An overview of main barriers to adopt the use of natural esters (NE) to replace mineral oils (MO) in the insulation of power transformers in Colombia

An overview of main barriers to adopt the use of natural esters (NE) to replace mineral oils (MO) in the insulation of power transformers in Colombia | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article An overview of main barriers to adopt the use of natural esters (NE) to replace mineral oils (MO) in the insulation of power transformers in Colombia Alejandro Paz Parra, Juan Carlos Aristizabal Giraldo, Diego Fernando García Gómez, and 1 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7935350/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 8 You are reading this latest preprint version Abstract From the beginning of the electrical industry, the use of power transformers became necessary for the expansion of electricity service wherever an electrification process began. The rapid growth of electricity demand forces the industry to use more power equipment and higher voltage power lines to transmit larger volumes of electric power over longer distances, making the use of power transformers indispensable. As the requirements for high voltage and power loss management increase, the use of new dielectrics to insulate the active parts of power equipment has become necessary to achieve higher insulation levels and efficient power loss mitigation. Mineral oils have provided a solution despite their inflammability and possible contamination of water and soil. Recently, natural esters have offered a modern solution to the same problems but with numerous advantages like their high water-solubility, fast degradation, and lower inflammability. Consequently, natural esters have been introduced into the electrical industry's manufacturing chain; however, their penetration in the power transformer market is still low and unrepresentative in terms of the number of operating units insulated with NE. Thus, the researchers decided to conduct a survey that involved a set of manufacturers, decision-makers, marketers, researchers, asset owners, and users of power transformers to identify the main barriers that slow the adoption of power transformers that use NE into the power grid. Subsequently, a review of the literature allows us to approximate some considerations that we expect will contribute to clarify some of the identified barriers and to establish a set of new research lines about the use of NE in power transformers. Natural esters power transformers liquid insulators mineral oil risk. I. INTRODUCTION The use of liquid insulators for power transformers and other power system equipment started since the early beginning of the power network expansion at the end of the 19th century. During all the 19th and the 20th centuries, the main source of liquid insulators was mineral oil (MO) obtained from petroleum, even though there were serious studies that warned about its toxicity and the low flash point, which made them risky and potentially dangerous for people, buildings, and structures.[ 1 ] [ 2 ]. Environmental regulations for the use of MO were basically oriented to preventive fire control equipment and the introduction of chemical substances like fire retardants into the oil to increase the flash point or delay the ignition process. These substances, introduced in the 1970s, were polychlorinated biphenyls (PCBs) that resulted in being more toxic than MO. Thus, the negative impact on the environment increased and the regulation became more restrictive. As a possible solution, the research about liquid insulators turned in different ways: the synthetic esters (SE) that consist of esters synthesized from oil, the use of high molecular weight hydrocarbons (HMWH), and the third way, which is the progressive substitution of MO by NE derived from vegetable oil. NEs have demonstrated advantages in comparison with MO, SE, and HMWH, such as a higher fire point of inflammability, a lower thermoelectric degradation rate, high water solubility, higher dielectric strength, and a lower carbon footprint because their origin is mostly from vegetable seeds. The more commonly used seeds are: Sunflower, Soybean, and rapeseed; but it's possible to use others like palm oil [ 1 ] [ 3 ] [ 4 ] [ 5 ] [ 6 ] [ 7 ]. In the increasing trend to use electric power for mobility to progressively replace fossil fuels in urban mobility and transport, the decarbonization of the production, transport, and end-use of electricity becomes a priority. In this way, the use of liquid insulators derived from oil is a contradiction, and there´s an increasing trend to replace them with alternatives with a lower carbon footprint like NE. This is reinforced by the idea that oil is a non-renewable resource, making it unsustainable to use oil to support the development of the electric industry. In the trend toward decarbonizing the electric industry, the use of NE arises as a good idea to increase the sustainability of the economic model and the growth of the electric industry. [ 7 ] [ 8 ] [ 6 ] [ 9 ]. Despite the technical advantages of NEs in comparison to MO, their recent introduction in the electric industry generates some distrust because there are not enough studies about the reliability obtained in the use of NE instead of MO in the electric equipment of the power systems. [ 10 ] [ 11 ]. The substitution of MO by NE is delayed because the lifespan of MO in a power transformer is longer than forty years. Therefore, there is a limited set of transformers insulated with NE in comparison with those insulated with MO [ 8 ] [ 12 ], their inclusion in distribution transformers is faster because the natural growth of the distribution network expands at a faster rate than the power network expansion. [ 13 ] [ 14 ]. A survey conducted among a wide set of stakeholders in the electrical sector of the economy, such as power transformer users, owners, engineers, universities, marketers, and manufacturers of power transformers and natural esters, asked about the main reasons to delay the decision to incorporate NE into the power grid supply chain and the associated power equipment. The survey showed that, although there is knowledge among professionals in the sector, its use remains limited caused by multiple doubts related to the lack of technical knowledge, implementation costs, and lack of success stories, while environmental benefits were the main motivator for adoption. The low trust in the use of NE instead of MO stems from three main considerations: the risk of losing the asset, the availability of a unit of the same characteristics to replace the asset, and the time of the replacement. In the case of a fault, the time of replacement is critical because a failed transformer puts out of service a critical set of other assets like transmission lines, power synchronous generators, or bus bars. This can cause a considerable loss of money during all the time until the replacement [ 8 ] [ 12 ]. In this paper, the authors provide a review of the comparative advantages of NE in comparison with MO in the insulation of power transformers to contribute to building some trust in the safe use of NE, in response to the barriers detected in the survey on barriers to the use of NE. II. METHODOLOGY At first, a set of skateholders in the use of NE for power transformers were identified: Asset Owners: Members of nineteen companies of the energy transmission, generation and distribution systems were included to explore the distrust of the asset owners regarding the use of NE in the insulation of power transformers. Transformer manufacturers: Members of a power transformer manufacturing company were asked about their perception of the future of the use of NE in power transformers. Maintenance service providers: Members of four maintenance service providers of power transformers were asked about their experience and knowledge about NE in comparison with MO in the maintenance of power transformers. Marketers of NE: A set of marketers of NE for its use in power transformers were asked about market respects and doubts expressed by customers related with the NE. Academy: Finally, universities, labs and research centers were asked about the state of the art in the development of NE and its incorporation in the productive chain of power transformers. The critical issues raised by survey respondents fall into the following categories: The need for specific thermal design for NE, adjusted technical standards, improved asset profitability, and NE conservation. They also call for greater technical outreach, coordination with manufacturers, and dissemination of tax and environmental benefits. In order to answer to the main issues revealed by the survey about the use of NE in power transformer insulation, the following research path was conducted: Review of the available technical normativity in the main standards organizations in reference with the use or maintenance of NE. As a result, two IEC, one ASTME and one IEEE standards were found and compared. There are a set of ASTM procedures that are applicable to natural esters that were reviewed and considered. Review of technical literature about experiences with the use of NE to insulate power transformers. This review used the main databases of research publications: SCOPUS, WOS, IEEE and GOOGLE SCHOLAR. The search was limited to publications in journals, conferences and technical standards. PhD thesis or monographies were not considered. The search was limited to publications in English and publication from the las 10 years referred to the following keywords: Natural esters, degradation of natural esters, dissolved gas analysis in natural esters, power transformers insulation. The results of the scientific evidence found in a wide set of research articles, application notes and technical standards are summarized in the results section. III. RESULTS As a result of the application of the survey there are seven categories to group the main barriers identified to avoid the use of NE in power transformers: A. Need for specifical thermal design of power transformers insulated with NE The need for a specific thermal design is related to two fundamental aspects, the dynamic viscosity and the pour point of NE compared to MO. Kinematic viscosity (ASTM D445) The kinematic viscosity is higher in NE than in MO so the power dissipation in forced and natural conditions is quite different. Consequently, the stable temperature point is different and the time to reach a new thermal equilibrium point could be longer in transformers insulated with NE in comparison with those insulated with MO [25] [27]. Pour point (ASTM D97) For transformers using natural esters with operational (particularly mechanical) internal accessories, a higher minimum temperature may be required before operation than required for mineral oil. In addition, it is possible for natural esters to cease to flow if left standing for long periods at low temperatures. Nevertheless, the pour point must be measured by test [25] [27]. The characteristics observed in the parameters of both types of insulating liquids (NE and MO) may vary depending on the composition and the chemical reactions that appear inherently in each liquid but the acceptance limits for these parameters are defined in Table 1 of Standard (STD) IEC 62770 and the IEEE STD C57-147 [25] [27]. The main differences registered in the