Enhancing the Performance of Natural Ester Insulating Liquids in Power Transformers: A Comprehensive Review on Antioxidant Additives for Improved Oxidation Stability

Abstract

The reliability of the electrical grid is vital to economic prosperity and quality of life. Power transformers, key components of transmission and distribution systems, represent major capital investments. Traditionally, these machines have relied on petroleum-based mineral oil as an insulating liquid. However, with a global shift toward sustainability, renewable insulating materials like natural esters are gaining attention due to their environmental and fire safety benefits. These biodegradable liquids are poised to replace hydrocarbon-based oils in transformers, aligning with Sustainable Development Goals 7 and 13 by promoting clean energy and climate action. Despite their advantages, natural esters face challenges in high-voltage applications, particularly due to oxidation stability issues linked to their fatty acid composition. Various antioxidants have been explored to address this, with synthetic antioxidants proving more effective than natural ones, especially under high-temperature conditions. Their superior thermal stability ensures that natural esters retain their cooling and dielectric properties, essential for transformer performance. Furthermore, integrating machine learning and artificial intelligence in antioxidant development and monitoring presents a transformative opportunity. This review provides insights into the role of antioxidants in natural ester-filled power equipment, supporting their broader adoption and contributing to a more sustainable energy future.

1. Introduction

Plant-extracted oils, often referred to as natural ester liquids, have found multiple applications within power equipment in the electrical industry, with a particular emphasis on high-voltage transformers. Natural esters have emerged as a highly attractive option in electrical insulation owing to their biodegradability, eco-friendliness, and enhanced fire safety characteristics [1,2]. Additionally, vegetable oils possess hydrophilic characteristics and can safeguard solid insulators, such as cellulose, from deterioration, a feature that sets them apart from mineral oils [3,4]. This moisture-attracting ability can be ascribed to their polar nature and the presence of hydroxyl groups, which readily enable the formation of water–oil emulsions [5,6,7,8,9]. Furthermore, natural ester-based insulating liquids have dielectric strength and dielectric permittivity higher than that of mineral oil [10,11]. The significant dielectric permittivity of natural esters is a crucial factor that, when combined with cellulose paper, ensures better distribution of electrical stress, effectively reducing partial discharges within the insulation system [12,13]. Figure 1 summarizes the qualities of natural esters as excellent insulating liquids for transformer insulation [14].

Natural esters, derived from plants, comprise a complex mixture of fatty acids that fundamentally define their properties. It is worth noting that the timing of planting can exert an influence on the characteristics of the oil extracted from the seeds [15]. During the cold test, it was observed that oils extracted from seeds grown in the dry season crystallize rapidly. This rapid crystallization can be attributed to the high content of saturated fatty acids produced in response to dry stress conditions, indicating that the planting time also has a significant influence on the oil’s properties [16]. The fatty acids can be categorized into three groups: saturated, monounsaturated, and polyunsaturated [17]. The green insulating liquids used in electrical appliances are derived from several plant seeds like soybean, rapeseed, canola, and sunflower [18,19,20,21,22,23]. A significant majority of these oils are characterized by a prevalence of mono or polyunsaturated fatty acids. Oils rich in long-chain unsaturated fatty acids are favored for their reliable performance in subzero climates. Nevertheless, the susceptibility of these oils to oxidation raises concerns that cannot be neglected, particularly when transitioning from monounsaturated to polyunsaturated compositions [24,25]. This is because the oxidation of vegetable-based insulating oils generates several unwanted products, which are detrimental to the lifespan of transformers [6,26,27,28]. Among the products generated during the oxidation reaction of vegetable-based insulating oils are moisture, alkane, ketones, alcohols, and aldehydes [6,29].

It is worth noting that not only does the oxidation process produce acids, but hydrolysis also plays a significant role in elevating the acidity of natural esters [30,31]. Short-chain fatty acids can be more susceptible to hydrolysis, in part due to the steric hindrance phenomenon, along with other factors such as chain length and saturation level [30]. There are three major stages in both oxidation and hydrolysis of natural esters. Figure 2a,b show the progressive oxidation reaction and the reversible hydrolysis reaction, respectively [32].

Acids are core chemical markers that are used for monitoring the degradation of both oil and insulating paper [33]. Unlike low molecular weight acids (LMAs), the acids generated by natural esters are high molecular weight acids (HMAs), which are hardly soluble in the oil with low impact on the insulating system of the transformer, especially the paper [34,35]. Nevertheless, the accumulation of the acids generated could affect the dielectric properties of both the oil and the paper insulation. Moreover, these high molecular weight acids can increase the viscosity of the oil, impeding its heat dissipation capabilities and potentially affecting its insulation performance and efficiency. While aging is influenced by time and various physical conditions, such as temperature, the introduction of antioxidants can help reduce the degradation process of the oil. Numerous antioxidants are documented in the literature, categorized into donors and acceptors. The donors provide hydrogen, whereas acceptors trap free radicals to create stable compounds [28]. Furthermore, the introduction of antioxidants into the base liquid positively influences specific dielectric properties of natural esters, such as the AC breakdown voltage [1,23]. However, the loading percentage is a crucial consideration when incorporating antioxidants into natural esters following the regulation by the CIGRE working group [36]. Excessive loading of antioxidants in the base sample (>0.3 wt.%) can potentially elevate critical oil parameters, such as conductivity and dielectric loss, which may adversely affect the insulating quality of transformer oil.

This review study aims to scrutinize the improvement of oxidation stability in natural esters. It reports on the efficacy of antioxidants utilized in recent studies. Additionally, it investigates the effects of antioxidants on physical properties and dielectric properties and examines their environmental impact. Improving the oxidation stability of natural ester insulating liquids is key to their broader adoption in transformer insulation. This transition will eliminate the use of environmentally harmful mineral-based insulating oils, reducing environmental pollution, lowering the carbon footprint of transformers, and supporting clean and renewable energy initiatives.

2. Oxidation Mechanism in Natural Esters

The degradation of natural esters via the oxidation process is initiated due to primary factors such as oxygen, heat, light, and potential impurities [28]. When natural esters are exposed to air at higher temperatures, oxygen molecules undergo hemolysis, generating free radicals within the oil. Additionally, an electrical insight into how radicals are formed in petroleum-based insulating liquids is explicitly reported in reference [37]. A free radical is defined as any molecular species that contains an unpaired electron in an atomic orbital capable of independent existence. Free radicals may also result from hydrogen detachment at the weak bond of the α carbon and hydrogen. The presence of copper windings is also among the factors that catalyze the process of oxidation. These radicals are highly reactive due to unpaired electrons, which react with oxygen, forming peroxide free radicals [38]. A laboratory testing technique by UV-vis spectroscopy techniques is proposed in [35] to assess the relative concentration of free radicals in insulating oil. Instability prompts their reaction with unsaturated bond sites in fatty acid chains, producing hydroperoxides and further radicals. This perpetuates a continuous chain reaction known as the propagation stage of oxidation [26]. The chain reaction is hindered in two distinct ways. Firstly, an antioxidant, a radical scavenger, is introduced, halting the chain reaction. These antioxidants act as donors by contributing hydrogen or acceptors by absorbing free radicals within the system. Secondly, natural inhibition occurs when two free radicals unite. The former process preserves oil properties unchanged, whereas the latter notably alters oil characteristics by elevating viscosity and acidity and exhibiting a distinct color change associated with oxidation byproducts like ketones [30]. Furthermore, increased viscosity poses a significant detriment to the transformer’s functionality, impacting oil circulation and subsequently compromising the cooling system. Hence, the incorporation of inhibitors into the base oil is crucial when contemplating the use of natural esters as a dielectric liquid.

Over time, diverse inhibiting materials have been used to improve the oxidation stability of natural esters for academic and industrial applications. They are classified as either natural or artificial inhibitors based on their sources. Indeed, despite their importance, reports suggest that the incorporation of inhibitors could affect the biodegradability of these liquids, notably through their biological oxygen demand, especially in the case of chemically synthesized inhibitors [31].

2.1. Effect of Oxidation on Transformer Useful Life and Sustainability

The lifespan of a transformer is tied to the integrity of its oil–paper insulation system [39,40]. Any contamination in the insulating liquids significantly impacts the overall stability of the transformer, posing a threat to its functionality. Contaminants, notably moisture and particulate matter, escalate the risk of electrical faults and impede proper heat dissipation within the transformer. The effect of oxidative reaction on both oil and paper are, therefore, discussed as follows.

2.1.1. Impact of Oxidation Reaction on Oil Properties

The oxidative degradation of natural ester insulating liquids is closely linked to their chemical structure, particularly the presence of carbon–carbon double bonds (C=C). These unsaturated sites, unlike mineral oils, render natural esters more susceptible to oxidation [41]. The extent of oxidation stability largely depends on the degree of unsaturation in the feedstock used for producing the esters. Esters derived from saturated, monounsaturated, di-unsaturated, and tri-unsaturated fatty acids exhibit oxidation stability ratios of approximately 1:10:100:200, respectively, meaning tri-unsaturated esters are the most prone to oxidation [32]. The oxidation process typically initiates at the allylic position, the carbon atom adjacent to a double bond, where hydrogen abstraction leads to the formation of free radicals. These radicals undergo a chain reaction involving oxygen, resulting in hydroperoxides, which eventually break down into a variety of secondary oxidation products. A significant consequence of oxidation in natural esters is the increase in oil viscosity [26], primarily due to oligomerization, where radical recombination leads to the formation of larger molecular structures [41]. This viscosity increase negatively affects the heat transfer efficiency of the transformer, as there is an inverse relationship between dynamic viscosity and the heat transfer coefficient [42,43,44]. Prolonged oxidation and viscosity buildup can lead to local overheating, impairing transformer performance and potentially triggering thermal failure. Additionally, when the viscosity rises beyond 35% of its original value, the reclamation of natural esters becomes technically and economically unfeasible [45], emphasizing the critical need for early detection and mitigation of oxidation processes.

