Flash Point Improvement of Mineral Oil Utilizing Nanoparticles to Reduce Fire Risk in Power Transformers: A Review

Abstract

Transformers are crucial equipment in electrical distribution systems but have a significant potential for failure. Insulation materials, including transformer oil (TO), play a primary role in transformer failures. A fire involving the TO can lead to a large explosion, causing the main tank to rupture and resulting in extensive damage to the entire transformer and the surrounding area. Mineral oil (MO) is the most widely used type due to its availability and relatively low cost compared to other types of oil. However, MO has a critical disadvantage, which is its very low flash point. The low flash point makes MO highly flammable. When the oil fires in an enclosed space, such as a transformer tank, the pressure inside the tank increases, leading to a large explosion. Therefore, research on increasing the flash point of MO is highly necessary. The application of nanotechnology is a promising approach to increasing the flash point of base fluids. Research on the effect of nanoparticles (NPs) on flash points is very limited in the literature; thus, there is significant potential for further research in this field. The majority of studies indicate an increase in flash points with the addition of NPs to MO. There is only one study that shows a decrease in flash point, which is −1.33% compared to MO. From all the reviewed studies, it can be concluded that NPs are a potential solution to increase the flash point of MO. Despite their benefits, NPs require a thorough examination of health and environmental impacts, along with proper waste management, to ensure their advantages.

1. Introduction

Electric energy is the fastest-growing type of energy compared to other forms, with an annual growth rate of approximately 1.9% per year [1]. This rapid growth is accompanied by an increasing demand for supporting equipment. One important piece of equipment used to transmit electrical energy at various voltage levels is a power transformer [2,3,4]. The general construction of a transformer is shown in Figure 1 [5]. Transformers are crucial equipment in electrical distribution systems but have a significant potential for failure. From 1987 to 2021, there were 1204 major failure cases reported by 66 utilities from 27 countries [6]. Insulation materials, including transformer oil (TO), play a primary role in transformer failures [7,8]. TO, indicated by the yellow area in Figure 1, occupies the majority of the transformer’s interior space. A fire involving the TO can lead to a large explosion, causing the main tank to rupture and resulting in extensive damage to the entire transformer and the surrounding area, as shown in Figure 2.

TO can be classified into three types: mineral oil (MO), synthetic oil, and organic oil [9]. MO is the most widely used type due to its availability and relatively low cost compared to other types of oil. However, MO has a critical disadvantage, which is its very low flash point. The low flash point makes MO highly flammable. When the oil fires in an enclosed space such as a transformer tank, the pressure inside the tank increases, leading to a large explosion. Therefore, research on increasing the flash point of MO is highly necessary.

The application of nanotechnology is a promising approach to increasing the flash point of base fluids. Current research mainly focuses on thermal properties such as thermal conductivity and viscosity. Only a few studies discuss the effect of nanofluids (NFs) on the flash point of MO. Hence, this review article focuses on this aspect [9,10,11]. This research aims to serve as a reference for future studies to make MO safer for use at high temperatures.

The novelty of this review lies in its focus on the use of nanoparticles (NPs) to enhance the flash point of MO, which is a relatively new and promising field of research. It provides a critical analysis of the current state of research, identifying gaps in knowledge and areas for future investigation. The review’s contribution is significant in the fields of power transformer safety and nanotechnology. It could serve as a valuable resource for researchers and industry professionals, guiding future research for the development of safer and more efficient power transformers. Additionally, identifying gaps in the current research could stimulate new studies and innovations in the field.

This review starts from Section 1 in the form of an introduction to their potential for enhancing flash points in MO. Section 2 describes the characteristics of TO. Section 3 explains NPs. Section 4 outlines the preparation of nano-insulating liquid. Section 5 discusses the impact of NPs on the flash points. Finally, Section 6 concludes the entire discussion.

2. Transformer Oil (TO)

The specifications of three different types of TO are shown in

Table 1. Classification of TOs [9].

Transformer Oil	Type	Specifications
MO	Paraffinic	Nonring long-chained
	Naphthenic	Saturated ring
	Aromatic	Nonsaturated ring
Synthetic oil	Polyalphaolefins	Polymerization of hydrocarbon molecules
	Polyglycols	Oxidation of ethylene and propylene
	Synthetic ester	Reaction of acids and alcohols with water
Organic oil	Soybean oil Coconut oil Cottonseed oil Rapeseed oil	Non-toxic, biodegradable, low inflammable, high breakdown voltage, flash point, acidity, viscosity, and pour point

. MO is processed from petroleum and has a complex chemical structure. The chemical structures of various TOs are shown in Figure 3. Synthetic oil is chemically processed to enhance its characteristics compared to MO, but it has the drawback of being environmentally hazardous. Organic oil is extracted from plants such as palm, castor, soybean, and coconut. The selection of TO is tailored to the operational needs of the transformer. For instance, in enclosed environments like underground mines, synthetic ester oil is commonly employed due to its high flash point, reducing the risk of combustion. In contrast, MO is typically used in open areas, like power generation and substations. The properties of TOs of various types can be observed in

Table 2. Properties of different TOs [9].