literature about the NE in contrast with the MO are referred to its viscosity, flash point, pour point, dielectric constant and its production and characterization of dissolved gases. [3] [5]. The first generation of NE´s was characterized by having high viscosity which results in low flow rate that makes heat exchange through convection difficult; As a consequence, higher temperatures were reached in power transformers, up to 4 o C higher than those reached in transformers insulated with MO [1] [30][8]. To improve this condition, lower viscosity NE´s are currently used, however, reducing viscosity requires using lower density oils whose flash point is lower and which therefore reduce the comparative advantage that was had in this aspect with MO’s [18]; an alternative solution is to modify the industrial process by increasing the impregnation temperature in the tank filling and lengthening the time to increase the paper impregnation [18] [31]. The pour point for NEs (-10C) is considerably higher than in MOs (-40C) thus, the selection of NEs to insulate transformers in extreme weather conditions must be carefully analyzed [24]. B. Need for specifical standards related with test and diagnostics of power transformers insulated with NE There are four standards to evaluate the performance of NE [12] [15][24]: IEC 62770:2013: Fluids for electro technical applications - Unused natural esters for transformers and similar electrical equipment. [25] ASTM 6871-17: Standard Specification for Natural (Vegetable Oil) Ester Fluids Used in Electrical Apparatus. [26] IEEE STD C57.147-2018 - IEEE Guide for Acceptance and Maintenance of Natural Ester Insulating Liquid in Transformers. [27] IEC 62975:2021 – Standard | Natural esters - Guidelines for maintenance and use in electrical equipment. [28] As a base of comparison there are a set of parameters defined by the STD IEEE C57:147 as suitable for comparison between the test results of the NE and MO: Table 1 Insulating liquid tests suitable for natural ester-based dielectric liquids. Adapted from Table 1 of the STD IEEE C57:147 [19] Test ASTM or IEC method number Acid number (neutralization) D664, D974 Dielectric breakdown voltage D1816 Dielectric breakdown voltage—impulse conditions D3300 AC loss characteristics—dissipation factor and relative permittivity D924 Interfacial tension D971 Color D1500 Kinematic viscosity D445 Flash point and fire point—Cleveland Open Cup Method D92 Relative density (specific gravity) D1298 Pour point D97, D5949, D5950 Volume resistivity D1169 Gas analysis D3284, D3612 Oxidation stability IEC 61125, Method C Water content—Karl Fischer Method D1533 Visual examination of used liquids D1524 Gassing under electrical stress and ionization D2300 Corrosive sulfur test D1275 Polychlorinated biphenyls (PCBs) D4059 Furanic compounds D5837 The most noticeable differences that can be expected depending on the parameter and the test method are listed below: Acid number (ASTM D974) The acid number is introduced like a general guide for determining when mineral oil should be replaced or reclaimed, but in the case of NE the acid number is associated with the content of fatty acids formed by hydrolysis, pyrolysis, and oxidation of several components of the NE that can be increased or decreased by the industrial process of the NE or from the seed or mix of seeds that originate the natural oil. Thus, the acid number in NE is not necessarily associated to adverse effects or the need to change the insulating liquid [27]. The IEC 62770 STD indicates than near neutral acidity in NE must be expected [25]. Dielectric breakdown voltage (DBV) and Dielectric breakdown voltage under impulse conditions (DBVI) The DBV and DBVI tests are conducted under the procedure defined by the ASTM D1816 and ASTM D3300 respectively. In NE as well as in MO the same procedure is followed, but the recommendation is to delay the realization of the test by 15 minutes after the full filling of the test cell to let flux the air bubbles trapped in the NE in comparison to the rest time used in MO because of the higher viscosity of NE in comparison with MO [17] [27]. AC loss characteristics—dissipation factor and relative permittivity (IEC Dielectric dissipation factor DD) Under the test conditions defined by the ASTM D924, the NE has shown a higher dissipation factor than MO by its own composition and humidity. Therefore, the limits defined by MO are not applicable to NE in the same test conditions. The electrical permittivity is usually higher in NE than in MO and the IEC recommends to use a temperature of 90C during the test [17] [27]. Interfacial tension IFT The procedure to define the interfacial tension in mN/m is defined by the ASTM D971, but just as in the dissipation factor the limits applicable to MO are not applicable to NE because its variable chemical composition and humidity contents, so there´s not a defined value applicable to NE but the expected value is lower than the expected in MO [27]. Flash point and fire point—Cleveland Open Cup Method (ASTM D92) The NE has higher flash point and fire point than the MO. The acceptance value for NE defined by ASTM D6871 is 275 o C for the flash point and 300 o C for the fire point of unused NE [27]. Volume resistivity (ASTM D1169) The test of the volume resistivity has inherently-lower values for the NE in comparison with MO because of its composition [27]. Gas analysis (ASTM D3284, D3612) While the gases produced in natural ester liquid and cellulose insulated systems are the same as those produced in mineral oil/cellulose systems, the circumstances and quantities in which they are produced are sometimes different. Therefore, there is a specific standard for the interpretation of the results of de DGA in NE [26] [27] [29]. Oxidation stability The IEEE STD C57-147 doesn´t recommend the use of the ASTM D6871test, because the limits of acceptance for NE has not been stablished and the use of the limits stablished for MO are not applicable to NE [26] [27]. Water content—Karl Fischer Method (ASTM D1533) The use of reagents required by the test to define the water content (mg/kg) stablishes a difference between the tests realized in NE in contrast with those in MO. The reagents are different because the composition of the NE is different and the results of the test when done with the same reagents used in MO to measure the water content in NE could be erratic and yield elevated. Then, the ASTM D1533 test guide introduces the Annex A1 “Alternative Solvent Systems” with recommended reagents to be used in NE [17] [27]. The water content and the DBV are closely related, in a high water content scenario, the BDV decreases. Thus the protection of the NE by using totally enclosed tanks are strongly recommended [28]. Gassing of insulating liquids under electrical stress and ionization (ASTM D2300) In presence of electrical stress NE have inherently lower gassing tendency than MO, below the lower range of mineral oils and generally fairly negative (gas absorbing from − 50uL/min to -90uL/min) [27]. Corrosive sulfur test (ASTM D1275) Natural esters naturally do not contain corrosive sulfur therefore they should not be corrosive [25] [27]. Polychlorinated biphenyls (PCBs) (ASTM D4059) Unused natural ester liquids should not contain PCBs but when present, their concentration shall not be more than 2mg/kg [25] [27]. The test classified into the category 1 of the IEC 62975 standard are the following: Checking of color and appearance, BDV, Water content, Viscosity, neutralization number (acidity), DDF and dissolved gases. In the category 2, complementary test is included the IFT, Fire point and density. In the category 3, research and informative test are included the flash point, pour point, antioxidant content, floating particles, Liquid Compatibility and miscibility and Materials compatibility (retro filled transformers) [28]. C. Improved asset profitability, and NE conservation. NEs due to their chemical composition tend to be hydrophilic and hygroscopic in contrast to MOs which are typically hydrophobic. This makes their use not recommended in high humidity environments unless the transformers are hermetically sealed [24] [25] [35]. However, this characteristic, which seems to be a disadvantage, ends up being an advantage since the relative humidity remains in the NE without impregnating the insulating paper, which reduces the aging rate of the paper [21] [36]. NE´s mechanical properties are particularly affected by temperature changes, so adequate control and monitoring of the tank temperature, as well as the operating regime, are decisive for their proper use [6][22] [23]. One of the most commonly used techniques to analyze the degradation of insulating oils is dissolved gas analysis (DGA) [10] [37]. By the use of DGA, the state of aging of the electrical insulation and its possibilities of electric fault can be recognized, either due to an increase in partial discharges, degradation of the insulating paper or chemical decomposition of the oil generated by polluting substances, humidity or power losses [16]. Each of the named process generates a different set an quantity of gases and the correlation between the concentration of gases and the total production of gases provides information about the dominant degradation process, the aging state of the insulation and the probability of electric fault in the short or medium term [2] [14] [37]. The DGA is used to diagnose the condition of the aging state of the insulation of a transformer and is complementary to the partial discharge test due to the ease of its two application modes: DGA OFFLINE: Oil samples are taken and sent to specialized laboratories which, through pyrolysis and mass spectrometry, allow the gases dissolved in the oil and their respective concentrations to be obtained. DGA ONLINE: The samples are taken directly on site and there is the equipment that performs the analysis and transmits the results periodically through cloud storage. Each of the application methods has its advantages and disadvantages: The off-line procedure has the advantage that the analysis is carried out in specialized laboratories that maintain the calibration of the equipment. In the sampling process, accidental contamination may occur that varies the results and provides imprecise information about the real state. of the oil. In addition, it involves the movement of specialized personnel and the programming of inspection routes. The online procedure facilitates systematization in taking samples and obtaining results, obviating the need to schedule inspection routes and associated personnel movements, but it also requires having equipment in the field associated with the transformer under analysis. Which limits its use to power transformers due to cost. Additionally, there is a risk of accidental fail on the calibration of the measuring equipment, then periodical calibration could be required. The main gases analyzed and that are present as a result of the natural degradation in operation of the two types of liquid insulation and of the cellulose of the insulating paper are: Hydrogen (\(\:{H}_{2}\)), methane (\(\:{CH}_{4}\)), ethane (\(\:{C}_{2}{H}_{6}\)), ethylene (\(\:{C}_{2}{H}_{4}\)), acetylene (\(\:{C}_{2}{H}_{2}\)), carbon monoxide (\(\:CO\)) and carbon dioxide (\(\:C{O}_{2}\)). Based on the concentrations of each of these gases, it is possible to identify a fault profile and