2.1.2. Impact of Liquid Oxidation on Insulating Paper

The degradation of natural esters due to oxidation significantly impacts the properties of the insulating paper, leading to a cascade of effects that jeopardize the transformer’s performance and longevity. As oxidation byproducts, carboxylic acids, and water accumulate, they interact with the solid insulation (typically impregnated cellulose paper), resulting in further degradation [40]. While natural esters tend to generate high molecular weight acids with lower absorption rates by paper compared to mineral oils [41], the combined presence of acidity and moisture remains detrimental to paper insulation. One of the most severe impacts is acid hydrolysis, a chemical process where the acidic oxidation byproducts degrade the cellulose fibers in the paper [32]. During this reaction, acidic compounds dissociate in the presence of water, forming hydronium ions (H₃O⁺) that can donate protons to the cellulose chains. This proton transfer induces a chain scission process, breaking down the cellulose structure, which weakens the paper’s mechanical properties and insulation effectiveness [34,46,47]. Furthermore, acid-catalyzed hydrolysis is self-perpetuating; for every linkage cleaved in the cellulose, one molecule of water is consumed and three water molecules are released, creating an auto-catalytic effect. This process accelerates the degradation of paper insulation, as each cycle of hydrolysis leads to more breakdown of the cellulose chains. In addition to acid hydrolysis, hydrolysis degradation reactions, which occur simultaneously with oxidation reactions in natural esters, can also lead to the formation of long-chain free fatty acids (FFAs) and diacetyl glycerol (DAG) [41]. These byproducts, which result from the hydrolysis of triglycerides in the natural esters, can further impact the dielectric properties of the paper insulation. When these products recombine with cellulose through transesterification, they alter the electrical characteristics of the paper, particularly its ability to resist electrical stress. Moreover, hydroxy radicals generated during the oxidation process, especially from the decomposition of hydrogen peroxide, can catalyze the oxidation of cellulose itself, generating carbon dioxide and water [32]. This process weakens the dielectric strength and reduces its ability to perform as an effective insulator under high voltage conditions. Therefore, while the oxidation of natural esters is a primary concern for the oil’s performance, the resulting byproducts, moisture and acids, further deteriorate the solid insulation system. This compounded effect ultimately compromises both the liquid and solid components of the transformer insulation system, making it essential to monitor and mitigate oxidation processes to safeguard transformer integrity, enhance sustainability, and extend the operational lifespan.

3. Antioxidants Classification

The primary function of antioxidants is to avert the activation and proliferation of oxidation mechanisms [38]. The antioxidants currently utilized for the enhancement of natural ester performance in transformer insulation systems fall into two distinct categories: natural and synthetic antioxidants. Natural antioxidants primarily originate from plant-based materials, often extracted from fruits, herbs, and plant seeds [48]. Conversely, synthetic antioxidants are synthetic compounds typically created in laboratory settings, devoid of natural occurrence in plants or any organic sources. While both types demonstrate efficacy in their respective roles, each holds specific advantages. Natural antioxidants align with environmentally conscious initiatives, being readily biodegradable when applied in transformer systems. However, the consistency and stability of natural antioxidants might pose challenges due to environmental influences. On the other hand, synthetic antioxidants offer predictable performance and stability, yet their usage may necessitate rigorous testing to ensure regulatory compliance and environmental compatibility. It is also important to know that the functionality of the antioxidants depends on the fatty acid source and the degree of oil unsaturation. This could be the reason for having a high percentage improvement in the enhancement of cotton seed biodiesel relative to that of palm biodiesel when an antioxidant like butylated hydroxytoluene (BHT) was considered [49].

3.1. Natural Antioxidant

The significance of natural antioxidants extends beyond their role in the food and pharmaceutical industries, finding application in the improvement of transformer insulating fluids. Derived from various plant sources like seeds and leaves, these natural antioxidants not only align with environmental regulations but also contribute to the eco-friendly nature of the industry [50]. For instance, tocopherol is sourced from soybean oil, ascorbic acid from citrus fruits, polyphenols from grapes and tea leaves, and carotenoids from the lycopene found in tomatoes [51,52]. Throughout plant routine metabolic processes, secondary metabolites emerge from various plant components such as the roots, stems, seeds, leaves, and nuts [53]. Unlike primary metabolites, which drive the plant’s photosynthesis, respiration, and growth, secondary metabolites encompass a diverse range of phytochemicals abundantly present in plants. These include phenolics, tocopherols, flavonoids, lignans, peptides, and more [54]. A comprehensive list of natural antioxidants is presented in

Table 1. A comprehensive list of natural antioxidants.

Sn	Antioxidants	Chemical Formula	Source	Reference
1	Quercetin	C15H10O7	Apples, onions and berries	[55]
2	Cardanol, epoxy cardanol	C15H27OH	Cashew nut shell liquid	[56]
3	Rosemary	C18H16O8	Rosmarinus species	[57]
4	Gallic acid, caffeic acid and ferulic acid	C7H6O5, C9H8O4, C10H10O4	Macerated barley waste and Moringa Oleifera leaves	[58,59]
5	Camphor	C10H16O	Camphor plant	[49]
6	Limonene	C10H16	Lemon and orange	[49]
7	Citric acid (anhydrous)	C6H8O7	Citrus fruits	[60,61]
8	L-Ascorbic acid	C6H8O6	Citrus fruits	[62,63]
9	α-tocopherol	C31H52O3	Vegetable oils, fruits	[64]
10	Carotenoids	C40H56	Fruits, vegetables	[65]
11	Lignin	-	Forestry residue	[66,67]

.

3.1.1. Phenolics

Phenolic compounds are mostly hydrogen donors or metallic chelators and it is a group of diverse secondary metabolites having over 8000 compounds [68]. They are mostly found in the roots, leaves, and fruits of a plant. The phenolics are classified into two different components which are the simple and complex phenolics. Benzoic acids, cinnamic acids, and caffeic acids are examples of simple phenolics, while tannins and flavonoids are examples of complex phenolics. The chemical structure with the IUPAC nomenclature of some simple phenolics can be seen in Figure 3.

3.1.2. Flavonoids

Flavonoids, classified as polyphenolic compounds, exhibit potent antioxidant properties [68]. Distributed throughout various parts of plants, these compounds play a significant role in medicinal applications by defending against diseases induced by oxidative stress. They achieve this by modulating enzymatic activities and interacting with specific receptors. Higher plant tissues consist of various types of flavonoids, including flavanols, flavanones, isoflavones, chalcones, and flavones [54].

3.1.3. Ascorbic Acid

Ascorbic acid, also known as vitamin C, serves as a potent antioxidant with radical-scavenging capabilities. Being water-soluble, it is present in all parts of plants [54]. This antioxidant functions as a reducing agent, hindering the oxidation process by contributing electrons (reducing equivalents) in chemical reactions. It demonstrates effectiveness against radicals such as ${\mathrm{O}}_2^{-}$ (superoxide), H₂O₂ (hydrogen peroxide), OH (hydroxyl), and ¹O₂ (singlet oxygen) [69,70]. Upon complete reduction, it transforms into a radical known as monodehydroascorbate, which acts as a scavenger in chemical reactions by interacting with radicals. Most importantly, monodehydroascorbate not only scavenges radicals but also terminates radical chain reactions, preventing the continuous propagation of radical reactions and thereby exerting control over the oxidation process. Within lipids, tocopherol, also recognized as vitamin E, serves to reduce free radicals. However, this reduction process results in the formation of a tocopheroxyl radical. The presence of ascorbic acid plays a role in reducing the tocopheroxyl radicals generated during this reaction. The structural formula of ascorbic acid is presented in Figure 4. It is important to know that this antioxidant is sensitive to some environmental conditions like temperature and light, in which temperature is considered the dominating factor that determines the stability of ascorbic acid. When a material containing ascorbic acid antioxidant is exposed to a temperature range above 100 °C for two hours, ascorbic antioxidant degradation occurs, which generates oxidation products like furfural, 2-furoic acid, 3-hydroxy-2-pyrone, and an unknown compound [70]. Applying this antioxidant in transformer insulation systems may pose challenges.

Moreover, the thermal degradation of the antioxidant produces by-products resembling those found in cellulose-based insulating materials in transformers. The oxidation product resulting from the antioxidant when the transformer operates at high temperatures during energization could be misconstrued as cellulose degradation during routine maintenance. Moreover, ascorbate is primarily derived from citrus fruits, melons, broccoli, tomatoes, and cabbage. Therefore, careful attention should be paid to the agricultural sector to avoid food scarcity and also prevent greenhouse gas emissions during large-scale cultivation when planning to adopt this antioxidant for enhancing oxidation stability in transformer-insulating oil.

3.2. Synthetic Antioxidants

The special attributes of synthetic antioxidants are the ability to withstand several conditions and a prolonged shelf life. They are considered superior to natural antioxidants due to their stability in products with prolonged storage times and consistent performance and effectiveness, as well as their high resistance to temperature degradation. In the context of enhancing transformer insulating liquids, several synthetic antioxidants have been explored and their effects documented. These include tert-butyl hydroquinone (TBHQ), butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), propyl gallate (PG), and lauryl tert butyl hydroquinone [23,71,72,73].

Table 2. Common synthetic antioxidants.

Sn	Antioxidants	Chemical Formula	Source	Reference
1	N,N′-disec-butyl-p-phenylenediamine	C22H38N2	Reaction of p-phenylenediamine with sec-butyl alcohol	[74,75]
2	Tert-butylhydroquinone	C10H14O2	Petroleum	[76,77]
3	Propyl gallate (PG)	C10H12O5	Esterification of gallic acid	[78,79]
4	Butylated hydroxy toluene	C15H24O	Petroleum	[80]
5	1,2,3-trihydroxybenzene (Pyrogallol)	C6H6O3	Hydrolysis of gallotannins or oxidation of gallic acid	[80]
6	Butylated hydroxyl anisole	C11H16O2	Reaction of p-methoxyphenol and isobutylene	[81]

gives a comprehensive list of synthetic antioxidants.