Properties	MO	Synthetic Oil	Organic Oil	Test Method
Breakdown voltage (kV)	30–85	45–70	82–97	IEC 60156 [12]
Relative permittivity at 25 °C	2.1–2.5	3.0–3.5	3.1–3.3	IEC 60247 [13]
Viscosity at 0 °C (mm2/s)	<76	26–50	77–143	ISO 3104 [14]
Viscosity at 40 °C (mm2/s)	3–16	14–29	16–37	ISO 3104 [14]
Viscosity at 100 °C (mm2/s)	2–2.5	4–6	4–8	ISO 3104 [14]
Pour point (°C)	−30 to −60	−40 to −50	−19 to −33	ISO 3016 [15]
Flash point (°C)	100–170	250–270	315–328	ISO 2592 [16]
Fire point (°C)	180–185	300–310	350–360	ISO 2592 [16]
Density at 20 °C (kg/dm3)	0.83–0.89	0.90–1.00	0.87–0.92	ISO 3675 [17]
Specific heat (J/g·K)	1.6–2.0	1.8–2.3	1.5–2.1	ASTM E1269 [18]
Thermal conductivity (W/m·K)	0.11–0.16	0.15	0.16–0.17	ASTM D7896 [19]

. It is evident that MO has the lowest flash point compared to other types. Therefore, it is important to observe this deeply because MO is the most widely used type of TO, although it has the highest risk of fire.

3. Nanoparticles (NPs)

NPs are materials characterized by having one dimension less than 100 nm [10]. Typically, NPs are selected based on their fundamental properties, such as conductivity and permittivity. NPs are broadly divided into various categories depending on their morphology, size, and chemical properties. As shown in Figure 4, NPs can be categorized based on their base material as well as their dimensions [11].

3.1. Material-Based NPs

NPs can be made from various materials, and the choice of material depends on specific properties and desired applications. Based on their material composition, NPs are classified into carbon, metal, semiconductor nanomaterials, and nanocomposites [11]. Carbon-based NPs are nanomaterials made from carbon content and exist in various morphologies. Examples of such NPs include C₆₀, graphene oxide (GO), and carbon nanotubes (CNTs) [20]. Metal-based NPs are made entirely from metal precursors. Examples of such NPs include iron(II,III) oxide (Fe₃O₄), gold NPs (AuNPs), and silver NPs (AgNPs) [21]. Semiconductor NPs are made from semiconductor materials. Semiconductors are solid substances that are neither good conductors nor insulators but have a crystalline structure and contain very few free electrons at room temperature. Examples of such NPs include zinc oxide (ZnO), titanium oxide (TiO₂), and cadmium selenide (CdSe) [22]. Nanocomposites are solid materials composed of multiple phases, where at least one of these phases has a size in the nanometer range. Based on their matrix material, they can be classified into ceramic matrix, polymer matrix, and metal matrix nanocomposites [23].

3.2. Dimension-Based NPs

Based on their dimensions, NPs are classified into zero, one, two, and three-dimensional [11,24]. The types of NPs in this category are shown in Figure 5. Zero-dimensional (0D) NPs refer to NPs where all three dimensions of the nanomaterial are within the nanoscale range. This category includes quantum dots, nanospheres, and nanoclusters. One-dimensional (1D) NPs are a type of NP where two dimensions are within the nanoscale range, but the third dimension is not. This classification includes nanorods, nanotubes, and nanowires. Two-dimensional (2D) NPs are a type of NPs where one dimension is in the nanoscale range, while the other two dimensions are outside of it. Examples include nanofilms, nanolayers, and nanosheets. Three-dimensional (3D) NPs are a type of NP where the nanomaterial is larger than the nanoscale dimension (1–100 nm). 0D, 1D, and 2D are fundamental structures within 3D nanomaterials. Examples of these NPs include nanocrystals, core-shell structures, nanowire bundles, nanotube bundles, and multi-nanolayers [11].

3.3. Synthesis of NPs

NPs are synthesized using two approaches: the top-down and bottom-up approaches [11,25]. If the starting material is larger than the NPs, it is referred to as the top-down approach, whereas if it is from a smaller material, it is called the bottom-up approach. By employing either of these approaches, NPs can be synthesized chemically, physically, physicochemically, or biologically. The chemical method involves the creation of NPs using a chemical reaction between precursor solutions that can lead to the formation of NPs, which are then precipitated, or the reduction of chemical compounds into NPs using a reducing agent. Chemical synthesis methods include sol-gel process, chemical etching, and chemical vapor synthesis. The physical method involves manufacturing using physical or mechanical methods, including mechanical grinding, melt mixing, and high-energy milling. The biological method involves manufacturing using living organisms or products of living organisms to reduce chemical compounds into NPs, including industrial and agriculture waste-assisted biogenesis, plant extract-assisted biogenesis, and bio-template biogenesis. Physical-chemical methods are an NP synthesis approach that combines physical and chemical aspects, including sonochemical and sono-electrochemical processes.

The approach for NP synthesis is shown in Figure 6a, while the methods for NP synthesis are illustrated in Figure 6b. After synthesis, the produced NPs can be analyzed and characterized using various techniques such as transmission electron microscopy (TEM), optical spectroscopy, X-ray diffraction, and other methods to ensure that the size, shape, and other properties align with the desired specifications [26,27,28].

4. Preparation of Nano Insulating Liquid (NIL)

NIL is a more specific term than NF, as it specifically refers to insulating liquid-based fluids. The initial step in the experimentation process involves the preparation of NF. Generally, two methods are employed for the preparation of TO-based NF, categorized as one-step and two-step methods [29]. A brief introduction to the preparation processes is given as follows.

4.1. One-Step Method

In the one-step method, NPs are synthesized and suspended in the base fluid simultaneously. Drawbacks associated with this method include high costs and challenges in large-scale production [29]. This process exhibits better dispersion stability compared to the two-step method [27]. Figure 7 illustrates the synthesis of NF through this method.

4.2. Two-Step Method

In the two-step method, solid NPs are prepared and then dispersed in the base fluid through ultrasonic methods, magnetic stirring, high-speed mixing, or ball milling. This method is widely employed for large-scale NF production due to its lower cost. However, there is a high possibility of NP agglomeration during the process due to the large surface area and high surface activity of NPs [29]. The generally adopted methods are elaborated in Figure 8.