determine the state of degradation of the oil and eventually its fault probability [13] [14] [38]. These analyses, which are effective in preventing faults in MO-insulated transformers, are not as effective in the NE-insulated transformers, mainly due to two clearly identified causes: The absence of sufficient data from NE-insulated transformers due to their relative recent penetration in the electric network and the high variability of gas concentrations depending on the source of the oil and its extractive process [4] [5] [39]. The high variability of the primary sources from which the oils are obtained include a wide spread of seeds, e.g., soybeans, olive oil and palm oil [3]. The chemical reactions that origin the different gases change depending on the original composition of the ester (the source), the manufacturing process, the environmental variables at the installation site and the operating regimes of the transformer under analysis[2][8] [38] [28]. In a study presented by Kraus and McPherson it was found in a sample of 17 power transformers insulated with NE that there was no correlation between the years of service of the transformer and the aging parameters calculated with the traditional DGA [15]. Similar results were found by researchers in Brazil in a sample of 21 transformers insulated with NE aged in a range from 2006–2009 which do not reflect significant signs of aging in the main physical parameters of the ester [16]. Despite this, it has been possible to identify that NE results in lower degradation rates of the impregnated paper than MO in all temperature ranges tested (130C, 150C and 170C) but this conclusion is limited by the type of ester analyzed [28][40]. Lower aging rates found in the literature review, but greater uncertainty in the prevention of electrical fault, require that complementary studies be conducted involving variables such as ambient temperature, the load cycles of the transformer, the operating regimes and the operating voltage level correlated together with the presence of partial discharges and aging indicators [35] [41]. The general perception is that the dissolved gas analysis technique is very effective, but that its scope in vegetable ester (NE) remains a field open to research, which requires greater refinement in the interpretation of results. D. Technical outreach Despite their recent entry into operation (less than 40 years), NE-insulated power transformers have already had enough time in service to evaluate their aging characteristics and to form a general idea of the electrochemical degradation mechanism of NEs in power transformers[15]. However, the studies found in the literature currently comprise mostly distribution transformers and very few power transformers whose operating regimes are very different. [13] [14] [16]. It has been shown that the general performance of vegetable oils is similar and in many cases better than that of mineral oils, but that their environmental impact in the replacement or recovery process is significantly lower, which makes them ideal for optimizing the operation of renewable energy integration systems into the grid and railway electric mobility systems [10] [17] [18] [19] [20] [21]. Regarding the electrical parameters, it has been demonstrated that the dielectric BDV of both types of oils is similar (35kV/2mm), but in new transformers, the impregnation times before entering service are twice in NE when compared to MO [6], and in modern NE´s the thermodynamic properties are quite similar to those in MOs [19] [32] [33] [34]. E. Dissemination of tax and environmental benefits. The comparison of the two types of oils from an environmental point of view is shown in the following Table, adapted from [24] [42]: Table 2 Environmental benefits on the use of NE´s in comparison with MO´s Property Applied STD MO NE Biodegradability in a 28 days period OECD 301 F 94% Soil eco-toxicity OECD Toxic Non Toxic Acute water toxicity OECD 203 Toxic Non Toxic Acute oral toxicity OECD 420 Toxic Non Toxic Groundwater pollution N/A Requires pit to avoid spills No pit required Total carbon footprint over the life cycle NIST BEES V4.0 NE´s present less than 2% of the total carbon footprint of MO´s during their life cycle. General environmental impact NIST BEES V4.0 NE´s have a negative environmental impact equivalent to 25% of that generated by MO´s. Greenhouse gases generated throughout the life cycle Tones by each 1000 gallons 4,18 0,075 Carbon neutrality N/A Non carbon neutral Carbon neutral Sulfur - corrosive ASTM D1275-06 Method B Non corrosive Undetected Although the amount of oil used for insulation in power transformers can vary from one manufacturer to another and from one design to another, ranges can be established that allow us to give an idea about the environmental benefits of NE´s, with based on the carbon footprint values identified in the Table 2. By replacing MO in a power transformer with a NE, one can avoid the emission of between 13 and 30 tons of CO2 into the atmosphere. Table 3 CO2 Tones avoided by replacing a MO with NE in the insulation of a power transformer Power (MVA) Capacity of the tank (liters) CO2 Tones Tones avoided MO NE 20–30 12000 13,25 0,24 13,01 40–70 18000 19,87 0,36 19,51 100 28000 30.92 0.55 30,37 Regarding the contamination of water, researchers from the University of West Parana took water samples where they kept cultures of fish and micro crustaceans and contaminated them with different concentration levels of NE and MO separately and compared the results. As a result of this comparison, the conclusion arises that NE are highly biodegradable compared to MO and that the water-soluble component of the former is significantly less toxic than that of the latter. [42]. Despite the fact that accidents involving accidental spillage of insulating oils are not common in power transformers and that environmental regulations require the construction of oil containment pits to prevent the possibility of an accidental spill occurring. that leaks into the soil and can cause contamination of bodies of water, researchers from different countries have studied the differences found in the environmental impacts generated when these oils come into contact with underground water bodies [21][42]. Both types of MO and NE insulation materials have the possibility of being renewed and recovered to be put into service again with a recovery rate close to 95%. However, in the case of MO, their production starts from the use of a non-renewable resource, so the sustainability of its use in the long term for new transformer units is debatable. Additionally, the 5% non-reusable oils can be highly polluting in MO compared to NE [7]. IV. DISCUSSION Natural esters represent an environmentally sustainable solution to the use of oil based mineral liquids for the insulation of electrical equipment in the power systems. They represent a tendency in the decarbonization of the electric industry and in the reduction of the oil dependence of the electric power transmission and distribution. Based on the higher fire point and flash point of the NE it is possible to see its contribution to increased fire safety in electrical installations in confined spaces, residential areas, and underground substations. Despite these advantages, the review of the literature indicates that there is still not enough information to identify their aging process in their operative conditions and the loss of dielectric properties based on external signs beyond the dissolved water content that can be stablished by periodical inspections. Thus, a more detailed research both under controlled conditions and in natural operating environments must be conducted to identify this mechanism and to correlate the indicators with the real condition of the insulation to give a more reliable approach to the real performance of these insulation liquids under thermal and electrical stresses. Results of this review suggest that the use of NE instead of MO in the refilling of operative transformers or in the production of new transformers can conduct to a reduction of the carbon footprint from 16 to 30 CO2 tones by each unit, although it may seem very small, in a cumulative impact it can significantly contribute to reducing the electricity industry's dependence on oil. The environmental benefits obtained from the replacement of mineral oils with vegetable esters in the manufacture or filling of power transformers are limited due to the lack of knowledge of their degradation processes and the signs of degradation of the dielectric properties, which must necessarily lead to a scientific effort aimed at improving said knowledge with a view to increasing the introduction of this type of liquid insulation in the electrical industry. V. CONCLUSIONS The lack of knowledge about the degradation or aging processes in NE is evident, but it must impel scientific efforts to increase the knowledge in order to take advantage of the benefits and reduce the uncertainty about their use in the electrical industry. There is a set of references and literature published about NE used in electrical power transmission and distribution, but the wide variety of NE available requires more detailed studies on their chemical properties and on the reactions that occur in their components due to thermal stress and electrical stress. The perceptions expressed by all stakeholders in the development of an NE industry for use in power transformers suggest that there is significant interest, awareness of the environmental benefits, and expectations regarding new applications and performance of natural esters in field applications, but there is significant uncertainty regarding specific aspects such as maintenance, operating costs, the supply chain, and safety. These issues must be resolved in the medium term to improve the penetration of this type of insulation in the national and international electrical markets. Declarations Author Contribution All authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by Alejandro Paz Parra and Juan Carlos Aristizabal. Diego García revised the article and complement the search of the information. Fernando Sebastian Villa is a support researcher and contribute to collect de data and process the results of the survey. The comments and depuration of the information included in the article where revised and approved by all authors. The first draft of the manuscript was written by Alejandro Paz Parra and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript. Acknowledgement Authors want to aknowledge the financial support ot the Universidad Santiago de Cali like the institution that offers financial support to the corresponsal author Alejandro Paz Parra to participate in the research team that produces the present article. 