3.3. Antioxidant Activity Assessment

Understanding the activity of antioxidants used in enhancing the oxidation stability of natural esters is crucial when it comes to the selection of appropriate antioxidants. In a controlled system, the rate and amplitude of oxidant formation should be neutralized by antioxidants. However, in natural systems, there is often an imbalance between pro-oxidants and antioxidants, leading to oxidative stress. An increase in reactive oxygen species (ROS) in natural ester insulating liquids accelerates the aging of these liquids, which can be detrimental to transformer life [82]. The common reactive oxygen species during the degradation of natural esters include peroxyl radicals, hydroxyl radicals, and alkoxyl radicals. An example of a non-free radical species is hydrogen peroxide. This imbalance may be attributed to the activity and effectiveness of the antioxidants themselves. Additionally, the activity of antioxidants depends on several factors, such as their chemical structure, which directly influences their intrinsic reactivity to free radicals and other ROS.

The enhancement of oxidation stability in natural ester insulating liquids can be fundamentally influenced by molecular-level interactions between antioxidant molecules and the fatty acid chains present in the oil matrix. These interactions are essential in interrupting the autoxidation chain reactions, which are typically initiated by the attack of free radicals on unsaturated fatty acids. Due to the presence of double bonds, particularly at the allylic positions, unsaturated fatty acids are especially susceptible to radical-induced degradation [41]. The effectiveness of antioxidants in scavenging these radicals is determined by several structural attributes, including aromaticity, polarity, steric configuration, and the presence of functional groups [83].

Among these structural features, aromaticity plays a particularly crucial role in radical scavenging efficiency. Many synthetic antioxidants, such as butylated hydroxytoluene (BHT), butylated hydroxyanisole (BHA), and hindered phenolic compounds like Irganox 1076 and Irganox L57, contain aromatic rings bearing phenolic hydroxyl groups [84,85]. These molecular structures facilitate resonance stabilization of the antioxidant radical that forms after hydrogen donation. Specifically, when a phenolic antioxidant donates a hydrogen atom to a lipid peroxyl radical (ROO^•), it forms a phenoxyl radical, which is subsequently stabilized by delocalization of the unpaired electron across the aromatic ring. This stabilization reduces the reactivity of the resulting radical, thereby hindering the propagation phase of the oxidative chain reaction [85]. Furthermore, the efficiency of these antioxidants is strongly influenced by the nature and positioning of substituent groups on the aromatic ring. Bulky alkyl substituents, such as tert-butyl or octadecyl groups, positioned at the ortho or para locations, enhance radical stability through steric hindrance and electronic effects, making such antioxidants particularly effective under elevated thermal conditions commonly encountered in transformer operation. Additionally, the presence of electron-donating groups like –OH or –OCH₃ further increases the antioxidant’s hydrogen-donating capacity, whereas electron-withdrawing groups may diminish the compound’s radical-quenching potential [85,86]. Polarity and solubility also play critical roles in determining antioxidant performance, particularly in relation to their compatibility with the ester matrix [87,88]. Polar antioxidants tend to associate more closely with ester functional groups and may preferentially localize in polar domains of the insulating fluid, such as at oil–paper interfaces or in regions rich in polar degradation byproducts. In contrast, non-polar antioxidants disperse more uniformly throughout the bulk oil, providing widespread oxidative protection. Consequently, the solubility and dispersion behavior of antioxidants significantly influence their spatial availability for effective radical scavenging [89,90,91].

The distribution and concentration of the antioxidants in the base liquid are also critical for their activity. Furthermore, the kinetics of the reaction, including the thermodynamics and the ability of antioxidants to react, play a significant role in determining their effectiveness [92].

The methods for determining antioxidant activity based on techniques are categorized into spectrometry, electrochemical assays, and chromatography [82,93].

Spectrometry-based assays: This includes several methods for evaluating antioxidant capacity. The ORAC (oxygen radical absorbance capacity) assay operates on the principle of an antioxidant’s reaction with peroxyl radicals, which are induced by 2,2′-azobis-2-amidino-propane (AAPH). The efficacy of the antioxidant is determined by observing the loss of fluorescence of fluorescein. The HORAC (hydroxyl radical averting capacity) assay measures the antioxidant’s ability to quench hydroxyl radicals generated by a cobalt (II)-based Fenton-like system, with the outcome also determined by the loss of fluorescein fluorescence. Similarly, the TRAP (total radical-trapping antioxidant parameter) assay evaluates the antioxidant’s capacity to scavenge luminol-derived radicals from AAPH decomposition, with chemiluminescence quenching serving as the determinant. Other spectrometric methods include CUPRAC (cupric ion reducing antioxidant capacity), FRAP (ferric reducing antioxidant power), PFRAP (potassium ferricyanide reduction antioxidant power), and DPPH (2,2-diphenyl-1-picrylhydrazyl), all of which involve colorimetric determinations based on the reduction of specific compounds by antioxidants [82].

Electrochemical techniques: These offer another approach to analyzing antioxidant activity. Voltammetry is a technique based on the reduction or oxidation of a compound at the working electrode’s surface when an appropriate potential is applied, with the assay’s effectiveness measured by the cathodic or anodic peak current. Another technique is amperometry, which involves setting the working electrode’s potential at a fixed value relative to a reference electrode, with the current produced by the oxidation or reduction of the electroactive analyte serving as the measure. Biamperometry, another electrochemical technique, assesses antioxidant activity by measuring the current passing between two identical working electrodes that are subjected to a slight potential difference and submerged in a solution containing the sample and a reversible redox couple [82].

Chromatography techniques: These include gas chromatography (GC) and high-performance liquid chromatography (HPLC). In gas chromatography, the separation of compounds in a mixture is achieved by their distribution between a liquid stationary phase and a gas mobile phase, with the results detected through flame ionization or thermal conductivity detection. High-performance liquid chromatography, on the other hand, involves the separation of compounds by their partitioning between a solid stationary phase and a liquid mobile phase under conditions of high flow rate and pressure. The separated compounds are detected using various methods, including UV-Vis detection, fluorometric detection, mass spectrometry, or electrochemical detection [82]. Determination of antioxidant activity can be categorized into two different types: hydrogen atom transfer (HAT) and single electron transfer (SET).

3.3.1. The Hydrogen Atom Transfer (HAT) Reaction Method

The efficiency of antioxidants can be determined by measuring the ability of antioxidants to remove a free radical through the donation of a hydrogen atom. In this process, the donating ability of antioxidants is determined by monitoring hydrogen atom transfer from the phenol to the peroxyl radical. It is, therefore, important for the antioxidant, the hydrogen donor, to react faster than the molecules being protected [94]. Several tests are available to evaluate the hydrogen atom transfer capability of a donor antioxidant. Among these are the oxygen radical absorption capacity (ORAC) and the total peroxyl radical trapping antioxidant parameter (TRAP) [95].

ORAC Test: The ORAC test assesses antioxidant efficiency by measuring how well antioxidants can inhibit the oxidation initiated by peroxyl radicals. The peroxyl used in this assay is generated from azo-compounds such as lipophilic α,α-azobisizobutyronitrile (AIBN), 2,2′-azobis (2-amidinopropane) hydrochloride (ABAP), 2,2′ azobis(2,4-dimethylvaleronitrile) (AMVN), and the hydrophilic 2,2′-azobis(2-amidinopropane) dihydrochloride (AAPH) [82]. The generated peroxyl radicals react with a fluorescent sample, resulting in a reduction in fluorescence. The data are then analyzed using the area-under-the-curve method, comparing results both in the presence and absence of the antioxidant.

TRAP Test: This test is performed by measuring the ability of the antioxidants to inhibit the reaction between the target molecules and the peroxyl radicals. The oxygen absorption rate is a significant factor in determining the efficiency of antioxidants in this method. When peroxyl radicals react with the target molecules, there is always an oxygen consumption; however, a retardation in the consumption time, i.e., the period in which the antioxidants reduce the reaction between the peroxyl and the target molecules, signifies the efficiency of the antioxidants. In this assay, the trap value is always expressed as the induction period following the time in which the antioxidant effectively delays the oxidation reaction.

3.3.2. The Single Electron Transfer (SET) Test

In this test, the reduction of metallic ions, carbonyl groups, and free radicals by the antioxidant through the transfer of electrons shows the ability of antioxidants as an oxidation inhibitor. The single electron transfer method depends on the pH value since reactivity in single electron transfer is based on the deprotonation and ionization potential of the reactive functional group [96]. Unlike the hydrogen atom transfer techniques that use peroxyl radicals when measuring antioxidant activity, this method uses fluorescent or colored samples. Examples of tests under this category are the ferric reduction of antioxidant power (FRAP) and the cupric antioxidant capacity (CUPRAC).

3.4. Natural Esters–Antioxidant Preparation

The preparation of natural insulating liquids with antioxidants can be seen in Figure 5. It is important to know that the preparation method could affect the percentage of oxidation stability enhancement. In some cases where the antioxidant is not soluble in natural ester, pre-dissolving the antioxidant in a suitable solvent is paramount as it ensures homogeneity and ready interaction between the antioxidant and the oil molecules. The solvent used in the pre-dissolving process can be removed through the process of distillation. The most commonly used polar solvent for dissolving antioxidants is ethanol which can be easily separated from the oil using a distillation process. Among the factors to be considered during the sample preparation for oxidation stability enhancement of natural esters are moisture content, the stirring time, and the temperature of the reaction.