4.3. Concentration

Concentration is a crucial aspect of NF preparation. The most commonly used methods for determining concentration are weight percent (wt%), volume percent (vol%), and mass-to-volume ratio (g/L). The mass-to-volume ratio is straightforward, involving weighing NPs and adding them to the base fluid used. Different equations can be used to calculate the gravimetric (wt%) and volumetric concentrations (vol%), which are summarized in

Table 3. The formula used to calculate the concentration of NFs.

References	Formula
Wilk et al. [30]	$w={\displaystyle \frac{m_{np}}{m_{np}+m_{bf}}}\left(wt\%\right)$	(1)
	$\varnothing ={\displaystyle \frac{1}{1+\left(\frac{1-w}{w}\right)\frac{{\rho }_{np}}{{\rho }_{bf}}}}\left(vol\%\right)$	(2)
Li et al. [31]	$w={\displaystyle \frac{m_{np}}{m_{np}+m_{bf}}}\times 100\mathrm{\%}\left(\mathrm{w}\mathrm{t}\mathrm{\%}\right)$	(3)
	$\varnothing ={\displaystyle \frac{{{\rho }_{bf}}_{}w}{{\rho }_{bf}w+{\rho }_{np}\left(1-w\right)}}\times 100\mathrm{\%}\left(\mathrm{v}\mathrm{o}\mathrm{l}\mathrm{\%}\right)$	(4)
Li et al. [32]	${\varnothing }_v={\displaystyle \frac{{{\rho }_p}_{}{\varnothing }_m}{{\rho }_p{\varnothing }_m+{\rho }_b\left(1-{\varnothing }_m\right)}}\left(\mathrm{v}\mathrm{o}\mathrm{l}\mathrm{\%}\right)$	(5)
Kannaiyan et al. [33]	$\varnothing ={\displaystyle \frac{\frac{w_{np}}{{\rho }_{np}}}{\frac{w_{np}}{{\rho }_{np}}+v_b}}\left(\mathrm{v}\mathrm{o}\mathrm{l}\mathrm{\%}\right)$	(6)
Moldoveanu et al. [34]	${\displaystyle \frac{1}{x_{vol}}}=1+{\displaystyle \frac{{\rho }_p}{{\rho }_f}}\left({\displaystyle \frac{1-x_{wt}}{x_{wt}}}\right)\left(\mathrm{v}\mathrm{o}\mathrm{l}\mathrm{\%}\right)$	(7)
Reddy et al. [35]	$\varnothing ={\displaystyle \frac{\frac{w_{nanofluid}}{{\rho }_{nanofluid}}}{\frac{w_{nanofluid}}{{\rho }_{nanofluid}}+\frac{w_{basefluid}}{{\rho }_{basefluid}}}}\times 100\left(\mathrm{v}\mathrm{o}\mathrm{l}\mathrm{\%}\right)$	(8)
Radkar et al. [36]	$Vol\%ofNF={\displaystyle \frac{\frac{W_{ZnO}\left(\mathrm{g}\right)}{{\rho }_{ZnO}\left(\frac{\mathrm{g}}{{\mathrm{c}\mathrm{m}}^3}\right)}}{Totalvol.ofNF}}\left(\mathrm{v}\mathrm{o}\mathrm{l}\mathrm{\%}\right)$	(9)
Dalkılıç et al. [37]	Hybrid NPs $\mathrm{\varnothing }={\displaystyle \frac{({\frac{m}{\rho })}_{{NP}_1}+({\frac{m}{\rho })}_{{NP}_2}}{({\frac{m}{\rho })}_{{NP}_1}+({\frac{m}{\rho })}_{{NP}_2}+({\frac{m}{\rho })}_{BF}}}\times 100\left(\mathrm{v}\mathrm{o}\mathrm{l}\mathrm{\%}\right)$	(10)
Kakavandi et al. [38]	Hybrid NPs and two base fluids $\varnothing ={\displaystyle \frac{({\frac{m}{\rho })}_{{NP}_1}+({\frac{m}{\rho })}_{{NP}_2}}{({\frac{m}{\rho })}_{{NP}_1}+({\frac{m}{\rho })}_{{NP}_2}+({\frac{m}{\rho })}_{{BF}_1}+({\frac{m}{\rho })}_{{BF}_2}}}\times 100\left(\mathrm{v}\mathrm{o}\mathrm{l}\mathrm{\%}\right)$	(11)

.

4.4. Stability of NIL

NF is considered stable if the particles do not agglomerate at a significant rate [28]. This section will discuss details about the causes of NF instability and methods to evaluate it. Several common factors influencing stability include Brownian motion, particle size, type of NPs and base fluid, sonication time, particle interaction, and thermophoresis [27,28]. The effect of Brownian motion (BM) is a key factor contributing to NF stability. BM refers to the random movement of particles in a liquid medium caused by constant collisions with surrounding molecules. In the context of NF, this effect plays a crucial role in preventing agglomeration or sedimentation of NPs. BM exhibits behavior inversely proportional to particle size [39] and NF concentration [40]. Thus, it can be concluded that size and concentration are inversely related to stability. This is evidenced by Primo et al. [41], as shown in Figure 9, who conveyed that increasing the concentration of graphene NF with an organic oil base decreases NF stability. The research results indicate that increasing the sonication time from 30 to 60 min resulted in a slight increase in stability by a few percent after 90 days of observation.