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Sbravati y R. I. Da Silva, «Long-Term Behavior of Natural Ester Filled Power Transformers in Eletronorte Transmission System,» Proceedings of the IEEE Power Engineering Society Transmission and Distribution Conference , Vols. %1 de %22020-October, 2020. R. I. Da Silva y H. Tatizawa, «A proposal of natural ester immersed GSU transformers for better efficiency of wind farms and its intermittences,» 2021 IEEE PES Innovative Smart Grid Technologies Conference - Latin America, ISGT Latin America 2021, 2021. J. E. Contreras, J. Rodriguez y C. Gaytán, «Analysis of the Impregnation Process of Cellulose Materials by Using Ester-Based Liquids and Mineral Oil,» IEEE Transactions on Dielectrics and Electrical Insulation , vol. 29, nº 3, 2022. M. Rajňák, K. Paulovičová, J. Kurimský, J. Tóthová, R. Cimbala, K. Kónyová, M. Dzida, M. Timko y P. Kopčanský, «Comparison of physical properties of ferrofluids based on mineral transformer oil and bio-degradable gas-to-liquid oil,» Journal of Magnetism and Magnetic Materials , vol. 589, p. 171628, 1 2024. S. Maneerot y N. Pattanadech, «Partial discharge characteristics of mineral oil immersed transformer compared with natural ester and palm oil immersed transformer under different periods of impregnation,» Proceedings of the International Symposium on Electrical Insulating Materials , Vols. %1 de %22020-September, 2020. T. Nogueira, J. Carvalho y J. Magano, «Eco-Friendly Ester Fluid for Power Transformers versus Mineral Oil: Design Considerations,» Energies , vol. 15, nº 15, p. 5418, 7 2022. D. M. Mehta, P. Kundu, A. Chowdhury, V. K. Lakhiani y A. S. Jhala, «A review on critical evaluation of natural ester vis-a-vis mineral oil insulating liquid for use in transformers: Part 1,» IEEE Transactions on Dielectrics and Electrical Insulation , vol. 23, nº 2, 2016. BS EN 62770:2014 Fluids for electrotechnical applications: unused natural esters for transformers and smiliar electrical equipment, 2013. ASTM_D6871–03, Standard Specification for Natural (Vegetable Oil) Ester Fluids Used in Electrical Apparatus. Annual Book of ASTM Standard, vol. 10, 2003. Transformers Committee IEEE, IEEE Std C57.147™-2008, IEEE Guide for Acceptance and Maintenance of Natural Ester Fluids in Transformers, 2008. IEC, IEC 62975:2021 - Natural esters – Guidelines for maintenance and use in electrical equipment, Geneva: Switzerland, 2021. IEEE, IEEE Draft Guide for Interpretation of Gases Generated in Natural Ester and Synthetic Ester Immersed Transformers, 1 ed., vol. 1, IEEE, 2014, pp. 1–50. J. Velandy, A. Garg y C. S. Narasimhan, «Continuous Thermal Overloading Capabilities of Ester Oil Transformers in Oil Directed Cooling Conditions,» 2020 IEEE 9th Power India International Conference (PIICON) , pp. 1–7, 2 2020. J. Sanz, O. Sancibrian, C. Olmo, C. Mendez, A. Ortiz y C. J. Renedo, «Study of the Impregnation of Power-Transformer Cellulosic Materials with Dielectric Ester Oils,» IEEE Access , vol. 9, 2021. D. M. Mehta, P. Kundu, A. Chowdhury, V. K. Lakhiani y A. S. Jhala, «A review of critical evaluation of natural ester vis-a-vis mineral oil insulating liquid for use in transformers: Part II,» IEEE Transactions on Dielectrics and Electrical Insulation , vol. 23, nº 3, 2016. N. Pattanadech, K. Jariyanurat, S. Maneerot y P. Nimsanong, «Electrical characteristic comparison of mineral oil and natural ester for transformer applications,» 2017 International Electrical Engineering Congress, iEECON 2017 , 2017. Z. Huang, R. Wang, K. J. Rapp y G. Liu, «Test Comparison Study on a Natural Ester Retro-filled 220kV Power Transformer,» 2020 IEEE International Conference on High Voltage Engineering and Application (ICHVE) , pp. 1–4, 9 2020. R. Villarroel, B. García de Burgos y D. F. García, «Moisture dynamics in natural-ester filled transformers,» International Journal of Electrical Power and Energy Systems , vol. 124, 2021. P. Przybylek, «Thermal Ageing of Dry Cellulose Paper Impregnated with Different Insulating Liquids—Comparative Studies of Materials Properties,» Energies , vol. 17, nº 4, p. 784, 2 2024. M. H. Hamid, M. T. Ishak, M. F. Din, N. F. S. Suhaimi y N. I. Katim, «Dielectric properties of natural ester oils used for transformer application under temperature variation,» PECON 2016–2016 IEEE 6th International Conference on Power and Energy, Conference Proceeding , 2017. P. Kurzweil, C. Schell, R. Haller, P. Trnka y J. Hornak, «Environmental Impact and Aging Properties of Natural and Synthetic Transformer Oils under Electrical Stress Conditions,» Advanced Sustainable Systems , vol. 5, nº 8, 8 2021. S. R. Kiran, T. Mariprasath, C. H. Basha, M. Murali y M. B. Reddy, «Thermal degrade analysis of solid insulating materials immersed in natural ester oil and mineral oil by DGA,» Materials Today: Proceedings , vol. 52, 2022. M. Meira, C. Verucchi, R. Alvarez y L. Catalano, «Dissolved Gas Analysis in Mineral Oil and Natural Ester Liquids from Thermal Faults,» IEEE Transactions on Dielectrics and Electrical Insulation , vol. 28, nº 4, 2021. C. M. Gutierrez, A. O. Fernandez, C. J. Renedo Estebanez, C. O. Salas y R. Maina, «Understanding the Ageing Performance of Alternative Dielectric Fluids,» IEEE Access , vol. 11, 2023. C. Xiang, Q. Zhou, J. Li, Q. Huang, H. Song y Z. Zhang, «Comparison of dissolved gases in mineral and vegetable insulating oils under typical electrical and thermal faults,» Energies , vol. 9, nº 5, 2016. O. Mazur, F. May y J. Mora, «Practical Experience of Natural Esters Quality Maintenance,» 2020 IEEE Electrical Insulation Conference, EIC 2020, 2020. A. N. Módenes, K. Sanderson, D. E. G. Trigueros, A. R. Schuelter, F. R. Espinoza-Quiñones, C. V. Neves, L. A. Zanão Junior y A. D. Kroumov, «Insights on the criteria of selection of vegetable and mineral dielectric fluids used in power transformers on the basis of their biodegradability and toxicity assessments,» Chemosphere , vol. 199, pp. 312–319, 5 2018. R. Asano y S. A. Page, «Reducing Environmental Impact and Improving Safety and Performance of Power Transformers With Natural Ester Dielectric Insulating Fluids,» IEEE Transactions on Industry Applications , vol. 50, nº 1, pp. 134–141, 1 2014. Additional Declarations No competing interests reported. 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Parra","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAABEklEQVRIie2QsWrDMBBALwicRbFWZajzCycMgeB8jEPBXlraKWQ0GNQl2Zu/yNjR4YYuIl7bzV26FdLNGQJ140AIOG7HQvWWk457Ot0BWCx/kS4AArDjDaVXH5wWhZ0ps7H/KwVOiokmyU+KSFl2v3sK7gT01p87TfEqpwK2UwLxkDQqkpxQLUw8Wibutexrul29RNh53BBIkzW3IY7Y04SYcZTqoHBgVaZaRdhoDEhs1b5W/HJSfQxzA2zfoiBx8I9dhjIzUYjZDbBOi6LIQf9Kx9hP3WiUzMZqWc2ynm9ifmkW7zl9Vx86QNFd0GuJcuDm9FaU08C7tLEKBw+B8VPq+3neWFzDijq21VgsFsu/5gvJPVv24JNWMAAAAABJRU5ErkJggg==","orcid":"","institution":"Universidad Santiago de Cali","correspondingAuthor":true,"prefix":"","firstName":"Alejandro","middleName":"Paz","lastName":"Parra","suffix":""},{"id":550259496,"identity":"2fcc5613-e57c-46a3-82b7-1c8257ec78d8","order_by":1,"name":"Juan Carlos Aristizabal Giraldo","email":"","orcid":"","institution":"University of Valle","correspondingAuthor":false,"prefix":"","firstName":"Juan","middleName":"Carlos Aristizabal","lastName":"Giraldo","suffix":""},{"id":550259497,"identity":"d28afd07-493b-4cf9-b588-a490fd1b94dd","order_by":2,"name":"Diego Fernando García Gómez","email":"","orcid":"","institution":"University of Valle","correspondingAuthor":false,"prefix":"","firstName":"Diego","middleName":"Fernando García","lastName":"Gómez","suffix":""},{"id":550259498,"identity":"b8b12931-4c1e-439f-98ad-8f0e078e2888","order_by":3,"name":"Fernando Sebastián Villa Díaz","email":"","orcid":"","institution":"University of Valle","correspondingAuthor":false,"prefix":"","firstName":"Fernando","middleName":"Sebastián Villa","lastName":"Díaz","suffix":""}],"badges":[],"createdAt":"2025-10-23 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14:20:28","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":777921,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7935350/v1/513a2cf3-0960-4e96-8b91-d8863a7c34de.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"An overview of main barriers to adopt the use of natural esters (NE) to replace mineral oils (MO) in the insulation of power transformers in Colombia","fulltext":[{"header":"I. INTRODUCTION","content":"\u003cp\u003eThe use of liquid insulators for power transformers and other power system equipment started since the early beginning of the power network expansion at the end of the 19th century. During all the 19th and the 20th centuries, the main source of liquid insulators was mineral oil (MO) obtained from petroleum, even though there were serious studies that warned about its toxicity and the low flash point, which made them risky and potentially dangerous for people, buildings, and structures.[\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e] [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eEnvironmental regulations for the use of MO were basically oriented to preventive fire control equipment and the introduction of chemical substances like fire retardants into the oil to increase the flash point or delay the ignition process. These substances, introduced in the 1970s, were polychlorinated biphenyls (PCBs) that resulted in being more toxic than MO. Thus, the negative impact on the environment increased and the regulation became more restrictive. As a possible solution, the research about liquid insulators turned in different ways: the synthetic esters (SE) that consist of esters synthesized from oil, the use of high molecular weight hydrocarbons (HMWH), and the third way, which is the progressive substitution of MO by NE derived from vegetable oil.\u003c/p\u003e\u003cp\u003eNEs have demonstrated advantages in comparison with MO, SE, and HMWH, such as a higher fire point of inflammability, a lower thermoelectric degradation rate, high water solubility, higher dielectric strength, and a lower carbon footprint because their origin is mostly from vegetable seeds. The more commonly used seeds are: Sunflower, Soybean, and rapeseed; but it's possible to use others like palm oil [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e] [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e] [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e] [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e] [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e] [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eIn the increasing trend to use electric power for mobility to progressively replace fossil fuels in urban mobility and transport, the decarbonization of the production, transport, and end-use of electricity becomes a priority. In this way, the use of liquid insulators derived from oil is a contradiction, and there\u0026acute;s an increasing trend to replace them with alternatives with a lower carbon footprint like NE. This is reinforced by the idea that oil is a non-renewable resource, making it unsustainable to use oil to support the development of the electric industry. In the trend toward decarbonizing the electric industry, the use of NE arises as a good idea to increase the sustainability of the economic model and the growth of the electric industry. [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e] [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e] [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e] [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eDespite the technical advantages of NEs in comparison to MO, their recent introduction in the electric industry generates some distrust because there are not enough studies about the reliability obtained in the use of NE instead of MO in the electric equipment of the power systems. [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e] [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]. The substitution of MO by NE is delayed because the lifespan of MO in a power transformer is longer than forty years. Therefore, there is a limited set of transformers insulated with NE in comparison with those insulated with MO [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e] [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e], their inclusion in distribution transformers is faster because the natural growth of the distribution network expands at a faster rate than the power network expansion. [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e] [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eA survey conducted among a wide set of stakeholders in the electrical sector of the economy, such as power transformer users, owners, engineers, universities, marketers, and manufacturers of power transformers and natural esters, asked about the main reasons to delay the decision to incorporate NE into the power grid supply chain and the associated power equipment. The survey showed that, although there is knowledge among professionals in the sector, its use remains limited caused by multiple doubts related to the lack of technical knowledge, implementation costs, and lack of success stories, while environmental benefits were the main motivator for adoption.