1. Introduction
According to the sustainable development, many challenges must be solved in the world’s industry, especially in power engineering. This important part of the industry faces great pressure to improve technologies to ensure the supply of electricity, with having regard for all strict regulations and environmental protection. The greatest attention is paid to the production and consumption of electricity, but also many other parts of this industry, such as transmission and distribution of electricity, are key to fulfil goals and needs regarding the sustainable development [
1]. One of the affected parts of this industry facing new challenges and modifications is the insulation of power transformers, especially insulating oils. Power transformers, as one of the most important devices in power engineering, are used for the transformation of voltage levels or for galvanic separation of electrical systems with the same voltage. To secure the operation of the power transformer, heat transfer and insulation of different parts have the highest priority. The most common failures in transformers are related to the quality of materials, random defects, and age. The age-related failures have a typical “bathtub” pattern depicted in
Figure 1 that shows the probability of failure related to transformer operating time. In the early years of service, there is a higher number of failures due to infant mortality. A low number of failures with constant failure rate lasts until the age when power transformers are affected for example by moisture or contamination by different kinds of insoluble particles and gases that cause a decrease in internal dielectric strength [
2]. The biggest number of failures are located in windings, bushings, tank, or tap chargers, and these failures are mainly electrical, lighting, insulation, and connection caused [
3,
4]. The lifespan of a power transformer is around 40 years, more precisely from 32 to 55 years with a standard deviation of 8 years, according to the design, loading, insulation, humidity, and working temperature [
5,
6]. According to [
7], the lifespan of a transformer that failed because of the insulation is 17.8 years, which is only half of the estimated lifetime. According to [
8], transformer oil lifetime prediction is 20.55 years, but in bad conditions, it can be lowered to 5.5 years. This information confirms the importance of progressive research on insulating fluids, their enhancement, analysis, and substitution.
Throughout history, there have been different kinds of insulating fluids used in power transformers. Rafiq et al. in [
9] offered an eloquent historical review. In short, the origin of this field of study is in the year 1890 when the first oil-immersed transformer was developed and filled with petroleum-based liquid, so-called paraffin oil [
10]. This oil had a low viscosity and outstanding insulation properties, but it was replaced after the year 1925 when the pour point was determined as too high and due to the insoluble particles (the result of oxidation) that affected its heat transfer and finally resulted in a shorter lifespan. For a few years, naphthenic oils replaced paraffin oils because of a higher pour point and better resistance to oxidation. However, this oil was very flammable and had environmental contaminant status, so it had to be replaced, too. PCBs oils (polychlorinated biphenyls) used for 40 years, till 1970, met the requirements for good liquid insulation, but the fact that they are toxic, changed its status to environmentally unacceptable. In year 1978 PCBs were banned [
9,
11].
The followers of PCB oils were mineral oils and silicone fluids as a new alternative, and their usage continues even today. The development of insulating fluids, later focused more on ecological aspects like biodegradability, led to the discovery of natural and synthetic esters as environmentally friendly fluids. All insulating fluids have to meet certain requirements and the most important features for categorization and rating of quality are thermal conduction, viscosity, fire point, biodegradability, dissipation factor breakdown voltage, and resistivity of fluids [
12]. Moreover, the presence of moisture in liquid affects the mentioned properties, so it is very important to find out the moisture content and how water in each state influences the properties of different fluids. For example, in [
13], there is a description of the influence of saturated moisture in transformer oils on the breakdown voltage, finding that 20% moisture saturation before and after the ageing process did not affect natural or synthetic esters, but a value (breakdown voltage) of N3X (high-grade mineral oil Nynas Nytro 3000X) was halved.