3.4.1. Moisture Content

The presence of high moisture concentration in natural esters can lead to a chemical process known as hydrolysis, as presented in Figure 2b. Moisture presence causes triglycerides to break down into their simplest forms, glycerol and fatty acids. The resulting fatty acids from hydrolysis are more susceptible to oxidation compared to the stable triglycerides, impacting the quality of insulating liquids. Therefore, it is crucial to minimize the moisture content of the base liquid during sample preparation. The American Society for Testing and Materials (ASTM) 6871 specifies that the moisture content of natural esters should be equal to or less than 200 ppm [97,98], which is a safety range to prevent the transformers from metal corrosion and prevent the insulators from degradation. In addition, it is important to know that when considering natural esters dominated by short-chain fatty acids, especially C8/C10, a very low concentration of moisture is required as the short-chain fatty acids are susceptible to hydrolysis [30].

3.4.2. Stirring Time and Temperature

Ensuring an optimal stirring duration is crucial in the preparation of natural esters and antioxidants. Adequate stirring time promotes uniform dispersion of antioxidant molecules, enabling a smooth integration with oil molecules. Insufficient distribution of antioxidants may result in localized areas with low concentrations, adversely affecting the desired properties of the oil. The reaction temperature is another critical factor in sample preparation. Various antioxidants exhibit distinct solubility temperatures in oil. It is imperative to carefully select and maintain an optimum temperature to prevent heat-induced degradation of the oil while ensuring the effective incorporation of antioxidants.

Several multiparameter optimization methods are available in the literature that have been used for natural ester–antioxidant preparation [72,73,97]. These methods can be used to optimize the stirring time and the temperature of the reaction during natural ester oxidation stability enhancement.

3.5. Antioxidant Mechanism in Insulating Liquids

Antioxidants operate according to their inherent properties. Their mechanism can be complex and varies based on the type of antioxidant and the condition under which they operate. Primarily, antioxidants protect the integrity of oils by scavenging free radicals, such as hydroperoxyl and alkyl radicals, as illustrated in Figure 2a [99]. An antioxidant can act either as a donor or an acceptor [28]. Donor antioxidants work by donating a hydrogen atom to free radicals, effectively neutralizing their harmful effects. Acceptor antioxidants, which are known as the chain breakers, stop the reaction by reacting with the free radicals. The classification of some of the common antioxidants is shown in

Table 3. Antioxidant classification according to behavior [103].

Antioxidants
Donor	Acceptor	Acceptor/Donor
Vitamin C (Ascorbic Acid)	Superoxide Dismutase (SOD)	Glutathione
Vitamin E (Tocopherol)	Catalase	Trolox
Butylated Hydroxytoluene (BHT)
Butylated Hydroxyanisole (BHA)
THBQ
N,N′-disec-butyl-p-phenylenediamine

, and the equation describing the scavenging activity of both the donor and the acceptor is presented in Figure 6. In operational transformers at elevated temperatures, the dissolved oxygen content in the oil increases, which can raise the concentration of metal ions due to the presence of transformer windings [100]. Antioxidants, particularly metal chelators, bind to these metal ions, inhibiting their catalytic activity and preventing the decomposition of hydroperoxides even under high-temperature conditions [101,102]. At elevated temperatures, the activation energy of the oil decreases, increasing the likelihood of oxidative reactions. However, antioxidants mitigate this by raising the activation energy required for radical chain reactions, thereby slowing down the oil’s degradation. Despite their benefits, the effectiveness of antioxidants is highly dependent on the transformer’s operating temperature. Higher temperatures often compromise their stability and efficiency. Consequently, further research is crucial to develop antioxidants with enhanced thermal stability and effectiveness for high-temperature applications, such as in transformer oils.

3.6. Natural Esters’ Oxidation Stability Assessment

The oxidation assessment of natural esters is still a point of concern as there is no established or approved method for evaluating the oxidation stability of natural esters [104]. Due to variations in the fatty acid composition of natural esters, which pose variations in oxidation stability, establishing a method of oxidation stability assessment may be a challenge. The variation in the fatty acids composition of natural esters is a result of different plant sources as presented in

Table 4. Percentage composition of oil fatty acids from different plants.

	Fatty Acid	Chemical Structure	Palm Kernel Oil	Coconut Oil	Canola Oil	Soybean Oil	Cotton Seed Oil	Neem Oil	Sunflower Oil	Rapeseed Oil	Safflower Oil
Oxidation stability decreases downward	Caprylic acid C8:0	s	3.30	9.16	-	-	-	-		-	-
	Capric acid C10:0	s	3.40	6.44	-	-	-	-		-	-
	Lauric acid C12:0	s	48.20	45.57	-	-	-	-		-	-
	Myristic acid C14:0	s	16.20	16.65	0.07	0.08	1.00	-		-	-
	Palmitic acid C16:0	s	8.40	8.21	1.29	14.04	23.70	18.10	7.00	4.00	7.00
	Stearic acid C18:0	s	2.50	3.42	2.40	4.07	3.40	18.10	5.00	2.00	2.00
	Palmitoleic acid C16:1	mus	-	-	0.29	0.09	0.60		-	-	-
	Oleic acid C18:1	mus	15.40	6.27	64.40	23.27	19.40	44.50	19.00	56.00	13.00
	Erucic acid C22:1	mus	-	-	0.50	-	-	-		-	-
	Linoleic acid C18:2	pus	2.30	1.40	17.40	52.18	53.20	18.30	68.00	22.00	78.00
	Linolenic acid C18:3	pus	-	-	9.60	5.63	0.50	0.20	1.00	10.00	-

Symbols: s = saturated; mus = monounsaturated; pus = polyunsaturated.

[17,105,106,107]. However, there are some proposed methodologies for the assessment of the oxidation stability of natural esters. It is important to note that some standards, which are primarily used to assess the stability of mineral-based insulating oils, may not be directly applicable to natural esters due to significant differences in their chemical compositions. IEC 61125 originally focused on methods for evaluating the oxidation stability of mineral oils [108]. However, recognizing the distinct chemical and physical properties of natural esters, IEC 62770 was introduced to specifically address the oxidation stability of these oils [109]. IEC 62770 is now the standard for natural esters, providing a more accurate and relevant assessment tailored to their unique characteristics [110]. Additionally, ASTM D2112 [111] and ASTM D2440 [112] were designed to assess the oxidation stability of mineral oils [104]. While some studies have used these methods for natural esters, modifications to the testing conditions, such as oxidation time and reagents, have been necessary [17,113]. Due to the polar nature and fatty acid composition of natural esters, their behavior during oxidation differs from that of mineral oils, making these tests less applicable or less accurate for evaluating the oxidation stability of natural esters.

i.

The IEC 62770 suggested that oxidation stability of natural esters could be performed at 120 °C, 48 hours of heating time [6,104]. It is to be mentioned that an investigation is still ongoing on the oxidation assessment of natural esters, which is to be reported in the near future.

ii.

RBOT: Oxidation assessment of natural esters can be determined using the rotating bomb oxidation test (RBOT). The natural ester sample is aged in a pressurized vessel at 120 °C in the presence of oxygen following the ASTM 2272 [114]. The completion of the test is indicated by the reduction in pressure within the vessel and the measurement of the corresponding time interval in minutes. The oxidation assessment value for mineral insulating liquids is 300 min, whereas for natural esters, it is less than 40 min [6,115]. A table comparing the oxidation induction time of different types of esters is presented in [116].

Table 5. RBOT Test results of different insulating liquids.

Types of Insulating Liquids	RBOT Test (Minutes)
Silicon fluids	>450
Natural esters	<40
High oleic natural esters	197
Synthetic esters	421
Mineral oils	300

shows the oxidation induction time of natural esters and other insulating liquids. Although the RBOT is an in-depth test for determining the oxidation stability of the insulating liquids, comparing the thermo–oxidative stability of different liquids when considering this method might be a challenge [115].

iii.

The turbine oil oxidation stability test (TOST): This approach efficiently assesses the oxidative stability of oils and has proven effective in evaluating the oxidation stability of natural ester insulating oils for transformer applications [30]. A schematic diagram illustrating the sequential stages of the turbine oil oxidation stability test (TOST) following ASTM D-943-04a [117] is depicted in Figure 7. As depicted in Figure 7, samples are taken every 168 h, and testing ceases upon reaching an acid value of 2.0 mgKOH/g. The oxidation lifetime of the liquid samples is calculated using the expression given in Equation (1).

$$Oxidation lifetime \left(H\right)=A+\left[{\displaystyle \frac{2.0-C}{D-C}}\right]\times \left(B-A\right)$$

(1)

where A represents the duration in test hours when the acid number was last recorded below 2.0; B represents the duration in test hours when the acid number was recorded above 2.0; C represents the acid number at A hours; and D represents the acid number at B hours [30].

iv.

Differential Scanning Calorimetry (DSC): DSC is a widely used technique for evaluating the oxidation stability of natural ester insulating liquids. During oxidation, oxygen reacts with the fatty acid chains in vegetable oils, leading to the generation of heat due to the energy released from the breaking and formation of chemical bonds. At the propagation stage of the oxidation process, the formation of lipid hydroperoxides is typically accompanied by heat release, which can be detected as an exothermic signal. Based on this principle, the oxidation induction time (OIT), defined according to ASTM E1858, is determined from the onset of the exothermic oxidation reaction and serves as a relative indicator of the oil’s oxidative stability [73]. An extended OIT implies improved oxidation resistance, especially when antioxidants are introduced, as their radical scavenging effect delays the onset of oxidation and increases the measured induction time.