\u003c/p\u003e\u003cp\u003eThe low trust in the use of NE instead of MO stems from three main considerations: the risk of losing the asset, the availability of a unit of the same characteristics to replace the asset, and the time of the replacement. In the case of a fault, the time of replacement is critical because a failed transformer puts out of service a critical set of other assets like transmission lines, power synchronous generators, or bus bars. This can cause a considerable loss of money during all the time until the replacement [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e] [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eIn this paper, the authors provide a review of the comparative advantages of NE in comparison with MO in the insulation of power transformers to contribute to building some trust in the safe use of NE, in response to the barriers detected in the survey on barriers to the use of NE.\u003c/p\u003e"},{"header":"II. METHODOLOGY","content":"\u003cp\u003eAt first, a set of skateholders in the use of NE for power transformers were identified:\u003c/p\u003e\u003cp\u003e\u003col\u003e\u003cspan\u003e\u003cli\u003e\u003cp\u003eAsset Owners: Members of nineteen companies of the energy transmission, generation and distribution systems were included to explore the distrust of the asset owners regarding the use of NE in the insulation of power transformers.\u003c/p\u003e\u003c/li\u003e\u003c/span\u003e\u003cspan\u003e\u003cli\u003e\u003cp\u003eTransformer manufacturers: Members of a power transformer manufacturing company were asked about their perception of the future of the use of NE in power transformers.\u003c/p\u003e\u003c/li\u003e\u003c/span\u003e\u003cspan\u003e\u003cli\u003e\u003cp\u003eMaintenance service providers: Members of four maintenance service providers of power transformers were asked about their experience and knowledge about NE in comparison with MO in the maintenance of power transformers.\u003c/p\u003e\u003c/li\u003e\u003c/span\u003e\u003cspan\u003e\u003cli\u003e\u003cp\u003eMarketers of NE: A set of marketers of NE for its use in power transformers were asked about market respects and doubts expressed by customers related with the NE.\u003c/p\u003e\u003c/li\u003e\u003c/span\u003e\u003cspan\u003e\u003cli\u003e\u003cp\u003eAcademy: Finally, universities, labs and research centers were asked about the state of the art in the development of NE and its incorporation in the productive chain of power transformers.\u003c/p\u003e\u003c/li\u003e\u003c/span\u003e\u003c/ol\u003e\u003c/p\u003e\u003cp\u003eThe critical issues raised by survey respondents fall into the following categories: The need for specific thermal design for NE, adjusted technical standards, improved asset profitability, and NE conservation. They also call for greater technical outreach, coordination with manufacturers, and dissemination of tax and environmental benefits.\u003c/p\u003e\u003cp\u003eIn order to answer to the main issues revealed by the survey about the use of NE in power transformer insulation, the following research path was conducted:\u003c/p\u003e\u003cp\u003e\u003col\u003e\u003cspan\u003e\u003cli\u003e\u003cp\u003eReview of the available technical normativity in the main standards organizations in reference with the use or maintenance of NE. As a result, two IEC, one ASTME and one IEEE standards were found and compared. There are a set of ASTM procedures that are applicable to natural esters that were reviewed and considered.\u003c/p\u003e\u003c/li\u003e\u003c/span\u003e\u003cspan\u003e\u003cli\u003e\u003cp\u003eReview of technical literature about experiences with the use of NE to insulate power transformers. This review used the main databases of research publications: SCOPUS, WOS, IEEE and GOOGLE SCHOLAR. The search was limited to publications in journals, conferences and technical standards. PhD thesis or monographies were not considered.\u003c/p\u003e\u003c/li\u003e\u003c/span\u003e\u003cspan\u003e\u003cli\u003e\u003cp\u003eThe search was limited to publications in English and publication from the las 10 years referred to the following keywords: Natural esters, degradation of natural esters, dissolved gas analysis in natural esters, power transformers insulation.\u003c/p\u003e\u003c/li\u003e\u003c/span\u003e\u003c/ol\u003e\u003c/p\u003e\u003cp\u003eThe results of the scientific evidence found in a wide set of research articles, application notes and technical standards are summarized in the results section.\u003c/p\u003e"},{"header":"III. RESULTS","content":"\u003cp\u003eAs a result of the application of the survey there are seven categories to group the main barriers identified to avoid the use of NE in power transformers:\u003c/p\u003e\n\u003cp\u003eA. Need for specifical thermal design of power transformers insulated with NE\u003c/p\u003e\n\u003cp\u003eThe need for a specific thermal design is related to two fundamental aspects, the dynamic viscosity and the pour point of NE compared to MO.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eKinematic viscosity (ASTM D445)\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe kinematic viscosity is higher in NE than in MO so the power dissipation in forced and natural conditions is quite different. Consequently, the stable temperature point is different and the time to reach a new thermal equilibrium point could be longer in transformers insulated with NE in comparison with those insulated with MO [25] [27].\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003ePour point (ASTM D97)\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eFor transformers using natural esters with operational (particularly mechanical) internal accessories, a higher minimum temperature may be required before operation than required for mineral oil. In addition, it is possible for natural esters to cease to flow if left standing for long periods at low temperatures. Nevertheless, the pour point must be measured by test [25] [27].\u003c/p\u003e\n\u003cp\u003eThe characteristics observed in the parameters of both types of insulating liquids (NE and MO) may vary depending on the composition and the chemical reactions that appear inherently in each liquid but the acceptance limits for these parameters are defined in Table\u0026nbsp;1 of Standard (STD) IEC 62770 and the IEEE STD C57-147 [25] [27].\u003c/p\u003e\n\u003cp\u003eThe main differences registered in the literature about the NE in contrast with the MO are referred to its viscosity, flash point, pour point, dielectric constant and its production and characterization of dissolved gases. [3] [5].\u003c/p\u003e\n\u003cp\u003eThe first generation of NE\u0026acute;s was characterized by having high viscosity which results in low flow rate that makes heat exchange through convection difficult; As a consequence, higher temperatures were reached in power transformers, up to 4\u003csup\u003eo\u003c/sup\u003eC higher than those reached in transformers insulated with MO [1] [30][8].\u003c/p\u003e\n\u003cp\u003eTo improve this condition, lower viscosity NE\u0026acute;s are currently used, however, reducing viscosity requires using lower density oils whose flash point is lower and which therefore reduce the comparative advantage that was had in this aspect with MO\u0026rsquo;s [18]; an alternative solution is to modify the industrial process by increasing the impregnation temperature in the tank filling and lengthening the time to increase the paper impregnation [18] [31].\u003c/p\u003e\n\u003cp\u003eThe pour point for NEs (-10C) is considerably higher than in MOs (-40C) thus, the selection of NEs to insulate transformers in extreme weather conditions must be carefully analyzed [24].\u003c/p\u003e\n\u003cp\u003eB. Need for specifical standards related with test and diagnostics of power transformers insulated with NE\u003c/p\u003e\n\u003cp\u003eThere are four standards to evaluate the performance of NE [12] [15][24]:\u003c/p\u003e\n\u003cul\u003e\n \u003cli\u003e\n \u003cp\u003eIEC 62770:2013: Fluids for electro technical applications - Unused natural esters for transformers and similar electrical equipment. [25]\u003c/p\u003e\n \u003c/li\u003e\n \u003cli\u003e\n \u003cp\u003eASTM 6871-17: Standard Specification for Natural (Vegetable Oil) Ester Fluids Used in Electrical Apparatus. [26]\u003c/p\u003e\n \u003c/li\u003e\n \u003cli\u003e\n \u003cp\u003eIEEE STD C57.147-2018 - IEEE Guide for Acceptance and Maintenance of Natural Ester Insulating Liquid in Transformers. [27]\u003c/p\u003e\n \u003c/li\u003e\n \u003cli\u003e\n \u003cp\u003eIEC 62975:2021 \u0026ndash; Standard | Natural esters - Guidelines for maintenance and use in electrical equipment. [28]\u003c/p\u003e\n \u003c/li\u003e\n\u003c/ul\u003e\n\u003cp\u003eAs a base of comparison there are a set of parameters defined by the STD IEEE C57:147 as suitable for comparison between the test results of the NE and MO:\u003c/p\u003e\n\u003cdiv\u003e\n \u003ctable id=\"Tab1\" border=\"1\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv\u003eTable 1\u003c/div\u003e\n \u003cdiv\u003e\n \u003cp\u003eInsulating liquid tests suitable for natural ester-based dielectric liquids.