4.5. Stability Enhancement Methods

Mechanical and chemical methods are used by some researchers to reduce the coagulation rate and produce stable NF [42,43]. The combination of these mechanical and chemical methods contributes to the overall stability of the NF, ensuring uniform dispersion of NPs and enhancing their properties. A schematic of enhancement methods using the mechanical and chemical methods is described in Figure 10.

Mechanical Method

Mechanical methods involve the use of mechanical forces to disperse and stabilize NPs in the fluid. As shown in Figure 10a, this method involves employing mechanical means, including ultrasonication, a magnetic stirrer, ball milling, and a high-pressure homogenizer, to enhance stability [44]. Ultrasonication is a process that involves the application of ultrasonic frequencies to a medium, often a liquid or a suspension, to achieve various effects. In the context of NP synthesis or dispersion in NF, ultrasonication is commonly used to break down agglomerates or clusters of particles and promote their uniform distribution in a liquid. A magnetic stirrer is a laboratory device that uses a rotating magnetic field to cause a stir bar immersed in a liquid to spin very quickly, thus stirring the liquid. The advantages of using a magnetic stirrer include the ability to stir solutions without the need for direct contact with a motor or external power source. Ball milling is a mechanical technique widely used to grind powders into fine particles and blend materials. In the context of NF, ball milling is often used to disperse NPs more uniformly in a base fluid.

The surface of antimony trioxide NPs is modified with dioctyl phthalate through ball milling, leading to the formation of O₂ and H₂ atoms on the NP surface, causing bridging action and steric hindrance that prevents agglomeration and increasing the zeta potential value [45]. A high-pressure homogenizer is a device used to break down and homogenize materials. It operates on the principle of forcing a fluid or substance through a narrow space or valve at high pressure, resulting in a more uniform and fine dispersion of particles.

As shown in Figure 10b, the chemical method is a process aimed at enhancing stability through chemical treatments, including electrostatic, steric, and electrosteric methods [44]. The electrostatic chemical method for stability is a technique used to enhance the stability of colloidal systems, such as NF, by utilizing electrostatic forces. In this method, the surface charge of NPs is manipulated to create repulsive forces that prevent agglomeration and improve stability. Steric stabilization is a method that involves the use of polymers or surfactants that adsorb onto the surface of NPs, creating a protective layer that prevents agglomeration and improves dispersion. Electrosteric stabilization refers to a combination of electrostatic and steric methods used in colloid and NP systems to enhance stability. This approach combines the principles of both electrostatic and steric stabilization to prevent agglomeration and improve the dispersion of particles. All three methods are carried out by adding a surfactant.

Although surfactants can enhance stability, they also have drawbacks, such as foam formation, which affects the authenticity of particles and base fluid, thermal conductivity, and viscosity [28]. Figure 11 presents the types of surfactants used to improve NF stability and some side effects of surfactant use. In addition to the addition of surfactants, another technique to enhance NF stability is known as pH control. Stable suspension is achieved by adjusting the pH of the NF due to the presence of strong repulsive forces [43,46].

Stability can be obtained when the pH value is far from the isoelectric point (IEP), which is the point of zero potential, thus providing maximum repulsion between particles. For example, Figure 12 and Figure 13 present the research results of Umar et al. [46], illustrating optical images of aluminum oxide (Al₂O₃) and copper oxide (CuO) water NF, respectively. These images depict that the Al₂O₃ NF with acetic acid shows better suspension compared to the basic pH adjusters, such as sodium hydroxide (NaOH) and ammonium hydroxide (NH₄OH), while the CuO NF with the same pH adjusters exhibits maximum agglomeration, leading to poor suspension of CuO in the base fluid.

4.6. Stability Enhancement Mechanism

The stability of colloidal suspensions is elucidated by the Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory, which is underpinned by several key assumptions. It posits a dilute dispersion of particles in the colloidal suspension, indicating a low particle concentration. The primary contributors to particle interactions are identified as Van der Waals attractive forces and electrostatic forces, with gravity and buoyancy forces intentionally omitted from the analysis due to their negligible impact on suspended particle behavior. The assumption of a homogeneous nature for the colloidal suspension ensures consistent properties and composition throughout. Furthermore, the distribution of ions in the colloidal system is intricately influenced by three factors: electrostatic force, Brownian motion, and entropy-induced dispersion.

4.7. Stability Evaluation Method

Various techniques are utilized in the literature to assess the stability of NFs. Sedimentation photoanalysis is a qualitative visual study used to evaluate the settling of NPs in newly synthesized NFs and report on sedimentation that may occur over a specific period. Zeta potential measurement assesses the electrostatic potential at the shear plane of particles, providing information about their surface charge and the potential for aggregation. The stability evaluation, based on the zeta potential values, is presented in

Table 4. Stability evaluation using Zeta potential [28].

Zeta Potential (±mV)	Stability
0	Lack of stability
0–15	Marginally stable
15–30	Moderate stability
30–45	Satisfactory stability (sedimentation occurs)
45–60	Outstanding stability (minimal sedimentation)
≥60	Exceptional stability

.

