Synopsis;
- Mineral oil has a history of usage since early transformers since 1890 however, Natural ester fluids were first introduced as a coolant and insulating fluid for transformers in 1998.Though, both are used as insulating oils in transformers, but they differ in several key characteristics. Mineral oil is a petroleum-based product, while natural ester fluid is derived from vegetable oils like soybean or corn oil. Natural esters offer advantages like biodegradability, higher flash and fire points, and better moisture absorption, but they also have higher viscosity and lower flow rates compared to mineral oil. Though, Mineral oil is widely available at lower cost, has lower viscosity (better flow and cooling), however, it is not environment- friendly ie not biodegradable, it has lower flash & fire point and is less water absorbent.
- Key Words: Transformer, Mineral oil, Natural ester fluid, Biodegradable, Fire point
1.0 Introduction;
Transformers are the most critical equipment in the transmission and distribution system. Insulating liquid is the back-bone in transformer insulation system. Insulating liquid has three important functions i.e. electrically insulate the active parts; effectively transfer the heat from conductors to the radiators and providing diagnostic support to assess the health of the equipment through regular monitoring.
In the present scenario, with the growth of power demand, reliability of Transformers has assumed greater importance. Due its lower flash & fire points, Mineral oil filled transformers are prone to failure, resulting in to transformer catching fire followed by explosion and splashing of Mineral oil, give rise to major environmental concern. oil is non-biodegradable. It can contaminate our soil and waterways if serious splashes occur. Large number of transformers are located in populated areas, shopping centres, and near waterways. Petroleum products are depleting and there could be shortage Mineral oil in the next few decades. Fore-seeing this situation and to mitigate the hazards as mentioned above alternate to Mineral oil was innovated as Natural ester Oil year 1998 and many Transformers manufacturers have switching-over to Natural ester fluid, which is categorised as environment friendly and class K liquid.
2.0 Comparison between Mineral oil and Natural Ester Oil as regards composion
2.1 Mineral oil ;
Mineral oil, is primarily composed of various hydrocarbons derived from crude petroleum. These hydrocarbons include alkanes (paraffins), (cycloalkanes) and aromatic compounds. The specific composition and properties of the oil are influenced by the crude oil source and refining process.
Brief description of composition;
Alkanes (Paraffins): These are saturated hydrocarbons with a general formula of CnH2n+2. They are characterized by their stability and ability to resist oxidation.
Aromatics: These are unsaturated hydrocarbons containing benzene rings. While they can contribute to the oil’s insulating properties, excessive amounts can be detrimental.
Additives: Transformer oil may also contain additives like antioxidants to improve its resistance to oxidation and prolong its service life.
2.2 Natural Ester fluid;
Natural ester fluids, commonly found in vegetable oils, are triglycerides, which are esters formed from glycerol and three fatty acids. The general formula for a natural ester fluid can be represented as (RCOO)₃C₃H₅, where ‘RCOO’ represents a fatty acid ester group and C₃H₅ represents glycerol.
Brief introduction is as follows;
Glycerol:
Glycerol is a triol (an alcohol with three hydroxyl groups) with the formula C₃H₅(OH)₃.
Fatty Acids:
Natural ester fluids are derived from fatty acids, which are long-chain carboxylic acids. Examples include palmitic acid, stearic acid, and oleic acid.
Ester Formation:
The esterification process involves the reaction between the carboxyl group (-COOH) of a fatty acid and the hydroxyl group (-OH) of glycerol, releasing water and forming an ester linkage (-COO-).
General Formula:
The overall structure of a natural ester fluid is a glycerol molecule with three fatty acid chains attached through ester linkages. This is why it’s referred to as a triglyceride or triacylglycerol.
2.0 CHEMISTRY AND APPEARANCE OF TRANSFORMER INSULATING LIQUIDS
Table I, gives the comparison of basic properties, atomic structure and appearance of mineral oil and natural ester insulating liquids. Transformer mineral oil is extracted from petroleum. Natural esters liquids can be extracted from crops like, canola, sun-flower, flax, soyabean, olive, poppy, rapeseed and others.
Table-1 Comparison of basic properties, atomic structure and appearance of mineral oil and natural ester insulating liquid.
3.0 PHYSIOCHEMICAL PROPERTIES COMPARISON
Table-2 compares the physio-chemical properties of different liquids. Acidity of transformer mineral oil is hazardous property for insulation in transformer. It deteriorates the insulation property of solid insulation. Acidic mineral oil, allows more water content to dissolve into oil. Acidity accelerates the oxidation process in the oil. Acid also impacts rusting of iron in presence of moisture. A higher relative acid value is inherent in natural esters; however, they are inert due to their long molecular chain.. Acidity of insulating liquid is expressed in mg of KOH required to neutralize the acid present in a gram of insulating liquid. This is, also known as “Neutralization Number”.