\u003c/p\u003e\n \u003cdiv\u003e\n \u003cp\u003eAdapted from Table\u0026nbsp;1 of the STD IEEE C57:147 [19]\u003c/p\u003e\n \u003c/div\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eTest\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eASTM or IEC\u003c/p\u003e\n \u003cp\u003emethod number\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eAcid number (neutralization)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eD664, D974\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eDielectric breakdown voltage\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eD1816\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eDielectric breakdown voltage\u0026mdash;impulse conditions\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eD3300\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eAC loss characteristics\u0026mdash;dissipation factor and relative permittivity\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eD924\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eInterfacial tension\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eD971\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eColor\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eD1500\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eKinematic viscosity\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eD445\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eFlash point and fire point\u0026mdash;Cleveland Open Cup Method\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eD92\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eRelative density (specific gravity)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eD1298\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003ePour point\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eD97, D5949, D5950\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eVolume resistivity\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eD1169\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eGas analysis\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eD3284, D3612\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eOxidation stability\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eIEC 61125, Method C\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eWater content\u0026mdash;Karl Fischer Method\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eD1533\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eVisual examination of used liquids\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eD1524\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eGassing under electrical stress and ionization\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eD2300\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eCorrosive sulfur test\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eD1275\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003ePolychlorinated biphenyls (PCBs)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eD4059\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eFuranic compounds\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eD5837\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n\u003c/div\u003e\n\u003cp\u003eThe most noticeable differences that can be expected depending on the parameter and the test method are listed below:\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcid number (ASTM D974)\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe acid number is introduced like a general guide for determining when mineral oil should be replaced or reclaimed, but in the case of NE the acid number is associated with the content of fatty acids formed by hydrolysis, pyrolysis, and oxidation of several components of the NE that can be increased or decreased by the industrial process of the NE or from the seed or mix of seeds that originate the natural oil. Thus, the acid number in NE is not necessarily associated to adverse effects or the need to change the insulating liquid [27]. The IEC 62770 STD indicates than near neutral acidity in NE must be expected [25].\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eDielectric breakdown voltage (DBV) and Dielectric breakdown voltage under impulse conditions (DBVI)\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe DBV and DBVI tests are conducted under the procedure defined by the ASTM D1816 and ASTM D3300 respectively. In NE as well as in MO the same procedure is followed, but the recommendation is to delay the realization of the test by 15 minutes after the full filling of the test cell to let flux the air bubbles trapped in the NE in comparison to the rest time used in MO because of the higher viscosity of NE in comparison with MO [17] [27].\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAC loss characteristics\u0026mdash;dissipation factor and relative permittivity (IEC Dielectric dissipation factor DD)\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eUnder the test conditions defined by the ASTM D924, the NE has shown a higher dissipation factor than MO by its own composition and humidity. Therefore, the limits defined by MO are not applicable to NE in the same test conditions. The electrical permittivity is usually higher in NE than in MO and the IEC recommends to use a temperature of 90C during the test [17] [27].\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eInterfacial tension IFT\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe procedure to define the interfacial tension in mN/m is defined by the ASTM D971, but just as in the dissipation factor the limits applicable to MO are not applicable to NE because its variable chemical composition and humidity contents, so there\u0026acute;s not a defined value applicable to NE but the expected value is lower than the expected in MO [27].\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFlash point and fire point\u0026mdash;Cleveland Open Cup Method (ASTM D92)\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe NE has higher flash point and fire point than the MO. The acceptance value for NE defined by ASTM D6871 is 275\u003csup\u003eo\u003c/sup\u003eC for the flash point and 300\u003csup\u003eo\u003c/sup\u003eC for the fire point of unused NE [27].\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eVolume resistivity (ASTM D1169)\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe test of the volume resistivity has inherently-lower values for the NE in comparison with MO because of its composition [27].\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eGas analysis (ASTM D3284, D3612)\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eWhile the gases produced in natural ester liquid and cellulose insulated systems are the same as those produced in mineral oil/cellulose systems, the circumstances and quantities in which they are produced are sometimes different. Therefore, there is a specific standard for the interpretation of the results of de DGA in NE [26] [27] [29].\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eOxidation stability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe IEEE STD C57-147 doesn\u0026acute;t recommend the use of the ASTM D6871test, because the limits of acceptance for NE has not been stablished and the use of the limits stablished for MO are not applicable to NE [26] [27].\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eWater content\u0026mdash;Karl Fischer Method (ASTM D1533)\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe use of reagents required by the test to define the water content (mg/kg) stablishes a difference between the tests realized in NE in contrast with those in MO. The reagents are different because the composition of the NE is different and the results of the test when done with the same reagents used in MO to measure the water content in NE could be erratic and yield elevated. Then, the ASTM D1533 test guide introduces the Annex A1 \u0026ldquo;Alternative Solvent Systems\u0026rdquo; with recommended reagents to be used in NE [17] [27]. The water content and the DBV are closely related, in a high water content scenario, the BDV decreases. Thus the protection of the NE by using totally enclosed tanks are strongly recommended [28].\u003c/p\u003e\n\u003cdiv\u003e\n \u003cp\u003e\u003cstrong\u003eGassing of insulating liquids under electrical stress and ionization\u003c/strong\u003e\u003c/p\u003e\n\u003c/div\u003e\n\u003cp\u003e\u003cstrong\u003e(ASTM D2300)\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eIn presence of electrical stress NE have inherently lower gassing tendency than MO, below the lower range of mineral oils and generally fairly negative (gas absorbing from \u0026minus;\u0026thinsp;50uL/min to -90uL/min) [27].\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCorrosive sulfur test (ASTM D1275)\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNatural esters naturally do not contain corrosive sulfur therefore they should not be corrosive [25] [27].\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003ePolychlorinated biphenyls (PCBs) (ASTM D4059)\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eUnused natural ester liquids should not contain PCBs but when present, their concentration shall not be more than 2mg/kg [25] [27].\u003c/p\u003e\n\u003cp\u003eThe test classified into the category 1 of the IEC 62975 standard are the following: Checking of color and appearance, BDV, Water content, Viscosity, neutralization number (acidity), DDF and dissolved gases.\u003c/p\u003e\n\u003cp\u003eIn the category 2, complementary test is included the IFT, Fire point and density.\u003c/p\u003e\n\u003cp\u003eIn the category 3, research and informative test are included the flash point, pour point, antioxidant content, floating particles, Liquid Compatibility and miscibility and Materials compatibility (retro filled transformers) [28].\u003c/p\u003e\n\u003cp\u003eC. Improved asset profitability, and NE conservation.\u003c/p\u003e\n\u003cp\u003eNEs due to their chemical composition tend to be hydrophilic and hygroscopic in contrast to MOs which are typically hydrophobic. This makes their use not recommended in high humidity environments unless the transformers are hermetically sealed [24] [25] [35]. However, this characteristic, which seems to be a disadvantage, ends up being an advantage since the relative humidity remains in the NE without impregnating the insulating paper, which reduces the aging rate of the paper [21] [36].\u003c/p\u003e\n\u003cp\u003eNE\u0026acute;s mechanical properties are particularly affected by temperature changes, so adequate control and monitoring of the tank temperature, as well as the operating regime, are decisive for their proper use [6][22] [23].\u003c/p\u003e\n\u003cp\u003eOne of the most commonly used techniques to analyze the degradation of insulating oils is dissolved gas analysis (DGA) [10] [37].\u003c/p\u003e\n\u003cp\u003eBy the use of DGA, the state of aging of the electrical insulation and its possibilities of electric fault can be recognized, either due to an increase in partial discharges, degradation of the insulating paper or chemical decomposition of the oil generated by polluting substances, humidity or power losses [16]. Each of the named process generates a different set an quantity of gases and the correlation between the concentration of gases and the total production of gases provides information about the dominant degradation process, the aging state of the insulation and the probability of electric fault in the short or medium term [2] [14] [37].\u003c/p\u003e\n\u003cp\u003eThe DGA is used to diagnose the condition of the aging state of the insulation of a transformer and is complementary to the partial discharge test due to the ease of its two application modes:\u003c/p\u003e\n\u003cul\u003e\n \u003cli\u003e\n \u003cp\u003eDGA OFFLINE: Oil samples are taken and sent to specialized laboratories which, through pyrolysis and mass spectrometry, allow the gases dissolved in the oil and their respective concentrations to be obtained.\u003c/p\u003e\n \u003c/li\u003e\n \u003cli\u003e\n \u003cp\u003eDGA ONLINE: The samples are taken directly on site and there is the equipment that performs the analysis and transmits the results periodically through cloud storage.\u003c/p\u003e\n \u003c/li\u003e\n\u003c/ul\u003e\n\u003cdiv\u003e\n \u003cp\u003eEach of the application methods has its advantages and disadvantages:\u003c/p\u003e\n\u003c/div\u003e\n\u003cul\u003e\n \u003cli\u003e\n \u003cp\u003eThe off-line procedure has the advantage that the analysis is carried out in specialized laboratories that maintain the calibration of the equipment. In the sampling process, accidental contamination may occur that varies the results and provides imprecise information about the real state. of the oil. In addition, it involves the movement of specialized personnel and the programming of inspection routes.