Spectral absorbance and transmittance measurements analyze light absorption and transmission in NFs to assess particle dispersion and stability. A decrease in absorbance over time indicates the instability of the NF suspension. According to Chakraborty and Panigrahi [44], absorbance (A_λ) is proportional to the concentration of particles in the solution and is given by Equation (12).

$$A_{\lambda }={\mathrm{l}\mathrm{o}\mathrm{g}}_{10}\left({\displaystyle \frac{I_o}{I}}\right)=\alpha \times 1\times c$$

(12)

where I, I_o, α, l, and c represent the intensity of the laser light beam after passing through the colloidal suspension, the intensity of the incident laser light, absorptivity, length of the light path, and concentration of particles, respectively. NF with a dark color and high particle concentration can be challenging to measure using this method [47,48]. Therefore, transmittance can also be employed to assess stability. Transmittance (T_λ) and absorbance (A_λ) are interrelated by Equation (13).

$$T_{\lambda }={\displaystyle \frac{I}{I_o}}$$

(13)

The 3ω (three omega) method utilizes the 3ω technique to measure thermal conductivity, indirectly indicating NP dispersion and stability. Transmission electron microscopy (TEM) captures high-resolution images of NPs, providing insights into their size, shape, and distribution. Dynamic light scattering (DLS) measures the Brownian motion of NPs in suspension to determine their size distribution and assess stability. The measured diffusion coefficient is then used to calculate particle size, as per the Stokes-Einstein equation [49] (Equation (14)).

$$R_H={\displaystyle \frac{k_{B}T}{6\pi \mathrm{\mu }D}}$$

(14)

where R_H is the hydrodynamic radius, k_B is Boltzmann’s constant, T is the temperature, µ is the dynamic viscosity, and D is the measured diffusion coefficient.

5. Encouraging Significant Discovery

The use of NIL for insulating oil applications was first conducted by Segal et al. [50] in 1998. The fluid was created by dispersing magnetic NPs into MO with the aim of enhancing its electrical characteristics, resulting in an improvement in breakdown voltage at high humidity levels. As a consequence, similar studies have become increasingly extensive, utilizing various NP materials, including conductive [51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68], insulative [69,70,71,72,73,74,75,76,77], and semiconductive NPs [78,79,80,81,82,83,84,85,86,87,88,89]. The scope of research extends beyond electrical properties such as BDV and tan δ to include thermal properties like thermal conductivity and viscosity.

Most studies indicate that the addition of NPs enhances both the BDV and thermal conductivity of the base fluid. A higher BDV mitigates the risk of flashover within transformers. Flashover represents a significant hazard as it can compromise insulation oil and generate hazardous gases, such as acetylene (C₂H₂) [90]. This gas is highly flammable and, if not properly managed, can lead to fires or even explosions within transformers. Furthermore, increased thermal conductivity facilitates more efficient heat dissipation, thereby reducing the formation of hot spots that could potentially ignite fires. Enhanced cooling efficiency contributes to maintaining a lower operational temperature, thus preventing the oil from reaching its flash point. It is crucial to elevate the flash point of transformer oil, as a higher flash point reduces the oil’s flammability.

Despite these advancements, there remains a limited number of studies addressing the impact of NFs on the flash point. The following section will present recent research findings on NFs, with a particular focus on the flash points of NILs.

5.1. Flash Point Prediction Method

Under operational conditions, TO will be exposed to high temperatures. Oil molecules will begin to evaporate from the fluid’s surface. At a certain temperature, the evaporated molecules will ignite if exposed to a flame. The minimum temperature at which the fluid emits a vapor that can ignite upon exposure to a flame is called the flash point [91]. The higher the flash point, the better the quality of the TO. For high-voltage transformers, using oil with a high flash point is crucial, especially for transformers operating in residential areas.

MO falls under the category of hydrocarbon compounds; hence, the discussion on prediction is focused solely on this type of fluid. Table 5 shows a collection of empirical correlations from several studies used to estimate the flash point of hydrocarbon compounds. These equations can serve as recommendations for research on the flash point of hydrocarbon compounds with various input parameters. Models are presented in their original unit systems with each range restriction and independent variables. Some methods include input parameters such as elemental composition, enthalpy of vaporization or heat of vaporization, and specific gravity. Some authors also consider volatility information by correlating vapor pressure with flash point. Unlike others, Equations (17) and (18) are recommended for both pure hydrocarbons and undefined petroleum fractions.

5.2. Flash Point Testing Method

Flash point testing methods are generally divided into two categories: open and closed-cups [92,93]. In the open cup method (Figure 14b), the sample is heated in an open container, while the closed cup method (Figure 14a) uses a small sealed cup with a small ignition point at the top of the cup. The open-cup method usually results in a higher flash point compared to the closed-cup method because it depends on the distance of the ignition source from the liquid surface and allows vapor loss to the atmosphere during testing [94]. The difference between the open-cup and closed-cup methods can reach tens of degrees. The closed-cup method is typically the preferred choice due to its higher accuracy [93].

Table 5. Empirical equations for flash point prediction of hydrocarbons.