Recent advancements have seen the application of pressure differential scanning calorimetry (PDSC) and thermogravimetric analysis-differential scanning calorimetry (TGA-DSC) in assessing the oxidation stability of natural esters, with promising results reported in the literature [28,118,119]. These techniques are also instrumental in analyzing thermal degradation behavior, where TGA monitors changes in the sample’s mass due to evaporation, decomposition, or oxidation, while DSC simultaneously captures thermal events such as exothermic and endothermic transitions [28,30,72,73,97]. However, there are significant analytical challenges associated with DSC and TGA measurements, especially when analyzing complex systems that contain antioxidants. One critical issue is the overlapping of exothermic peaks, which arises from the simultaneous occurrence of oxidation of the base ester and degradation of antioxidant molecules. Antioxidants, particularly phenolic types, may thermally degrade within a temperature range close to that of ester oxidation, producing their own exothermic signals. This overlap can result in broad or indistinct thermal peaks, making it difficult to differentiate between the thermal contributions of the base oil and the additives. Consequently, accurate determination of OIT and interpretation of oxidative behavior become less straightforward.

Similarly, in TGA analysis, mass loss associated with antioxidant volatilization or decomposition may overlap with the degradation of the ester matrix, leading to ambiguous multi-stage weight loss profiles. These challenges indicate the need for careful data interpretation and, in some cases, the use of complementary analytical techniques, such as Fourier transform infrared spectroscopy (FTIR) or gas chromatography-mass spectrometry (GC-MS), to validate the identity of degradation products and assign thermal events more precisely. Furthermore, modulated DSC or isothermal DSC protocols can help improve resolution and separate overlapping thermal signals. Thus, while DSC and TGA remain powerful tools for assessing oxidation stability and thermal behavior, their limitations in complex systems must be acknowledged and addressed through a more comprehensive analytical approach.

4. Antioxidants and Effects on Some Imperative Properties of Natural Ester-Based Insulating Liquids

In this section, the effect of antioxidants on some fundamental properties of natural ester insulating liquid is investigated. Among the parameters considered are the dielectric strength, fire properties, and viscosity.

4.1. AC Breakdown Voltage of Natural Esters Containing Antioxidants

Improving the oxidation stability of insulating liquids, particularly natural esters, requires careful consideration of how antioxidants impact the electrical properties of the oil.

Table 6. Influence of composite antioxidants on the AC breakdown voltage of natural esters [23,121].

Antioxidants	Ratio of Antioxidants	Soybean Oil (% Increment)	Pongamia Pinnata Oil	Sunflower Oil (% Increment)	Rice Bran Oil (% Increment)	Corn Oil (% Increment)	Rapeseed Oil (% Increment)
α-Tocopherol + CA	1.0:1.0	+40.0	-	+44.0	+23.0	+50.0	-
	0.5:0.5	+37.0	-	+59.0	0	+47.0	-
α-Tocopherol + BHT	2.5:2.5	-	+60	-	-	-	+63.0
	1.25:1.25	-	+41	-	-	-	+53.0
BHA + BHT	2.5:2.5	-	+88	-	-	-	+53.0
	1.25:1.25	-	+50	-	-	-	+59.0
BHT+ (citric acid) CA	1.0:1.0	+48.0		+32	−3.0	+53.0
	0.5:0.5	+44.0		+12	+15.0	+34.0
PG (1 mg) + CA (1 mg) + α-Tocopherol (1 mg)	500 mL of the composite antioxidants in 500 mL of oil (90 °C)	-	-	+20	-	-	-
PG (1 mg) + CA (1 mg) + α-Tocopherol (1 mg)	1000 mL of the composite antioxidants in 500 mL of oil (90 °C)	+19.2	-	-	-	-	-
PG (1 mg) + CA (1 mg) + α-Tocopherol (1 mg)	500 mL of the composite antioxidants in 500 mL of oil (90 °C)	-	-	-	−12.0	-	-

and

Table 7. Effect of standalone antioxidants on the AC breakdown voltage of natural esters [71,81,122].

Base oil	Antioxidants	Ratio of Antioxidants	Percentage Increment
25% Olive oil and 75% of Rice bran oil	BHT	0.25%	+46.6
75% Olive oil and 25% of Soya bean oil	BHT	1.00%	+52.54
50% Olive oil and 50% of Sunflower	BHT	1.00%	+48.50
25% Olive oil and 75% of Corn oil	BHT	1.00%	+48.50
75% Olive oil and 25% of Rice bran oil	BHA	1.00%	+62.50
50% Olive oil and 50% of Soya bean oil	BHA	1.00%	57.14
25% Olive oil and 75% of Sunflower	BHA	1.00%	+40.63
50% Olive oil and 50% of Corn oil	BHA	1.00%	+51.61
Vegetable oil	DBPC	9 g in 1108 mL of oil	+27.58
Vegetable oil	BHA	9 g in 1108 mL of oil	+48.27
Vegetable oil	TBHQ	9 g in 1108 mL of oil	+51.72
Honge oil	GA	0.50%	+12.14
Neem oil	GA	0.50%	+11.63
Mustard oil	GA	0.25%	+25.00
Punna oil	GA	0.50%	+20.56
Castor oil	GA	0.25%	+51.61
Sunflower oil	AA	1.00 g	+24.00
Soya bean oil	AA	1.00 g, 5.00 g	+56.00, +96.00
Corn oil	PG	1.00 g, 5.00 g	+72.00, +72.00

provide a summary of the impact of various commonly used natural and synthetic antioxidants on the AC breakdown voltage of natural esters. Table 6 illustrates the influence of composite antioxidants determined through experimental design, while Table 7 showcases the effect of individual antioxidants on the AC breakdown voltage of various natural esters. The tables demonstrate that incorporating antioxidants into the base liquids leads to an increase in the breakdown voltage. This enhancement can be attributed to the monoaromatic nature of the antioxidants, which have been found to enhance gas-absorbing behavior when natural esters are exposed to electrical stresses [23,120].

4.2. Ignition Properties of Natural Esters Containing Antioxidants

Effective asset protection in any industry is of great importance, especially in the electrical sector, where assets are expensive and interconnected. The application of high ignition point liquids in oil-filled transformers significantly protects the asset and prevents downtime, financial implications, and safety of the personnel and the environment. Natural esters are known to have higher flash and fire point temperatures relative to mineral oil and synthetic esters [123]; however, understanding the effect of antioxidants on the fire property of the base liquids is paramount.

Table 8. Effect of antioxidants on flash and fire points of natural esters [71,81,121].

Base Oil	Antioxidants	Ratio of Antioxidants	Flash Point Percentage Increment	Fire Point Percentage Increment
Sunflower	PG (1 mg) + CA (1 mg) + α-Tocopherol (1 mg)	500 ppm in 500 mL	+8.70	+8.00
Soybean	BHT (1 mg) + CA (1 mg) + α-Tocopherol (1 mg)	500 ppm in 500 mL	−3.70	−9.67
Rice bran	BHT (1 mg) + CA (1 mg) + α-Tocopherol (1 mg)	500 ppm in 500 mL	+8.00	+7.14
25% Olive oil and 75% of Rice bran oil	BHT	0.25%	+8.57	+3.66
50% Olive oil and 50% of Sunflower	BHT	0.50%	+5.26	+3.33
75% Olive oil and 25% of Corn oil	BHA	1.00%	+1.66	+1.94
75% Olive oil and 25% of Sunflower oil	BHA	1.00%	+2.75	+4.33
Honge oil	BHT	0.75%	+3.36	+5.81
Neem oil	BHT	0.75%	+6.94	+13.80
Mustard oil	BHT	0.5%	+7.27	+10
Punna oil	BHT	0.75%	+12.14	+8.20
Castor oil	BHT	0.75%	+6.43	+9.66
Neem oil	GA	0.5%	+8.33	+9.43
Mustard oil	GA	0.25%	+7.96	+11.00
Punna oil	GA	0.50%	+6.43	+4.26
Castor oil	GA	0.50%	+13.57	+14.48
Corn oil	PG	5.00g	+7.00	+6.00
Sunflower	AA	5.00g	+8.00	+19.00
Sunflower	α-Tocopherol + CA	1:1	+12.00	+11.00
Sunflower	BHT + CA	0.5:0.5	0	+11.00

gives a summary of recent investigations reported on the effect of antioxidants on the ignition and combustion properties of natural esters.

It can be observed from Table 8 that the addition of antioxidants to natural esters generally increases both the flash point and fire point, albeit with some exceptions. This improvement can be attributed to antioxidants’ ability to curtail the thermal decomposition of natural esters, thus impeding the release of flammable gases at lower temperatures. However, when introducing new antioxidants to enhance natural ester-insulating liquids, careful consideration of the loading concentration is crucial. Studies have demonstrated that at elevated concentrations of antioxidants, both the flash point and fire point begin to decline. This decrease might be attributed to the formation of higher ignition combinations within the oil [59].

4.3. Effect of Antioxidants on Cooling Properties of Natural Esters

In transformer insulation systems, dielectric liquids play a crucial role in facilitating cooling and dissipating heat. The cooling efficiency of these insulating liquids depends directly on their viscosity [124]. Consequently, investigating the impact of antioxidants on the viscosity of these base liquids is paramount.

Table 9. Influence of antioxidants on the viscosity of natural ester insulating liquids [59,71].