\u003c/p\u003e\n \u003c/li\u003e\n \u003cli\u003e\n \u003cp\u003eThe online procedure facilitates systematization in taking samples and obtaining results, obviating the need to schedule inspection routes and associated personnel movements, but it also requires having equipment in the field associated with the transformer under analysis. Which limits its use to power transformers due to cost. Additionally, there is a risk of accidental fail on the calibration of the measuring equipment, then periodical calibration could be required.\u003c/p\u003e\n \u003c/li\u003e\n\u003c/ul\u003e\n\u003cp\u003eThe main gases analyzed and that are present as a result of the natural degradation in operation of the two types of liquid insulation and of the cellulose of the insulating paper are: Hydrogen (\\(\\:{H}_{2}\\)), methane (\\(\\:{CH}_{4}\\)), ethane (\\(\\:{C}_{2}{H}_{6}\\)), ethylene (\\(\\:{C}_{2}{H}_{4}\\)), acetylene (\\(\\:{C}_{2}{H}_{2}\\)), carbon monoxide (\\(\\:CO\\)) and carbon dioxide (\\(\\:C{O}_{2}\\)). Based on the concentrations of each of these gases, it is possible to identify a fault profile and determine the state of degradation of the oil and eventually its fault probability [13] [14] [38].\u003c/p\u003e\n\u003cp\u003eThese analyses, which are effective in preventing faults in MO-insulated transformers, are not as effective in the NE-insulated transformers, mainly due to two clearly identified causes: The absence of sufficient data from NE-insulated transformers due to their relative recent penetration in the electric network and the high variability of gas concentrations depending on the source of the oil and its extractive process [4] [5] [39].\u003c/p\u003e\n\u003cp\u003eThe high variability of the primary sources from which the oils are obtained include a wide spread of seeds, e.g., soybeans, olive oil and palm oil [3]. The chemical reactions that origin the different gases change depending on the original composition of the ester (the source), the manufacturing process, the environmental variables at the installation site and the operating regimes of the transformer under analysis[2][8] [38] [28].\u003c/p\u003e\n\u003cp\u003eIn a study presented by Kraus and McPherson it was found in a sample of 17 power transformers insulated with NE that there was no correlation between the years of service of the transformer and the aging parameters calculated with the traditional DGA [15]. Similar results were found by researchers in Brazil in a sample of 21 transformers insulated with NE aged in a range from 2006\u0026ndash;2009 which do not reflect significant signs of aging in the main physical parameters of the ester [16].\u003c/p\u003e\n\u003cp\u003eDespite this, it has been possible to identify that NE results in lower degradation rates of the impregnated paper than MO in all temperature ranges tested (130C, 150C and 170C) but this conclusion is limited by the type of ester analyzed [28][40].\u003c/p\u003e\n\u003cp\u003eLower aging rates found in the literature review, but greater uncertainty in the prevention of electrical fault, require that complementary studies be conducted involving variables such as ambient temperature, the load cycles of the transformer, the operating regimes and the operating voltage level correlated together with the presence of partial discharges and aging indicators [35] [41].\u003c/p\u003e\n\u003cp\u003eThe general perception is that the dissolved gas analysis technique is very effective, but that its scope in vegetable ester (NE) remains a field open to research, which requires greater refinement in the interpretation of results.\u003c/p\u003e\n\u003cp\u003eD. Technical outreach\u003c/p\u003e\n\u003cp\u003eDespite their recent entry into operation (less than 40 years), NE-insulated power transformers have already had enough time in service to evaluate their aging characteristics and to form a general idea of the electrochemical degradation mechanism of NEs in power transformers[15]. However, the studies found in the literature currently comprise mostly distribution transformers and very few power transformers whose operating regimes are very different. [13] [14] [16].\u003c/p\u003e\n\u003cp\u003eIt has been shown that the general performance of vegetable oils is similar and in many cases better than that of mineral oils, but that their environmental impact in the replacement or recovery process is significantly lower, which makes them ideal for optimizing the operation of renewable energy integration systems into the grid and railway electric mobility systems [10] [17] [18] [19] [20] [21].\u003c/p\u003e\n\u003cp\u003eRegarding the electrical parameters, it has been demonstrated that the dielectric BDV of both types of oils is similar (35kV/2mm), but in new transformers, the impregnation times before entering service are twice in NE when compared to MO [6], and in modern NE\u0026acute;s the thermodynamic properties are quite similar to those in MOs [19] [32] [33] [34].\u003c/p\u003e\n\u003cp\u003eE. Dissemination of tax and environmental benefits.\u003c/p\u003e\n\u003cp\u003eThe comparison of the two types of oils from an environmental point of view is shown in the following Table, adapted from [24] [42]:\u003c/p\u003e\n\u003cdiv\u003e\n \u003ctable id=\"Tab2\" border=\"1\" class=\"fr-table-selection-hover\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv\u003eTable 2\u003c/div\u003e\n \u003cdiv\u003e\n \u003cp\u003eEnvironmental benefits on the use of NE\u0026acute;s in comparison with MO\u0026acute;s\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eProperty\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eApplied STD\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eMO\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eNE\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eBiodegradability in a 28 days period\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eOECD 301 F\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u0026lt;\u0026thinsp;10%\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u0026gt;\u0026thinsp;94%\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eSoil eco-toxicity\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eOECD\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eToxic\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eNon Toxic\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eAcute water toxicity\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eOECD 203\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eToxic\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eNon Toxic\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eAcute oral toxicity\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eOECD 420\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eToxic\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eNon Toxic\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eGroundwater pollution\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eN/A\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eRequires pit to avoid spills\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eNo pit required\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eTotal carbon footprint over the life cycle\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eNIST BEES V4.0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colspan=\"2\"\u003e\n \u003cp\u003eNE\u0026acute;s present less than 2% of the total carbon footprint of MO\u0026acute;s during their life cycle.\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eGeneral environmental impact\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eNIST BEES V4.0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colspan=\"2\"\u003e\n \u003cp\u003eNE\u0026acute;s have a negative environmental impact equivalent to 25% of that generated by MO\u0026acute;s.\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eGreenhouse gases generated throughout the life cycle\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eTones by each 1000 gallons\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e4,18\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0,075\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eCarbon neutrality\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eN/A\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eNon carbon neutral\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eCarbon neutral\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eSulfur - corrosive\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eASTM\u003c/p\u003e\n \u003cp\u003eD1275-06\u003c/p\u003e\n \u003cp\u003eMethod B\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eNon corrosive\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eUndetected\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n\u003c/div\u003e\n\u003cp\u003eAlthough the amount of oil used for insulation in power transformers can vary from one manufacturer to another and from one design to another, ranges can be established that allow us to give an idea about the environmental benefits of NE\u0026acute;s, with based on the carbon footprint values identified in the Table\u0026nbsp;2. By replacing MO in a power transformer with a NE, one can avoid the emission of between 13 and 30 tons of CO2 into the atmosphere.\u003c/p\u003e\n\u003cdiv\u003e\n \u003ctable id=\"Tab3\" border=\"1\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv\u003eTable 3\u003c/div\u003e\n \u003cdiv\u003e\n \u003cp\u003eCO2 Tones avoided by replacing a MO with NE in the insulation of a power transformer\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\" rowspan=\"2\"\u003e\n \u003cp\u003ePower\u003c/p\u003e\n \u003cp\u003e(MVA)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" rowspan=\"2\"\u003e\n \u003cp\u003eCapacity of the tank\u003c/p\u003e\n \u003cp\u003e(liters)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colspan=\"2\"\u003e\n \u003cp\u003eCO2 Tones\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" rowspan=\"2\"\u003e\n \u003cp\u003eTones\u003c/p\u003e\n \u003cp\u003eavoided\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eMO\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eNE\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e20\u0026ndash;30\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e12000\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e13,25\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0,24\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e13,01\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e40\u0026ndash;70\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e18000\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e19,87\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0,36\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e19,51\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e100\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e28000\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e30.92\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.55\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e30,37\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n\u003c/div\u003e\n\u003cp\u003eRegarding the contamination of water, researchers from the University of West Parana took water samples where they kept cultures of fish and micro crustaceans and contaminated them with different concentration levels of NE and MO separately and compared the results. As a result of this comparison, the conclusion arises that NE are highly biodegradable compared to MO and that the water-soluble component of the former is significantly less toxic than that of the latter. [42].