Ref.	Formula	Aplicability	Variable	Eq. No.
[95]	$FP={\displaystyle \frac{T_b}{1.4420-0.08512\times \mathrm{L}\mathrm{n}\left(CST\right)}}$ $CST={\displaystyle \frac{83.8}{4\mathrm{C}+4\mathrm{S}+\mathrm{H}-\mathrm{X}-2\mathrm{O}+0.84}}$	Compounds (FP: 124–204 °C)	- Chemical composition (vol%) of C, S, O, H, and X - Tb (normal BP in K)	(15)
[96]	$FP=-83.3362+0.5811\times T_b+0.1118\times {\displaystyle \frac{{10}^{-3}}{{T_b}^2}}+38.734\times sg$	Hydrocarbons (BP: 20–340 °C)	- Tb (normal BP in °C) - sg (specific gravity)	(16)
[97]	${\displaystyle \frac{1}{FP}}=-0.02421+{\displaystyle \frac{2.84947}{T_{10}}}+0.0034\times \mathrm{L}\mathrm{n}{T}_{10}$	Hydrocarbon and petroleum fraction (BP greater than 500 °F)	- T10 (normal BP in K for pure hydrocarbon) - Temp. of 10% evaporated volume in K	(17)
[97]	$FP=-124.72+0.7070\times T_{10}$	Hydrocarbon and petroleum fraction (BP greater than 500 °F)	- T10 (Temperature of 10% evaporated volume in K)	(18)
[98]	$FP=4.656+0.844\times T_b-0.234\times {10}^{-3}\times {T_b}^2$	Organic compounds	- Tb (normal BP in K)	(19)
[99]	$FP=-54.5377+0.5883\times T_b+0.00022\times {T_b}^2$	Organic compounds (Tb range: −13.7–365 °C and FP: −69–193 °C)	- Tb (normal BP in °C)	(20)
[100]	$FP=225.1+{\displaystyle \frac{{537.6\times \left(\frac{2217}{T_b}\right)}^2\times {\mathrm{e}}^{\frac{-2217}{T_b}}}{({1-{\mathrm{e}}^{\frac{-2217}{T_b}})}^2}}$	Hydrocarbons	- Tb (normal BP in K)	(21)
[101]	$FP=0.3544\times T_b^{1.14711}\times {\mathrm{n}}^{-0.07677}$	Organic liquid	- Tb (normal BP in K) - n (no. of carbon atoms in the molecule)	(22)
[102]	$FP=-84.794+0.6208\times T_b+37.8127\times sg$	Hydrocarbons (BP: 20–340 °C)	- Tb (normal BP in °C) - sg (specific gravity)	(23)
[103]	$FP=33.176+0.67456\times T_b$	Hydrocarbons	- Tb (normal BP in K)	(24)
[104]	$FP=-18.44+0.8493\times T_b-3.723\times \mathrm{n}$	Compounds	- Tb (normal BP in K) - n (no. of carbon atoms in the molecule)	(25)
[105]	$FP=0.683\times T_b-119$	Hydrocarbons (BP: 200–700 °F)	- Tb (normal BP in °F)	(26)
[106]	$FP=0.714\times T_b$	Hydrocarbons	- Tb (normal BP in K)	(27)

Table 5. Empirical equations for flash point prediction of hydrocarbons.

Ref.	Formula	Aplicability	Variable	Eq. No.
[95]	$FP={\displaystyle \frac{T_b}{1.4420-0.08512\times \mathrm{L}\mathrm{n}\left(CST\right)}}$ $CST={\displaystyle \frac{83.8}{4\mathrm{C}+4\mathrm{S}+\mathrm{H}-\mathrm{X}-2\mathrm{O}+0.84}}$	Compounds (FP: 124–204 °C)	- Chemical composition (vol%) of C, S, O, H, and X - Tb (normal BP in K)	(15)
[96]	$FP=-83.3362+0.5811\times T_b+0.1118\times {\displaystyle \frac{{10}^{-3}}{{T_b}^2}}+38.734\times sg$	Hydrocarbons (BP: 20–340 °C)	- Tb (normal BP in °C) - sg (specific gravity)	(16)
[97]	${\displaystyle \frac{1}{FP}}=-0.02421+{\displaystyle \frac{2.84947}{T_{10}}}+0.0034\times \mathrm{L}\mathrm{n}{T}_{10}$	Hydrocarbon and petroleum fraction (BP greater than 500 °F)	- T10 (normal BP in K for pure hydrocarbon) - Temp. of 10% evaporated volume in K	(17)
[97]	$FP=-124.72+0.7070\times T_{10}$	Hydrocarbon and petroleum fraction (BP greater than 500 °F)	- T10 (Temperature of 10% evaporated volume in K)	(18)
[98]	$FP=4.656+0.844\times T_b-0.234\times {10}^{-3}\times {T_b}^2$	Organic compounds	- Tb (normal BP in K)	(19)
[99]	$FP=-54.5377+0.5883\times T_b+0.00022\times {T_b}^2$	Organic compounds (Tb range: −13.7–365 °C and FP: −69–193 °C)	- Tb (normal BP in °C)	(20)
[100]	$FP=225.1+{\displaystyle \frac{{537.6\times \left(\frac{2217}{T_b}\right)}^2\times {\mathrm{e}}^{\frac{-2217}{T_b}}}{({1-{\mathrm{e}}^{\frac{-2217}{T_b}})}^2}}$	Hydrocarbons	- Tb (normal BP in K)	(21)
[101]	$FP=0.3544\times T_b^{1.14711}\times {\mathrm{n}}^{-0.07677}$	Organic liquid	- Tb (normal BP in K) - n (no. of carbon atoms in the molecule)	(22)
[102]	$FP=-84.794+0.6208\times T_b+37.8127\times sg$	Hydrocarbons (BP: 20–340 °C)	- Tb (normal BP in °C) - sg (specific gravity)	(23)
[103]	$FP=33.176+0.67456\times T_b$	Hydrocarbons	- Tb (normal BP in K)	(24)
[104]	$FP=-18.44+0.8493\times T_b-3.723\times \mathrm{n}$	Compounds	- Tb (normal BP in K) - n (no. of carbon atoms in the molecule)	(25)
[105]	$FP=0.683\times T_b-119$	Hydrocarbons (BP: 200–700 °F)	- Tb (normal BP in °F)	(26)
[106]	$FP=0.714\times T_b$	Hydrocarbons	- Tb (normal BP in K)	(27)

5.3. The Influence of NPs on Flash Point

One way to increase the flash point is by utilizing nanotechnology. The use of metal oxide NPs, such as TiO₂, Al₂O₃, and silicon dioxide (SiO₂) in MO, can provide additional thermal stability and play a role in increasing the effective boiling point of the MO, thereby increasing the flash point. TiO₂ at a concentration of 0.00006 wt% can increase the flash point by up to 22.73% compared to the base fluid. A downward trend starts when the concentration is increased, resulting in only a 15.91% increase at a concentration of 0.0001 wt% [78]. For Al₂O₃, the increase can reach 17.86% at a concentration of 0.005 wt%, and a decrease occurs when the concentration is increased to 0.002 wt%, with only a 14.29% increase in the flash point. Interestingly, the flash point increase stagnates at 14.29% when the concentration is increased to 0.05 wt% [107]. For SiO₂, there is a relatively small increase of only 1.46% at a concentration of 0.032 wt% [66].