Base Liquids	Antioxidants	Ratio of Antioxidants	Percentage Increment/Decrement at Room Temperature
Sunflower oil	PG	1.00 g	−15.00
	AA	1.00 g	−8.00
	BHT + CA	0.5:0.5	−17.00
	α-Tocopherol + CA	0.5:0.5	−17.00
Corn oil	PG	1.00 g	−19.00
	α-Tocopherol + CA	0.5:0.5	−22.00
	BHT + CA	0.5:0.5	−7.00
	AA	5.00 g	−7.00
Castor oil	BHT	0.25%	+35.12
	BHA	0.25%	+40.13
	GA	0.25%	+35.12
Punna oil	BHT	0.25%	−20.12
	BHA	1.00%	−32.22
	GA	0.50%	−14.48
Mustard oil	BHT	1.00%	+21.81
	BHA	0.25%	−10.93
	GA	1.00%	+9.82
Neem oil	BHT	0.50%	+35.95
	BHA	0.25%	−21.61
	GA	0.25%	+0.79
Honge oil	BHT	1.00%	−14.55
	BHA	1.00%	+8.95
	GA	1.00%	+6.71

presents the effect of various antioxidants on the viscosity of natural ester insulating liquids. Antioxidants exhibit diverse behaviors regarding the viscosity of the base liquid. While some antioxidants decrease viscosity, others increase it. References [81,121] highlight the irregular nature of the effect of antioxidant loading on base liquid viscosity. Thus, it can be inferred that the behavior of antioxidants in base liquids is contingent upon the chemical composition of the antioxidants and their interaction with the base liquid. In situations where antioxidants reduce the viscosity of the base liquids, they may be categorized as solvents or dispersants. These antioxidants work by breaking molecular bonds, thereby reducing the attractive forces between oil molecules and, ultimately, decreasing viscosity [71]. Conversely, if antioxidants increase the viscosity of the base liquid, they may have formed chemical bonds with the oil molecules, leading to an elevated viscosity.

It is worth noting that the concentration of antioxidants added to the base liquid can also impact viscosity [59]. Additionally, whether antioxidants increase or decrease viscosity, these scenarios may not necessarily negatively affect the oxidation enhancement of the base liquids. Increased viscosity could impede oxygen diffusion processes, slowing down the rate of oxidation. Conversely, decreased viscosity may facilitate better dispersion of antioxidant molecules throughout the base liquid, potentially enhancing antioxidant effectiveness.

Low-viscosity liquids are preferred for efficient cooling in transformers, as supported by established literature regarding their favorable Reynolds number and heat transfer coefficient [27,125]. However, during transformer operation, various factors like thermal, electrical, and environmental contribute to the deterioration of insulating liquids. Specifically, the oxidation of natural esters leads to the formation of polymeric materials, which, in turn, increases the viscosity of the insulating liquids. This rise in viscosity is attributed to the higher molecular weight of the polymeric materials generated during the oxidation process. The relationship between viscosity and the average molecular weight of the aged liquid is described by the following expression in Equation (2) [43].

$$\left[\mu \right]=K{\overline{M}}^{\propto }$$

(2)

where µ is the viscosity of the liquid, M is the molecular weight, and K and α are constants that depend on solvent and temperature [126].

Incorporating antioxidants into natural esters during transformer operation is crucial for maintaining efficient cooling. Antioxidants, whether donors or acceptors interrupt the oxidation process before it reaches the termination stage, where harmful polymeric materials are formed. By preventing this progression, antioxidants help preserve the integrity of the insulating oil and protect against thermal breakdown, consequently enhancing the reliability and longevity of the transformers.

4.4. Effect of Antioxidants on the Dielectric Dissipation Factor of Natural Esters

The dielectric dissipation factor is a crucial parameter for assessing the quality of insulating liquids. It is influenced by factors such as ionic conduction and dipolar interaction when materials are subjected to alternating electric fields [127]. During the aging process of natural esters, particularly when transformers are in operation at high temperatures, aging byproducts such as moisture, polymeric materials, acids, and ketones gradually form. These byproducts lead to an increase in the dielectric loss of the insulating liquids. At low frequencies, it is possible to monitor charge transport within the liquids, a means of identifying the presence of foreign materials in the liquid. As aging progresses, the accumulation of byproducts enhances ionic conduction and interfacial polarization, consequently increasing the dielectric loss of the insulating liquids. When the imaginary part of the dielectric response exhibits a slope of −1, it indicates that the loss process is dominated by DC conduction. Furthermore, a dielectric loss exceeding 1 is detrimental to the transformer’s operational lifespan due to the associated dielectric heating. This heating can adversely affect both the dielectric and mechanical properties of the cellulose insulation material within the transformer by reducing its degree of polymerization.

The increase in dielectric loss of natural esters during transformer operation can be minimized by the addition of antioxidants since it prevents or reduces the formation of thermo-oxidation byproducts. The early influence of antioxidants on unaged natural esters is also important as it gives the compatibility of the antioxidants with the liquids. When antioxidants are introduced into natural esters, understanding ionic conduction and dipolar interaction due to the alignment and reorientation of molecular dipoles within the insulating material in response to the electric field is crucial.

The impact of butylated hydroxytoluene (BHT) antioxidants was studied in [128], employing mineral oil, sunflower oil, and rapeseed oil at concentrations of 0.25 wt.%, 0.3 wt.%, and 0.35 wt.%. It was observed that the dielectric dissipation factor of both uninhibited and inhibited oils remained unchanged. In reference [129], the effect of antioxidants, including propyl gallate (PG), citric acid (CA), tert-butyl hydroquinone (BTHQ), and butylated hydroxyanisole (BTHX) on rapeseed oil was examined. At 30 °C, no notable changes in the dissipation factor value were detected across all prepared samples. However, with rising temperatures, an increase in the dielectric dissipation factor was observed, particularly in samples with higher concentrations of antioxidants, which exhibited the highest values. For samples containing 1 wt.% of PG, BTHQ, CA, and BTHX at 90 °C, the dielectric loss of the oil increased by 126.4%, 71.42%, 335.71%, and 128.57%, respectively. This implies that an increase in temperature increases the disorderliness in both oil and antioxidant molecules, leading to an increase in dielectric loss. Among the antioxidants considered, BTHQ demonstrated the least percentage increment in dielectric loss, indicating its superior stability even at elevated temperatures. The rise in dielectric dissipation factor values may be ascribed to heightened molecular mobility within the oil, facilitating easier interaction between oil molecules and antioxidant molecules.

It is, therefore, important to optimize the loading of antioxidants that can improve the oxidation stability of natural esters without compromising the dielectric loss.

4.5. Antioxidant Monitoring in Online Transformers

Over the operational life of a transformer, the concentration of antioxidants in insulating oils gradually decreases due to the aging process and the ongoing reaction with free radicals. These antioxidants are crucial for maintaining the stability and effectiveness of the insulating liquid by preventing the formation of harmful byproducts that can lead to deterioration. Monitoring the depletion of antioxidants is, therefore, essential for assessing the health and longevity of the insulating system. A reduction in antioxidant levels can indicate increased oxidative stress and impending failure, making it critical for transformer owners to track these changes over time. Various analytical techniques can be employed to monitor the concentration of antioxidants in insulating oils, each with its unique advantages and applications.

Analytical measurements like high-performance liquid chromatography (HPLC), gas chromatography-mass spectrometry (GC-MS), Fourier transform infrared spectroscopy (FTIR), and nuclear magnetic resonance (NMR) spectroscopy are highly sensitive techniques that can be employed in antioxidant monitoring [130,131].

i.

High-Performance Liquid Chromatography (HPLC)

High-performance liquid chromatography (HPLC) is a powerful analytical technique widely used for the identification, quantification, and resolution of compounds [132]. This liquid chromatography method has significant applications across various fields, including research, manufacturing, biomonitoring of pollutants, and pharmaceuticals. The basic principle of HPLC involves injecting a small amount of liquid sample into a flowing stream of liquid, known as the mobile phase [132,133]. As this mobile phase moves through a column packed with particles of the stationary phase, the mixture separates into its components based on their degree of retention within the column. The retention of each component is determined by its partitioning between the liquid mobile phase and the stationary phase, allowing for effective separation and analysis [132]. In the context of monitoring antioxidants, natural ester insulating oil is passed through a stationary phase, where the antioxidants are separated based on their interactions with the stationary phase. Detectors, such as UV-Vis, are then used to identify and quantify the antioxidant components. HPLC is particularly useful for tracking the concentration of antioxidants and determining the remaining quantity over a specific aging period. The advantages of this analytical technique include its high sensitivity, precision, and capability to analyze a wide range of antioxidants, making it an ideal tool for this purpose [134].

ii.

Gas Chromatography-Mass Spectrometry (GC-MS)

Gas chromatography-mass spectrometry (GC-MS) is also a powerful technique used for the separation and identification of compounds. It involves the vaporization of samples, followed by the separation of components based on their volatility using gas chromatography [135]. The separated components are then further identified and quantified using mass spectrometry. When applied to insulating liquids containing antioxidants, mass spectrometry provides detailed molecular information about the quantity of antioxidants present [136]. Among the numerous advantages of GC-MS are high sensitivity, the ability to provide structural information about antioxidants and the capability to identify an unknown compound in an insulating liquid.

iii.

Fourier Transform Infrared Spectroscopy (FTIR)

Fourier transform infrared (FTIR) spectroscopy is widely used in material synthesis and characterization, particularly for the identification of functional groups in various materials [125]. This technique measures the absorption of infrared radiation by samples at different wavelengths. Since each antioxidant, whether synthetic or natural, possesses a unique infrared spectrum, FTIR can be employed to identify and quantify their presence in natural ester insulating liquids [137,138]. Additionally, FTIR is valuable for monitoring the rate of degradation of these liquids in real-time [26,113,137]. Furthermore, FTIR is a non-destructive and fast measurement capable of analyzing the quality of both liquid and solid insulators in transformers. It can be used to monitor the early degradation of insulating materials [129].

iv.

Nuclear Magnetic Resonance (NMR) Spectroscopy

NMR can be used in understanding the degradation of natural ester insulating liquids [139,140]. It involves the interaction of nuclear spins with an external magnetic field. This interaction causes the nuclei to resonate at specific frequencies, depending on their chemical environment. The resulting NMR spectra provide detailed information about the molecular structure of antioxidants [141,142]. NMR spectroscopy is not destructive and provides detailed molecular information by detecting the interaction between antioxidants and oil molecules.