\u003c/p\u003e\n\u003cp\u003eDespite the fact that accidents involving accidental spillage of insulating oils are not common in power transformers and that environmental regulations require the construction of oil containment pits to prevent the possibility of an accidental spill occurring. that leaks into the soil and can cause contamination of bodies of water, researchers from different countries have studied the differences found in the environmental impacts generated when these oils come into contact with underground water bodies [21][42].\u003c/p\u003e\n\u003cp\u003eBoth types of MO and NE insulation materials have the possibility of being renewed and recovered to be put into service again with a recovery rate close to 95%. However, in the case of MO, their production starts from the use of a non-renewable resource, so the sustainability of its use in the long term for new transformer units is debatable. Additionally, the 5% non-reusable oils can be highly polluting in MO compared to NE [7].\u003c/p\u003e"},{"header":"IV. DISCUSSION","content":"\u003cp\u003e\u003cdiv class=\"BlockQuote\"\u003e\u003cp\u003eNatural esters represent an environmentally sustainable solution to the use of oil based mineral liquids for the insulation of electrical equipment in the power systems. They represent a tendency in the decarbonization of the electric industry and in the reduction of the oil dependence of the electric power transmission and distribution. Based on the higher fire point and flash point of the NE it is possible to see its contribution to increased fire safety in electrical installations in confined spaces, residential areas, and underground substations. Despite these advantages, the review of the literature indicates that there is still not enough information to identify their aging process in their operative conditions and the loss of dielectric properties based on external signs beyond the dissolved water content that can be stablished by periodical inspections. Thus, a more detailed research both under controlled conditions and in natural operating environments must be conducted to identify this mechanism and to correlate the indicators with the real condition of the insulation to give a more reliable approach to the real performance of these insulation liquids under thermal and electrical stresses.\u003c/p\u003e\u003cp\u003eResults of this review suggest that the use of NE instead of MO in the refilling of operative transformers or in the production of new transformers can conduct to a reduction of the carbon footprint from 16 to 30 CO2 tones by each unit, although it may seem very small, in a cumulative impact it can significantly contribute to reducing the electricity industry's dependence on oil.\u003c/p\u003e\u003cp\u003eThe environmental benefits obtained from the replacement of mineral oils with vegetable esters in the manufacture or filling of power transformers are limited due to the lack of knowledge of their degradation processes and the signs of degradation of the dielectric properties, which must necessarily lead to a scientific effort aimed at improving said knowledge with a view to increasing the introduction of this type of liquid insulation in the electrical industry.\u003c/p\u003e\u003c/div\u003e\u003c/p\u003e"},{"header":"V. CONCLUSIONS","content":"\u003cp\u003eThe lack of knowledge about the degradation or aging processes in NE is evident, but it must impel scientific efforts to increase the knowledge in order to take advantage of the benefits and reduce the uncertainty about their use in the electrical industry.\u003c/p\u003e\u003cp\u003eThere is a set of references and literature published about NE used in electrical power transmission and distribution, but the wide variety of NE available requires more detailed studies on their chemical properties and on the reactions that occur in their components due to thermal stress and electrical stress.\u003c/p\u003e\u003cp\u003eThe perceptions expressed by all stakeholders in the development of an NE industry for use in power transformers suggest that there is significant interest, awareness of the environmental benefits, and expectations regarding new applications and performance of natural esters in field applications, but there is significant uncertainty regarding specific aspects such as maintenance, operating costs, the supply chain, and safety. These issues must be resolved in the medium term to improve the penetration of this type of insulation in the national and international electrical markets.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eAll authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by Alejandro Paz Parra and Juan Carlos Aristizabal. Diego Garc\u0026iacute;a revised the article and complement the search of the information. Fernando Sebastian Villa is a support researcher and contribute to collect de data and process the results of the survey. The comments and depuration of the information included in the article where revised and approved by all authors. The first draft of the manuscript was written by Alejandro Paz Parra and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.\u003c/p\u003e\u003ch2\u003eAcknowledgement\u003c/h2\u003e\u003cp\u003eAuthors want to aknowledge the financial support ot the Universidad Santiago de Cali like the institution that offers financial support to the corresponsal author Alejandro Paz Parra to participate in the research team that produces the present article. Authors aknowledge the financial support of the Universidad del Valle represented in the dedication time of the authors Diego Garc\u0026iacute;a, Juan Carlos Aristizabal and Fernando Sebastian Villa. Authors aknowledge the financial support of the company Geiico that is a private electrical sector company that provides the logistic to collect the data of the survey, to contact the businessmen of the other companies of the electrical sector that participates in the survey and that contribute to improve the findings and conclusions of the research.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eZ. Shen, F. Wang, Z. Wang y J. Li, \u0026laquo;A critical review of plant-based insulating fluids for transformer: 30-year development,\u0026raquo; \u003cem\u003eRenewable and Sustainable Energy Reviews\u003c/em\u003e, vol. 141, 2021.\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eR. Soni y B. 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Page, \u0026laquo;Reducing Environmental Impact and Improving Safety and Performance of Power Transformers With Natural Ester Dielectric Insulating Fluids,\u0026raquo; \u003cem\u003eIEEE Transactions on Industry Applications\u003c/em\u003e, vol. 50, n\u0026ordm; 1, pp. 134\u0026ndash;141, 1 2014.\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"electrical-engineering","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"elen","sideBox":"Learn more about [Electrical Engineering](http://link.springer.com/journal/202)","snPcode":"202","submissionUrl":"https://submission.nature.com/new-submission/202/3","title":"Electrical Engineering","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Natural esters, power transformers, liquid insulators, mineral oil, risk.","lastPublishedDoi":"10.21203/rs.3.rs-7935350/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7935350/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eFrom the beginning of the electrical industry, the use of power transformers became necessary for the expansion of electricity service wherever an electrification process began. The rapid growth of electricity demand forces the industry to use more power equipment and higher voltage power lines to transmit larger volumes of electric power over longer distances, making the use of power transformers indispensable. As the requirements for high voltage and power loss management increase, the use of new dielectrics to insulate the active parts of power equipment has become necessary to achieve higher insulation levels and efficient power loss mitigation. Mineral oils have provided a solution despite their inflammability and possible contamination of water and soil. Recently, natural esters have offered a modern solution to the same problems but with numerous advantages like their high water-solubility, fast degradation, and lower inflammability. Consequently, natural esters have been introduced into the electrical industry's manufacturing chain; however, their penetration in the power transformer market is still low and unrepresentative in terms of the number of operating units insulated with NE. Thus, the researchers decided to conduct a survey that involved a set of manufacturers, decision-makers, marketers, researchers, asset owners, and users of power transformers to identify the main barriers that slow the adoption of power transformers that use NE into the power grid. Subsequently, a review of the literature allows us to approximate some considerations that we expect will contribute to clarify some of the identified barriers and to establish a set of new research lines about the use of NE in power transformers.\u003c/p\u003e","manuscriptTitle":"An overview of main barriers to adopt the use of natural esters (NE) to replace mineral oils (MO) in the insulation of power transformers in Colombia","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-11-26 13:04:47","doi":"10.21203/rs.3.rs-7935350/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2026-02-25T00:30:56+00:00","index":"","fulltext":""},{"type":"reviewerAgreed","content":"310551197580767297345688857008550496357","date":"2025-11-19T19:23:05+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-11-18T10:34:03+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"286346962290596880165182286434555660820","date":"2025-11-16T06:21:43+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-11-15T02:35:02+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-11-04T05:51:56+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-10-25T07:14:40+00:00","index":"","fulltext":""},{"type":"submitted","content":"Electrical Engineering","date":"2025-10-23T21:52:31+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"electrical-engineering","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"elen","sideBox":"Learn more about [Electrical Engineering](http://link.springer.com/journal/202)","snPcode":"202","submissionUrl":"https://submission.nature.com/new-submission/202/3","title":"Electrical Engineering","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"814e3e9e-5dc1-48c6-b284-88f2c06448dd","owner":[],"postedDate":"November 26th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2026-05-18T02:53:22+00:00","versionOfRecord":[],"versionCreatedAt":"2025-11-26 13:04:47","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7935350","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7935350","identity":"rs-7935350","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}