The most frequently used insulation fluid in power transformers is mineral oil due to its good dielectric and thermal properties. However, due to their environmental footprint, they are currently considered unsatisfactory. The tendency of actual research is to replace mineral oils with natural (vegetable oils) or synthetic esters as more environmentally friendly alternatives [
14]. If one compares the biodegradability of mineral oil and natural or synthetic esters, it is obvious that the esters are much more environmentally safer with biodegradability more than 80%, while mineral oils reach the values of less than 30% [
15]. Many studies deal with the comparison of environmentally friendly liquids with mineral oils, for example [
9,
12,
16,
17]. To sum up, the biggest advantage of mineral oils is a low viscosity, low dissipation factor, and high resistivity and breakdown voltage that shows its predestination for high voltage equipment. On the other hand, the weakest point is the safety, because of its low fire and flame point and low biodegradability that can cause environmental damage and problems during liquidation of used oils.
Recent research has revealed that there is a possibility to improve the heat transfer and dielectric properties of the insulating liquids by introducing various nanoparticles. A mixture of nanoparticles and a base fluid is called a nanofluid. Nanofluids are one of the possible options as a substitution for the currently most used insulating fluids for high voltage equipment [
9]. Power engineering, as one of the most important industries, is heading to increase the transmission voltage to keep the distribution of electricity sufficient and reliable, so the need for a new and more effective insulation is justified [
18]. Furthermore, high voltage substations and equipment are of large area and because of that, there is a need to smaller the size of this equipment that goes hand in hand with a more efficient insulation and heat transfer medium. Nanofluids are a possible option for reducing the size of equipment because of their better resistivity to overvoltages, and degradation caused by humidity in comparison to base fluids [
19]. Moreover, the fact that nanoparticles may improve the heat transfer properties confirm the possibility to construct smaller high voltage equipment filled with oil. A prototype of a ferrofluid-cooled transformer with reduced sizes is published in [
20]. Concerning the environmental aspects, the mixture of environmentally friendly oils and nanoparticles may be a good option for maintaining the sustainable development and quality requirements [
21].
2. Different Kinds of Base Fluids
Generally, the most used transformer oils are mineral oils (MO). The reason for their usage is the fact they have excellent performance in practice and they are very reliable with a long tradition. Mineral oil is created from crude oils (fossil fuels) that contain hydrocarbon compounds of different bonds [
16,
22]. It is a translucent liquid “composed of hydrocarbons, among which are straight-chain alkanes, branched alkanes, cyclic paraffin and aromatic hydrocarbons” [
23]. The disadvantage of paraffinic oils is their high pour point that must be treated by additives to reach the required values. The difference between them is not very significant and they are classified as very naphthenic if there is a minimum of 50% of the naphthenic structure, intermediate—containing 44–50% of the naphthenic structure, and below 44% of the naphthenic structure in MO is considered to be paraffinic [
23].
Mineral oil as a potential environmental contaminator has low biodegradability that is a possible critical issue for its usage in future. Mineral oil has a relatively low fire and flash point that may be a safety issue for the environment and people and it could cause serious environmental damage after ignition and leakage from the power transformer, finally resulting in an ecological problem for the affected region [
17,
24]. However,
Table 1 shows high values of viscosity and dissipation factor as important properties for insulating oil, concretely viscosity for heat transfer and dissipation factor for economic issues (power loss). To sum up, MO is an excellent insulating fluid tested in practice, but it conflicts with the sustainable development.
The most convincing difference between MO and silicone fluids (SF) is flammability. SF have significantly higher fire and flame points with similar values of insulation properties as MO. A big contrast in safety between SF and MO can be seen from
Table 1, pointing out a difference in fire point up to 240 °C. SF is chemically known as polydimethylsiloxane and it is not frequently used mainly because of a higher price than MO, and because of biodegradability, which is at the lowest level in comparison with other base fluids that makes the biggest environmental impact [
16,
25].
A major component of synthetic esters (SE) is pentaerythritol tetra ester developed from chemicals with biodegradability at a level from 80 to 89%. Firstly, SE as an insulation liquid in power transformer was used in 1976. Synthetic esters are mostly used in special and distribution transformers with a relatively low level of voltage [
9,
21]. Comparison between SE and MO in
Table 1 indicates that the properties are comparable and, in some cases, even better for SE. The most noticeable disadvantage of SE is the high dissipation factor [
26]. The most significant commercial SE are MIDEL 7131, NYCODIEL 1244, BecFluid 9902, ECO-FR PDS, Naturelle Transformer Oil S4, Envirotemp 200 [
16].