Machine Learning and Artificial Intelligence in Oxidation and Antioxidant Monitoring

The introduction of machine learning (ML) and artificial intelligence (AI) is revolutionizing materials research in the new industrial era. These advanced approaches are increasingly being applied in transformer monitoring and condition assessment, with a particular focus on transformer insulation systems [143]. Unlike traditional methods of detecting, diagnosing, and predicting the condition of transformer insulation, ML offers a more cost-effective and efficient technique for transformer health monitoring [144]. The practical applications of ML and AI in transformer oxidative assessment are given as follows.

i.

Predictive Models for Oxidation Stability

A key application of ML is the development of predictive models that can forecast the oxidation stability of natural ester insulating liquids. Popular ML models such as adaptive boosting (AdaBoost), random forest (RF), k-nearest neighbor (kNN), artificial neural networks (ANNs), and support vector machines (SVMs) have demonstrated utility in the area of predictive analysis on transformers [145]. These models analyze the chemical composition of insulating liquids alongside key operating conditions like temperature, moisture content, and ambient environment to provide accurate predictions about oxidation behavior and stability.

ii.

Integration with Molecular Dynamics

By integrating ML with molecular dynamics simulations, researchers can model the interactions between antioxidant molecules and natural ester insulating liquids at the atomic level. This enables the prediction and optimization of novel antioxidant additives tailored for specific operating conditions. Such approaches have the potential to accelerate research and reduce costs by minimizing the need for extensive laboratory testing [127].

iii.

Real-Time Monitoring of Antioxidants

AI-powered systems can play a critical role in the real-time monitoring of antioxidant depletion during transformer operation. Smart sensors embedded in transformers can continuously track antioxidant levels and other parameters, feeding data into AI algorithms for analysis. This allows operators to maximize operational stability, reduce the frequency of offline testing, and prevent rapid degradation of insulating liquids [146]. For example, the application of convolutional neural networks (CNNs) has been applied for analyzing infrared spectroscopic data, enabling precise monitoring of antioxidant levels [147,148].

iv.

Fault Detection and Maintenance Optimization

ML algorithms, when combined with data from transformer sensors, can identify patterns that indicate early signs of oil degradation or antioxidant depletion. Predictive maintenance strategies powered by AI can, thus, reduce downtime and extend transformer lifespan.

The integration of machine learning (ML) and artificial intelligence (AI) into antioxidant monitoring is still in its early stages, but several promising future directions are emerging. Embedding transformers with the Internet of Things (IoT)-)-enabled sensors and integrating them with AI systems could create a comprehensive, real-time monitoring network capable of automatically adjusting antioxidant levels or recommending corrective measures to maintain optimal insulation performance. Digital twin technology, which involves creating virtual replicas of physical systems, could be extended to include the dynamics of antioxidant behavior in transformer oil. These virtual models would simulate real-time transformer operations, providing predictive insights into antioxidant performance and enabling the timely mitigation of risks. Additionally, the future of transformer operation may involve fully autonomous systems where AI not only monitors antioxidant levels but also manages their replenishment or formulation adjustments in real-time, ensuring optimal performance. However, despite these promising applications, several challenges must be addressed. These include collecting high-quality and representative data from transformers across varying operational conditions, ensuring a sufficient volume of data to train robust ML models, and validating predictive models across diverse transformer systems and insulation types. Addressing these challenges is crucial for fully realizing the potential of ML and AI in oxidation stability and antioxidant monitoring.

5. Antioxidants and Other Additives in Insulating Liquids

Natural ester insulating liquids are often formulated with several functional additives, including antioxidants, pour point depressants, metal deactivators (passivators), anti-foaming agents, and viscosity modifiers. The total concentration of these additives is typically limited to no more than 5% to maintain optimal performance and chemical compatibility. It is, therefore, essential to understand the interactions between antioxidants and other additives, as these interactions can significantly impact the oxidation stability and overall performance of the insulating liquid [6]. Among these additives, metal passivators play a crucial role in protecting both the oil and metallic components within the transformer. These compounds form a chemical varnish on the surface of copper, creating an impermeable barrier between the metal and the surrounding oil or insulating cellulose. This dual action prevents the catalytic degradation of oil by copper ions and also protects the copper from chemical attack by the oil. A widely used example of a metal passivator is Irgamet 39™, a benzotriazole-based copper passivator commonly recommended for transformer applications [149]. Studies have shown a strong synergistic relationship between antioxidants and metal passivators [150,151,152,153]. Metal deactivators help reduce the catalytic initiation of oxidation reactions, thereby decreasing the oxidative load on antioxidants and enhancing their efficacy. This complementary action contributes to improved long-term oxidation stability of the insulating liquid. Although antioxidants and pour point depressants serve distinct functional roles, recent reports [13,14] have highlighted potential synergistic effects when antioxidants are grafted onto pour point depressants. Such formulations not only improve oxidation resistance but also enhance the low-temperature flow properties of the insulating liquid. However, it is important to note that polar additives such as pour point depressants and viscosity modifiers can influence the solubility, dispersion, and reactivity of antioxidants, particularly in complex multi-component ester systems. These interactions may impact the diffusion and mobility of antioxidant molecules, thereby affecting their radical scavenging performance. Furthermore, potential antagonistic effects must be considered. Certain surfactant-like additives, for instance, may encapsulate antioxidant molecules, reducing their availability to interact with oxidative radicals and diminishing their overall effectiveness. Therefore, understanding and optimizing the compatibility and combined behavior of antioxidants with other functional additives is critical for enhancing the oxidative stability, dielectric performance, and long-term sustainability of natural ester insulating liquids used in transformers.

6. Environmental Impact of Antioxidants Used in Natural Ester Oxidation Stability Enhancement

Having an environmental understanding of the effect of antioxidant addition to natural ester insulating liquids is paramount. The chemical composition and biodegradability of antioxidants could pose environmental challenges in the case of accidental release, such as leakage, spills, or during antioxidant refilling to natural ester in service [137]. Natural antioxidants, which are mainly derived from plants, may not pose any significant challenge to the environment since they are non-toxic and readily biodegradable. However, the application of these antioxidants in transformer application is limited due to their poor stability at high temperatures. Synthetic antioxidants, on the other hand, are chemically synthesized, and proper understanding of their interaction with the environment is crucial. The wide application of synthetic antioxidants has exposed the environment (air, soil, and water) to pollution. It was recently reported that synthetic phenolic antioxidants and some of their by-products are found in humans (tissue, serum, urine, breast milk) and the environment [154,155]. Some are found in mollusks and shrimps [156].

Toxicity studies have revealed that some synthetic antioxidants can be carcinogenic and also cause endocrine disruption effects [157,158]. Some of the secondary products from synthetic antioxidants during degradation are more dangerous and can cause DNA damage at low concentrations [159]. These secondary products are often from DNA adducts, where electrophilic reactants generated during degradation covalently bind to nucleophilic sites on DNA. This process disrupts the normal structure and function of DNA, leading to mutations if the damage is not properly repaired [160]. The mechanism involves the metabolic activation of these secondary products into a reactive intermediate, which then interacts with the DNA base. For example, the oxygen and nitrogen atoms in DNA bases serve as nucleophilic targets for the electrophilic intermediates forming stable adducts. The adducts can mispair during DNA replication or create structural distortion that blocks replication entirely. If not addressed by DNA repair systems, the resulting damage can contribute to carcinogenesis [161]. While data on the toxicology of some synthetic antioxidants, such as BHT, are limited, studies have shown that sub-lethal doses (0.1–60 µM) of BHT significantly impact zebrafish embryos. Specifically, heart rate was reduced by 25–30%, and doses above 0.1 µM led to increased movement both in terms of distance traveled and speed. Furthermore, exposure to 0.01 µM BHT resulted in increased expression of the dopamine transporter gene. It was also reported in reference [162] concerning zebrafish that the effective concentration (EC50) was 1.375 mg/L, causing cardiotoxicity and teratogenic effects and potentially causing developmental defects in aquatic organisms. These suggest that BHT not only causes acute toxicity but also interferes with the dopamine system in aquatic organisms [163,164].

Although synthetic antioxidants have been observed to biodegrade rapidly in soil and aquatic environments, with over 95% of some antioxidants degrading within 24 days [159], it is still important to exercise caution to prevent exposure. Synthetic antioxidants have the potential to cause significant health damage if proper precautions are not taken. Therefore, it remains crucial to handle and dispose of synthetic antioxidants responsibly to minimize the risk of adverse health effects and environmental contamination.

6.1. Antioxidant-Related Contamination Mitigation Strategy

The adoption of antioxidants to enhance the oxidation stability of natural esters has been crucial in the power industry. This enhancement has prolonged the usability and lifespan of natural esters in liquid-filled transformers, leading to a reduced reliance on mineral-based alternatives [165]. Consequently, this mitigation strategy helps prevent environmental damage caused by hydrocarbon spillage. However, proper handling of natural esters containing antioxidants is needed to prevent environmental and human exposure to these compounds and their by-products, particularly synthetic antioxidants. There are several ways to mitigate environmental contamination.

i.

It is of utmost importance to understand the environmental effect of specific antioxidants that are being considered in natural ester enhancement by conducting thorough environmental impact assessments before use. This could involve assessing the toxicity of both the antioxidants and their secondary products in the ecosystem.

ii.

Effective handling and storage of natural esters containing antioxidants is important regardless of the toxicity of the antioxidants used. This helps prevent spills or leaks of the liquids by ensuring that the containers are securely sealed, labeled, and stored appropriately.

iii.

In the event of accidental oil spills or leaks, it is important to have effective means of oil detection and excellent cleaning measures in place to prevent the release of antioxidant-containing natural esters. Installing sensors or oil detection alarms in transformer units enables swift response to oil leakages. Furthermore, regular transformer maintenance and monitoring are vital for detecting wear, corrosion, and damage, allowing a prompt intervention to prevent oil leakage or spills.

iv.