Carbon-based NPs such as MWCNTs have high conductivity, which helps modify the thermal properties of the base fluid [108]. The addition of MWCNTs to MO at the right concentration will increase the flash point. At a concentration of 0.001 wt%, the flash point increases by up to 4.6%, but if the concentration is raised to 0.01 wt%, the flash point drops below that of the base fluid [108]. A similar condition was demonstrated by [109], showing an increase in flash point of up to 13% at a concentration of 0.2 wt%, but a downward trend occurs when the concentration is increased to 0.5 wt%, although it still remains above the base fluid.

Research on the effect of NPs on flash points is very limited in the literature, indicating significant potential for further exploration in this field. Figure 15 illustrates that the majority of studies have resulted in increased flash points by adding NPs to MO. Only one study, conducted by Behesti et al. [108], showed a decrease in flash point by −1.33% using MWCNTs NPs at a concentration of 0.01 wt%. This highlights the substantial potential of NP use in advancing research on flash points in MO.

Table 6. Summary of research on the effect of NPs on the flash point of MO.

No.	Ref.	Year	NPs	Concentrations (wt%)	Results
1	[108]	2014	MWCNTs	0.001; 0.01	The optimal concentration of 0.001 wt% resulted in a 4.6% increase in flash point, whereas concentrations higher than 0.01 wt% led to a decrease in the flash point.
2	[109]	2013	MWCNTs	0.1; 0.2; 0.5	Increasing the concentration of carbon nanotubes up to 0.2 wt% resulted in a flash point increase of about 13%. However, with concentrations up to 0.5 wt%, the flash point showed a decreasing trend.
3	[111]	2017	AGQD	0.001	A significant increase in flash point was observed when 0.001 wt% of AGQD was added to the transformer oil.
4	[78]	2021	TiO2; ZnO	0.00001; 0.00003; 0.00006; 0.00008; 0.0001	The addition of both types of nanomaterials caused an increase in the flash point of the oil blend. However, the increase caused by ZnO NPs was greater compared to the increase caused by TiO2 NPs.
5	[107]	2013	Al; Cu; CuO; Al2O3	0.005; 0.023; 0.045; 0.45; 0.9	The flash point values of Al2O3, aluminum, CuO, and copper-based NFs show significant increases for various concentration. Copper NP-based transformer oil shows the highest enhancement of flash point.
6	[66]	2017	Fe3O4; ZnO; SiO2	0.032; 0.064; 0.096; 0.128; 0.16	The flash point increases with the rising concentration of all types of NPs.
7	[110]	2017	CCTO	0.003; 0.006; 0.028	The flash point increases as the concentration of CCTO in the MO increases.

also shows that concentration is a crucial parameter affecting the increase in flash points. Most studies indicate that increasing concentration will enhance the flash point. However, some studies have shown that the flash point increase will reach a maximum and then trend downwards [109], even falling below that of the base fluid [108]. This is intriguing because none of the studies so far have identified the truly optimum concentration for increasing the flash point. For example, Behesti et al. [108] found a decrease in flash points at concentrations higher than 0.01 wt%, a result that is contradictory to the findings of Ettefaghi et al. [109], where a much higher concentration of up to 0.5 wt% still resulted in a flash point higher than that of the base fluid. Despite both studies showing a decrease in flash points at certain concentrations, there is no clear consensus.

In addition to concentration, the type of NP also affects the flash point. This is evident from studies where the same concentration with different NPs shows varying flash point increases. According to Siddique et al. [78], the flash point enhancement characteristics of TiO₂ and ZnO are quite different. Both NPs reach their maximum flash point at a concentration of 0.00006 wt%, with ZnO performing better than TiO₂. Conversely, at higher concentrations, ZnO shows a more pronounced downward trend compared to TiO₂. A different observation was made by Karthik et al. [107], which reported a stagnation in flash point values. Increasing the concentration from 0.005 to 0.05 wt% of CuO resulted in the same flash point value, and no difference in flash point was observed for Al₂O₃ at concentrations of 0.002 and 0.05 wt%. These variations across different studies present a significant opportunity for future research to explore other parameters that may influence the flash point with NP addition, such as particle size, purity, preparation methods, and other factors.

There are numerous positive aspects of NPs, but the study must also comprehensively address other aspects. The impact on health and the environment requires further in-depth research. Once high-quality NFs are successfully developed, it is crucial to evaluate their potential effects on health and the environment. Furthermore, the management of waste from used NFs must be carefully considered. If these aspects are not addressed properly, the benefits of using NFs could be overshadowed by their negative impacts, rendering their advantages negligible.

5.4. Minimizing Fire Risk in Transformers

No matter how high the flash point of TO is, the risk of fire remains as long as the insulating liquid used is flammable. Protection and preventive measures must be taken to minimize this risk. When focusing on insulating oil, actions related to protection, handling and early detection of TO must be implemented.

5.4.1. Detection of Gas Accumulation in Oil

The earliest detection method involves early indication of potential gas presence in the oil. Under normal conditions, TO will not produce gas. If a failure or flashing occurs inside the transformer, the TO will decompose into gas. A protection signal is needed for early detection of this occurrence. This instrument is typically installed on the pipe between the main tank and the conservator, as this area is certain to be traversed by gas from the main tank to the highest point of the transformer.