Inter facial tension (IFT) between water and oil interface is the way to measure molecular attractive force between water and oil. It is measured in Dyne/cm or mN/m. Inter-facial tension is exactly useful for determining the presence of polar contaminants and mineral oil decay products. New mineral oil generally exhibits relatively higher inter-facial tension than new natural ester liquid. Insulating liquid oxidation contaminants lower the IFT.
Table- 2 Physiochemical properties of transformer insulating liquids.
| Properties | Mineral oil | Natural ester liquid |
| Total Acidity(mgKOH/g) | 1.2 | Max.0.6 |
| Neutralization Number | 0.015 | 0.015 |
| Inter facial Tension[dyne/cm] mN/m | 38 | Between 25 to30 |
4.0 FIRE ASSURANCE.
The fire and flash points are measures of the liquids resistance to catch fire. The flash point of a flammable liquid is the lowest temperature at which the vapor pressure is sufficient to form an ignitable mixture with air near the surface of the liquid. The fire point is the lowest temperature at which a liquid in an open container will attain a vapor pressure sufficient to continue to burn when once ignited. Table 3 compares the fire resistive property of mineral oil and natural ester liquid. Mineral oil has a much lower flash and fire point in comparison with natural ester liquid. Natural Ester fluid has fire point of 360 °C to classify as “less flammable liquid”. Natural ester liquid has significantly higher flash and fire points than that of conventional mineral oil. Calorific value of the insulating liquid is a measure of the amount of energy produced by the complete combustion of the material tested.
No cases of natural ester liquid filled transformer fires, have been reported.
Table 3 Comparison the fire resistive property of mineral oil and natural ester liquid
| Properties | Mineral oil | Natural Ester liquid | |
| Fire safety class | O1 | K2 | |
| Flash point(oC) | ≥135 | Min.250 | |
| Fire point(oC) | 170 | 360 | |
| Calorific value MJ/kg | 46 | 37.5 | |
| Requirement of Fire extinguishing system | Needed | Not required even for indoor application | |
5.0 ENVIRONMENTAL IMPACTS
Table 4 gives the significant advantage of natural ester liquid over mineral oil in terms of environmental concerns. Two major environmental impacts lead towards use of alternate natural resources in transformers:
1. Petroleum products are sooner or later going to run out, and there could be severe shortage by the mid twenty-first century.
2. Transformer oil is poorly biodegradable. It could pollute our
soil and waterways if severe explosion happen resulting into splashing of Mineral oil. Government supervisory representatives are now looking into this issue Government is likely to impose rigid punishments on splashing of Mineral oil. Thousands of transformers filled with mineral oil are located in populated areas, shopping centres and near waterways which are potentially dangerous from points of view of fire safety and environment issues.
Table 4 Environmental impact of Transformer insulating liquids.
| Properties | Mineral oil | Natural Ester liquid |
| Biodegradability In 28 days | <10% | >94% |
| Soil Eco-toxicity | Toxic/forms toxic products | Non-Toxic fresh as well as aged oil |
| Acute aquatic toxicity | Toxic | Non-Toxic |
| Acute oral | Toxic | Non-Toxic |
| Over all Environmental impact | 1/4 of impact of Mineral oil |
6.0 WATER SATURATION
Water presence in transformer insulation is dangerous to overall life of transformers. It may be present in insulating liquids in several forms. Presence of water in the form of distinct droplets or as a cloud detached throughout liquid can be noticed by visual examination. This type of water presence results into decreased dielectric strength of the liquid. It is important to note that water saturation limit for mineral oil is not directly applicable to natural ester liquid as it is hygroscopic in nature and can handle more water and still can maintain the dielectric strength to permissible value. However, the effect of water content on dielectric strength as a function of percent of saturation is the same for both natural ester and mineral oil. Table 5 gives the water content limits for mineral and natural ester liquid in as received condition from suppliers.
Table 5 Water content limits of Transformer insulating liquids
| Water content limits | Mineral oil | Natural Ester liquid |
| Allowable water content in liquid as received from suppliers [mg/kg] |
7.0 OXIDATION STABILITY
The presence of Carbon – Carbon double bond makes insulating liquids prone to oxidation. In a natural ester more carbon – carbon double bonds are present in comparison with mineral oil and it is more susceptible to oxidation. The oxidation process is irreversible and in reaction process, oxygen is consumed. In case of natural ester liquids, continuous exposure to oxygen will produce complex molecules that can slightly increase the viscosity of natural ester in the tank and/or produces oxygen containing byproducts such as alcohols, aldehyde, acids, and ketones.
