fluids, it is still a bit risky to throw everything overboard and fill all your transformers with these new fluids. Mineral oil has been with us for a very long period of time and has a proven track record. There are no adverse long-term effects on the transformer operation and lifetime. We have established a trustworthy set of analysis parameters for mineral oils that can very accurately give an indication of the status and predict the remaining lifetime of a unit. This type of monitoring is not available for the synthetic and vegetable oils. We still have to monitor units filled with alternative fluids for a very long period of time before we can safely say there would be no negative long-term implications associated with the use of these fluids, as well as to effectively apply predictive maintenance models to the units filled with these new types of fluids.

We still have to monitor units filled with alternative fluids for a very long period of time vefore we can safely say there would be no negative long-term implications associated with the use of these fluids.

Fire and flashpoint

Research has shown that vegetable oil has better thermal behavior than mineral oil. In the research, palm and coconut oils were used. High temperature or high molecular weight hydrocarbons (HMWH) are alternatives to PCB liquids. They have higher boiling points and higher molecular weight, and they are classified as paraffinic consisting mainly of saturated compounds of long, straight-chain structures. They also have higher viscosity which reduces their heat transfer capabilities. They are used in collaboration with Aramid paper insulated transformers and especially in conditions with very high operating temperatures.

Arc quenching property

Vegetable oil has better arc quenching. Less acetylene and hydrogen was formed under arcing conditions in vegetable oil than in mineral oil, and CO and CO2 production was higher for vegetable oil than mineral oil.

Dielectric breakdown strenght

Vegetable oil came out as the winner in this category. The effect of moisture on the dielectric behavior in ester fluids is smaller, compared to that of mineral oil. Palm and coconut oils were used in the test.

Oxidation stability

Vegetable oil has superior oxidation stability compared to mineral oil due to the fact that there is little change in breakdown voltage and acid value over time.

Test of insulating liquid

Insulating liquids are subjected to thermal, electrical and mechanical stresses during the operating cycle of the transformer. There are also chemical reactions that take place between all the phases and materials in the transformer and these reactions are catalyzed by operating temperature: the higher the temperature, the faster these reactions take place.

Dissolved Gas Analysis

DGA is used as a specialized diagnostic test to determine the internal condition of the transformer. This method has been used with great success with the analysis of mineral oil in transformers.

However, the various new types of oils coming on the market are complicating the diagnosis, and the DGA methods used in esters, synthetic esters and other insulating liquids are not yet 100% dependable. There are still a lot of uncertainties in the interpretation at this point in time.

Often a customer is not even sure what type of insulating liquid has been used to fill the transformer and the information about specific concentrations of the insulating liquids in the transformer is not available. Great caution should be taken when interpreting DGA diagnosis in both synthetic and natural fluids as this can cause extreme confusion and unnecessary cost to the client. The methods for interpretation of fault diagnosis, abnormal combustible gas formation limits and dissolving of gases in liquids other than mineral oil are very different from those for regular mineral oil and the laboratory teams should familiarize themselves with the new standards and interpretations.

The search for a reliable, biodegradable, highly effective, and chemically inacitive insulating fluid is still ongoing.

Mineral oil has a low cost, availability and excellent performance. However, the growing demand for fire safety, material sustainability, environmental fiendliness and extender asset lifetimes have driven the identification and development of alternative liquids for insulation purposes.

Conclusions and Perspectives

Various additives are available to prolong the life and efficiency of insulating liquids. Each application – transformers, capacitors, bushings or cables – requires an insulating liquid with specific electrical, chemical, and physical characteristics.

Mineral oils have been used in electrical apparatus for over a century, and they have a long and proven track record. The requirements on quality and stability have become more stringent and improvement in refining technology has advanced. Mineral oil has a low cost, availability and excellent performance. However, the growing demand for fire safety, material sustainability, environmental friendliness and extended asset lifetimes have driven the identification and development of alternative liquids for insulation purposes.

The search for a reliable, biodegradable, highly effective, and chemically inactive insulating fluid is still ongoing.

The inherent properties of mineral oils have ensured their use as electrical insulant over the last century and will probably ensure their use for decades to come. Unfortunately, the growing demand for petroleum products may lead to a shortage in the mid-21st century. Also, given the growing environmental concern, fully biodegradable oils, improved with additives that are sustainable and biodegradable, will become increasingly important.

Natural and synthetic ester fluids are only limited to use in distribution, traction and mobile transformers as the outcomes of using these liquids over a lengthy period of time might have adverse effects on the power units. Collection of data and track records for in-service equipment and continued fundamental investigations will provide a knowledge data base to ensure the engineer is equipped with the information needed to make informed decisions on selecting the best fitted insulating liquid for the application at hand.

References:

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[2] I. Fofana, “50 years in the development of insulating liquids,” IEEE Electrical Insulation Magazine, Vol. 29, no. 5, p. 13, 2013.
[3] D. Martin, H. Ma, C. Ekanayake, T. Saha, N. Lelekakis, K. Williams, “State of the art review on managing vegetable oil filled transformers,” E Cigre Study Committee B2 & Study Committee D1.
[4] Vishal, Saurabh, Vikas and Prashant, “Transformer’s History and Its Insulating Oil,” Proceedings of the 5th National Conference, India 2011.
[5] Envirotemp FR Fluid, Cargill industrial Specialists, August 17, 2006
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[7] A.C.M. Wilson, Insulating Liquids: Their Use, Manufacture and Properties, Peter Peregrinus LTD 1980.
[8] A. Sierota, J. Rungis, “Electrical insulating oils, Part 1, Characterization and pre-treatment of new transformer oils,” IEEE Electrical Insulation Magazine, Vol 11, no. 1, pp 8-20, 1995.
[9] S. Prabkahar Karthikeyan, Sarvesh Rathi, Narala Anudeep Reddy “Transformer Oil and its Evolution,” July 5, 2019.
[10] Z. Wang. X Yi, J. Huang, J.V. Hinshaw, J. Noakhes, “Fault gas generation in natural-ester fluid under localized thermal faults,” E Cigre Study Committee, Vol 28, no. 6, pp 45-56, 2012