When insulating oil reaches the end of its life, it is essential to ensure proper disposal in accordance with recommended regulations and guidelines for used oil disposal. Additionally, if the used oils are being considered for recycling, such as conversion into lubricants and biodiesel, it is important to take proper precautions, especially for workers, by providing reliable personal protective equipment.

6.2. Oxidation Stability Enhancement of Natural Esters for Sustainable Energy

Electricity is fundamental to modern life, powering essential services across medical, security, industrial, and social sectors. Even brief power interruptions can have severe consequences, including loss of life, security breaches, and significant financial losses for both private and governmental entities. Ensuring a reliable and sustainable power supply is, therefore, a critical necessity. As global electricity demand rises with population growth, power equipment, particularly transformers, is increasingly burdened. The longevity and reliability of transformers are directly tied to the quality of their insulating materials.

In alignment with the United Nations’ Sustainable Development Goals, particularly those related to affordable and clean energy, there is a growing shift towards using natural esters as an alternative to traditional mineral insulating oils in transformers. Natural esters are more environmentally friendly, but they have a key drawback, which is low resistance to oxidation, especially at elevated temperatures. This susceptibility to reactive oxygen leads to rapid degradation of the insulating liquids, which in turn compromises transformer reliability and efficiency. This degradation not only diminishes the effectiveness of transformers but also undermines the environmental benefits of using natural esters. In renewable energy applications, such as wind and solar power installations, where sustainability is paramount, the failure of transformers due to poor oxidation stability can result in increased maintenance costs, heightened environmental risks, and reduced overall efficiency. These issues directly conflict with sustainability objectives. Therefore, improving the oxidation stability of natural esters is crucial for enhancing their performance, reliability, and alignment with the goals of sustainable energy. To achieve this, it is essential to thoroughly investigate the compatibility of antioxidants with natural esters and optimize their use in transformer insulation. The benefits of antioxidants in enhancing the oxidation stability of natural esters are as follows.

i.

Inhibition of Oxidation Reactions: Antioxidants play a critical role in interrupting the oxidation chain reaction within natural esters. By neutralizing free radicals, which are highly reactive species responsible for initiating and propagating oxidation, antioxidants help prevent the degradation of the insulating liquid.

ii.

Thermal Stability and Extended Transformer Lifespan: Antioxidants significantly enhance the thermal stability of transformer oils, allowing natural esters to function effectively at high temperatures without experiencing pronounced degradation or thermal aging of insulating components. This improvement in thermo–oxidative stability enables transformers to handle higher and fluctuating loads without compromising efficiency, which is vital for ensuring reliable and sustainable energy.

iii.

Increased Compatibility with Existing Transformer Designs: By enhancing the oxidation stability of natural esters, antioxidants also improve their compatibility with existing transformer designs. This increased compatibility supports the broader adoption of natural esters as a replacement for mineral oils, promoting a shift towards more sustainable and environmentally friendly insulating liquids. To achieve sustainable, reliable, and efficient electricity generation and distribution, it is essential to focus on enhancing the oxidation stability of natural esters. The introduction of antioxidants extends the life and performance of transformers and also aligns with the global sustainability goals, making it a key strategy for the future of energy.

6.3. Economic Importance of Antioxidants in Natural Ester Enhancement

The application of antioxidants in oxidation stability enhancement of natural esters offers numerous benefits to transformers, including prolonging transformer lifespan and reducing maintenance and replacement frequency, thus saving costs for utilities and operators [166]. The addition of antioxidants to natural esters prevents the degradation of the liquids, which consequently reduces the probability of downtime and associated maintenance expenses, thereby improving operational efficiency and productivity [167]. Furthermore, transformers insulated with natural esters containing antioxidants may achieve improved energy efficiency, resulting in long-term cost savings through reduced energy consumption and enhanced system performance.

7. Challenges and Outlook

Despite the proven technical and economic benefits of incorporating antioxidants into natural ester insulating liquids, several challenges persist. The application of antioxidants for enhancing the performance of natural esters remains an evolving area of research, particularly regarding the use of multiple antioxidants, as recommended by CIGRE [168]. One of the key considerations is the variability in fatty acid composition among different natural esters, which directly influences antioxidant behavior and efficacy. Antioxidants may demonstrate varying degrees of effectiveness depending on whether the base liquid is rich in saturated, monounsaturated, or polyunsaturated fatty acids. Generally, antioxidant performance tends to be more pronounced in esters with a higher degree of unsaturation. Consequently, the selection of appropriate antioxidants must be tailored to the specific characteristics of each base liquid to achieve optimal enhancement. Environmental sustainability is another critical concern. Certain synthetic antioxidants are known to exhibit persistence in the environment and pose potential ecological toxicity risks. Thus, minimizing the environmental impact of antioxidant-containing insulating liquids is a vital consideration for green transformer technologies. To mitigate environmental concerns, proper disposal and recycling of used insulating liquids are essential. Oil reclamation processes provide a cost-effective and environmentally responsible approach to reduce the ecological footprint of antioxidants and minimize waste generation. Furthermore, achieving a balance between antioxidant efficacy and preserving other essential physicochemical and dielectric properties of the base liquid presents a significant challenge. Antioxidants must be optimized in a way that enhances oxidation stability without adversely affecting other performance parameters of the liquid. Additionally, ongoing research should focus on advancing the chemistry of antioxidant formulations to develop environmentally benign, biodegradable, and low-persistence alternatives that align with sustainability goals.

Looking toward the future, the development of antioxidants for natural ester insulating liquids should focus on several key directions. First (i), the design of next-generation bio-based antioxidants derived from natural sources such as lignin, tocopherols, and polyphenols, which offer high efficiency with minimal environmental impact, should be prioritized. Second (ii), there is a need to develop multifunctional antioxidants that can simultaneously act as radical scavengers, metal passivators, and stabilizers against hydrolytic degradation. Third (iii), exploring synergistic antioxidant systems, where multiple antioxidants or additives are combined to provide broader and more robust oxidative stability across various operating conditions, is essential. Fourth (iv), the application of machine learning and computational chemistry tools to enable predictive modeling and screening of antioxidant performance based on ester composition and aging profiles can significantly accelerate formulation development. Fifth (v), the incorporation of encapsulated or controlled-release antioxidant technologies, which can deliver antioxidant compounds progressively during transformer operation, may offer a promising potential for extending the service life of insulating liquids. Sixth (vi), the formulation of tailor-made antioxidant solutions that are customized for different transformer designs, operating conditions, and geographical locations, particularly in harsh or subpolar climates, should be considered. Finally (vii), a comprehensive evaluation of antioxidant compatibility within complex multi-additive formulations is necessary to prevent potential antagonistic interactions and enhance additive synergy. Therefore, while antioxidants hold immense promise in improving the longevity and performance of natural ester insulating liquids, further research is needed to resolve compatibility issues, optimize formulation strategies, and ensure long-term environmental safety. Advancing antioxidant science through innovative chemistry and sustainable engineering will be central to supporting the global transition toward more reliable and eco-friendly transformer insulation systems.

8. Conclusions

The oxidation of natural ester insulating liquids in transformers poses a significant threat to their functionality, ultimately impacting the socioeconomic activities within a society. This degradation, resulting from oxidation, can lead to the breakdown of the transformer system. While natural esters are viewed as promising alternatives to mineral oil, their susceptibility to oxidation presents certain limitations. As a result, they are primarily utilized in sealed transformers. When natural esters undergo oxidation, they undergo a thickening process and may eventually polymerize. This, in turn, adversely affects both the cooling and dielectric activity of the liquids within the transformer. The importance of antioxidants in the enhancement of natural esters used in green transformers cannot be overemphasized. These materials have boosted the long-term stability of the green alternative insulating liquids, consequently promoting carbon net zero. This ultimately extends the lifespan of transformers and decreases maintenance costs. In addition, antioxidants have reportedly increased the properties of natural ester insulating liquids like AC breakdown voltage and fire safety. However, concerns persist regarding the environmental impact and potential toxicity of antioxidants.

Thus, it is imperative to carefully manage the disposal and recycling of natural esters containing antioxidants in accordance with environmental regulations. Furthermore, there is a pressing need for the development of new environmentally friendly antioxidants that exhibit low persistence and high performance at low concentrations. These advancements are crucial not only for improving the oxidation stability of natural ester insulating liquids but also for minimizing associated environmental risks. Overcoming these challenges can pave the way for broader adoption of natural ester-based insulating liquids in transformers, significantly reducing the power sector’s carbon footprint. This aligns with the Sustainable Development Goals (SDG 7: Affordable and Clean Energy, and SDG 13: Climate Action), promoting a cleaner, more sustainable energy infrastructure while minimizing the environmental impact of transformer operations.

9. Recommendation

The focus on clean and affordable energy is growing rapidly in both industrial and academic sectors, particularly within the power industry. This trend is driven by increasing environmental and social responsibilities. To ensure the widespread adoption of natural esters as insulating liquids in transformers, it is crucial to address the challenges associated with their poor oxidation stability and the aspect of copper dissolution in the liquid. Despite the advancements in technology, the integration of machine learning (ML) and artificial intelligence (AI) with molecular simulation for the development and monitoring of antioxidants remains largely unexplored in the literature. The application of cutting-edge technologies like ML and AI presents a transformative opportunity for the energy sector. These technologies could play an important role in designing novel, more effective antioxidants that significantly enhance the oxidation stability of natural esters. Furthermore, leveraging AI to optimize the concentration of antioxidants in natural ester formulations could lead to substantial improvements in the performance and reliability of transformer insulation systems. This approach would mark a significant advancement in the industry, aligning with the ongoing transition towards more sustainable energy solutions.