With continuous exposer to oxygen, thin film of gel is formed in natural ester. To avoid the oxidation of the natural ester liquid during manufacturing process as well as during operation, it is necessary to limit exposure of the insulating liquid to the atmosphere. Due to this reason ester liquid immersed transformers are constructed with sealed type transformer.
Large transformers design often uses nitrogen in the headspace or those with an insulating liquid conservator require a diaphragm barrier between the insulating liquid and external venting. Table 6 provides the oxidation stability details mentioned in different references for mineral oil and natural ester liquid.
Table 6 provides the oxidation stability of insulation liquids
| Oxidation stability limits | Mineral oil | Natural Ester liquid |
| Duration of accelerated aging | 164 hrs at 120oC | 48 hrs at 120oC |
| Oxidation Stability | Mineral oil Natural Ester liquid |
8.0 DENSITY
The relative density of an insulating liquid is the ratio of the weights of equal volumes of liquid and water at 15 °C. In certain cold climates, ice may form in equipment exposed to sub-zero (0 °C) temperatures and may float on liquids that may have densities higher than 0.917. Table 8 provides the relative density limits for mineral oil and natural ester liquids.
Table 7 Density details
| Property | Mineral oil | Natural Ester liquid |
| Relative Density [g/ml] at 15oC | ≤ 0.92 | |
| Density at 20oC | Max.0.895 | Max.1.0 |
9.0
CONCLUSION
Considering environmental risks, fire safety, health hazard, insulating liquid based on Natural ester liquid is the next-generation transformer liquid that is going to substitute the mineral oil. Natural ester score over mineral oil on environment concerns with fully biodegradability, non-toxicity, as well as environmentally favourable. Natural ester is the insulating liquid which has properties desired for transformer. Natural ester proves as fire safe with high fire point (“K” class liquid). Water saturation limits of natural ester being very high it can hold more water, also, due to hygroscopic nature water remains in liquid instead of migrating to solid insulation which extends the life of the solid insulation.
Poor oxidation stability of natural ester requires a special care and sealed construction of the tank. Considering, Natural ester liquid’s higher temperature withstand capability, overall space requirement of transformer can be reduced by usage of high temperature insulating materials.
10.0 REFERENCES
[1] D.P. Stockton, et al., “Natural Ester Transformers Fluids: Safety, Reliability and Environmental Performance”, IEEE Petroleum and Chem. Industry Technical Conf. (PCIC), pp. 1-7, 2007.
[2] R. Asano and S. A. Page, “Reducing Environmental Impact and Improving Safety and Performance of Power Transformers with Natural Ester Dielectric Insulating Fluids”, IEEE Trans. Industry Appl., Vol. 50, No. 1, pp. 134-141, 2014.
[3] T.V. Oommen, “Vegetable Oils for Liquid-Filled Transformers”, IEEE Electr. Insul. Mag., Vol. 18, No. 1, pp. 6-11, 2002.
[4] K. Baburao, N. Nanavati and P.N. Narayanan, “Eco-Friendly Modern Dielectric Fluids – A Boon to Power Sector”, 2nd Int’l. Conf. Innovation and Best Practices in Transformer Design, Testing and Maintenance, CPRI, pp. 211-217, 2014.
[5] R. Martin, “Esters–Their Structure and Their Properties”, CPRI National Conf. Environmentally Friendly Insulating Liquids (EFIL 2013)”, New Delhi, pp. 1-16, 2013.
11.0 The Author
Er.K.K.Murty, B.E.(Hons) Elec. Engg , FIE(India) & CE(India), Member-CGRE.
•Former Chief Engineer & HOD (Testing & Commun.), M.P. Power Transmission Co. Ltd. Jabalpur. (Served in MPSEB & MPPTCL, Jabalpur for 33 years at different levels from the Assistant Engineer to the Chief Engineer)
•Empanelled as an Expert Professional on the panel of CPRI, Bangalore and carried out TPI of Power equipments on behalf of CPRI, Bangalore/Bhopal.( from 2008 to 2012)
•Worked as Advisor (Testing) in SOUTHCO, Berhampur. Odisha (A DISCOM in Odisha of Anil Dhirubhai Ambani group). (from 2004 to 2006).
•Worked as Metering Consultant to M. P. Electricity Regulatory Commission, Bhopal, for 1year. In 2005).
•Awarded plaque in recognition of eminence and contribution to the profession of Electrical Engineering at the National level by the Institute of Engineers India, Kolkata in Oct 2015.
•Author of a Book titled as “COMPENDIUM OF ARTICLES ON EHV SUBSTATIONS & PROTECTIONS FOR BUDDING AND PRACTICING ENGINEERS OF TRANSMISSION UTILITIES.