This instrument is called a Buchholz relay, which will provide an alarm signal if the amount of accumulated gas reaches the maximum limit. This equipment will also command the transformer to shut down if there is a sudden significant gas accumulation. Additionally, the Buchholz relay will signal if there is a sudden oil drop, which occurs if there is a significant leak in the transformer’s main tank [112].

5.4.2. Monitoring Dangerous Gases

The decomposition of MO in the temperature range of 150–500 °C produces low molecular weight gases such as hydrogen and methane, along with small amounts of higher molecular weight gases like ethylene and ethane. When the temperature decreases from high to normal, heavier molecules such as acetylene, carbon monoxide, and carbon dioxide are formed [113].

Some of the gases produced during oil decomposition are highly dangerous and flammable. These gases cannot be avoided but can be monitored regularly. If the gas content exceeds the maximum threshold, special treatment of the TO must be performed immediately. Monitoring the amount of these gases can also be used to predict the transformer’s lifespan, allowing for preparation for replacement when the transformer nears the end of its usable life. This gas content testing is called dissolved gas analysis (DGA) [90,114,115,116].

5.4.3. Pressure Protection

If a transformer catches fire, there is a significant risk of a large explosion, leading to more severe damage. Explosions occur due to the accumulation of pressure inside the main tank as the temperature rises. To avoid explosions, the pressure must be reduced. The quickest way to reduce pressure is by releasing oil from the tank.

Manually releasing oil in the event of a transformer fault is very dangerous. Therefore, an automatic protection device that can discharge oil while tripping the transformer is needed. A commonly used device is the pressure relief device, which will disconnect the transformer and release the pressure from the tank if it exceeds the maximum operating pressure.

5.4.4. Fire Prevention

The final defense for a transformer is to extinguish the fire quickly if one occurs. This equipment is called fire prevention or fire protection. In the event of a fire and excessive pressure, a ruptured disk will break, evacuating hot oil to reduce the pressure. Simultaneously, nitrogen will be injected into the main tank to extinguish the fire and cool the remaining oil in the main tank. This equipment acts as the last line of protection if the initial protection systems fail. If this equipment is activated, it indicates that the transformer’s protection system is very poor, as it had to go through several layers of protection. An explosion in the transformer can be avoided if all protection systems function perfectly. Essentially, a transformer is a static piece of equipment with a lifespan of up to 30 years. Good maintenance and protection systems are necessary to avoid significant losses.

5.5. Research Potential and Future Challenges

Among all types of transformer oils, MO has the poorest quality in terms of flash point. MO is classified as a less flammable liquid because it has a flash point below 300 °C [117]. MO can ignite very quickly, within approximately 5 s of exposure to fire, and will continue to burn continuously [118]. Explosions in transformers using MO cause severe damage and significant safety issues [119]. This indicates the significant danger of MO, making it unsuitable for the design requirements of electrical equipment with high fire resistance. Although nanofluids have great potential to improve the flash point of MO, they are still in the research and development stage [120]. Therefore, the potential for research in this area is substantial. Future research needs to critically analyze the strategies reviewed related to enhancing the flash point of MO using NPs. This critical analysis should include a thorough evaluation of the advantages and disadvantages of each strategy identified in the literature.

The majority of research shows that the addition of NPs can significantly increase the flash point of MO, which is a significant advantage in practical applications. However, there are several drawbacks that must be critically considered. One major concern is the potential negative impact on health and the environment due to the use of NPs. Although preliminary research indicates benefits, potential toxicity and challenges in managing NP waste require further attention. Additionally, there is variability in research results, with some studies showing insignificant increases in flash point and even some reporting a decrease. This suggests that more research is needed to understand the conditions and factors affecting the effectiveness of NPs. This critical analysis should also consider the costs involved in implementing this technology. The costs of developing, producing, and applying NPs in transformer oil could be a constraint, especially when compared to other solutions that might be more economical and practical.

Overall, critical analysis of these strategies helps in understanding the extent to which the benefits of using NPs can be balanced with their potential risks. Future research should also ensure that the proposed strategies are not only technically effective but also safe, sustainable, and cost-effective in the long term. This is important to ensure that the research and implementation of this technology yield optimal results without introducing new problems in the future.

6. Conclusions

Transformers are crucial equipment in the electric power distribution system. Transformer failures can cause significant economic and safety losses. The insulation system contributes significantly to transformer failures, necessitating reliable insulation. Nanotechnology utilization is very promising in addressing this issue. The use of NPs can improve the quality of TO, especially MO, which has a lower flash point compared to other types.

MO is the most widely used type of oil but has the highest fire risk compared to other types. One way to reduce the fire risk is by increasing the flash point of MO. Research on the utilization of NPs to increase flash points is still rare, and thus, the literature is limited. From the collected literature, it is concluded that adding NPs to MO will increase the flash point of the base fluid. An increased flash point will reduce the risk of fire in TO.

No matter how high the flash point of TO is, there is still a potential for fire as long as the type of oil used is flammable. To minimize this occurrence, additional layered protection is necessary. The initial protection involves the indication of gas formation in TO. Subsequently, the types of gases in the oil are identified as references for preventive actions. If all protections fail, quick extinguishing equipment is needed to minimize further significant losses.

Future research should investigate how factors like particle size, purity, and preparation methods affect the flash point of NFs. While NPs offer many benefits, their health and environmental impacts require thorough examination, and proper waste management is essential to ensure their advantages are not outweighed by negative effects.