Natural esters (NE) are the best option as environmentally friendly insulating fluids for high voltage equipment. Commercial availability of natural esters has a rising tendency that makes it one of the most likely options for the power transformers refilling. NE is defined as a refined vegetable oil or a plant-based ester derived from crops. For the extraction of oil, seeds or other parts of a suitable plant are used [
9]. Biodegradability at a level from 97 to 99% makes the most significant difference in comparison to other insulating oils. This high level of biodegradability is connected with the safety of this insulating liquid mainly in the case of a possible leakage. The advantage of high fire and flame points is confirmed by statistics from 2014 when there was no reported fire or environmental issue in over 600,000 transformers using NE [
27]. Disadvantages of NE are relatively high pour point that is in some cases −10 °C and high dissipation factor according to the type of NE up to 0.4 [
28]. The most significant commercial NE are BIOTEMP, Envirotemp FR3, ambient insulating fluid, ambient prime insulating fluid, NeuGen 1540 [
16]. SE have a more stable chemical structure than NE and it results in a better oxidation stability. NE have the worst oxidation stability among the mentioned base fluids [
24]. However, the addition of nanoparticles into a base fluid may enhance the electrical properties and NE as an environmentally friendly fluid may be a suitable option for the refinement. Moreover, the enhanced properties may assure the ecologically friendly insulation fluid that could meet the requirements for sustainable development as the future most widely used insulation liquid.
Table 1 compares various properties of the mentioned base fluids. The range of values is connected with different references with various experimental results.
Table 1.
Different properties of base fluids.
Table 1.
Different properties of base fluids.
Properties | MO | SF | NE | SE | Reference |
---|
Biodegradability in 21 days [%] | <30 | very low | 97–99 | 80–89 | [9,16,17] |
Viscosity at 40 °C [mm2/s] | 3–16 | 35–40 | 16–37 | 14–30 | [14,26] |
Flash point [°C] | 100–175 | 300–310 | 250–316 | 260–343 | [16,17,26] |
Fire point [°C] | 110–185 | 330–350 | 300–370 | 300–322 | [16,17,26] |
Pour point [°C] | −30 to −63 | −50 to −60 | −10 to −33 | −40 to −60 | [16,17,26] |
AC breakdown voltage [kV] | 30–85 | 35–60 | 49–97 | 43–75 | [16,17,26,29] |
Dissipation factor at 90 °C [–] | <0.001 | 0.0016 | 0.0001–0.009 | 0.001–0.03 | [13,16] |
Resistivity at 90 °C [GΩm] | 1013–1015 | 1014 | 1013–1014 | 1013 | [9,16] |
3. Nanofluids
There are many different mixtures tested in research history with different insulation fluids and nanoparticles, but there are also different ways of preparing nanofluids categorized in a one-step process and a two-step process. According to [
9], the origin of the word “nanofluid” is connected with the work by Choi et al. back in 1995, where they researched enhancement of thermal conductivity of fluids mixed with nanoparticles, determined the direction of further research, described the theoretical study of these fluids, and set potential benefits of nanofluids [
30]. From that point, the number of scientific papers that deal with nanofluids has been exponentially increasing. Till 2008, publications on the topic of nanofluids did not exceed number 100 throughout the year. Since 2008 there has been a more significant increase of scientific papers year by year [
31].
Figure 2 shows the number of publications on the web of science core collection year by year made by basic search of word “nanofluid”. The biggest growth of scientific papers was in recent years when the topic of enhancement of heat transfer and insulating fluid is more discussed, because of increasing demand on life and industry. More than 36% of these papers are in the category of thermodynamics, 22% mechanics and 21% belong to mechanical engineering. Of course, the paper may be in more categories at once. The highest increase of papers was in 2019 when the number of scientific papers dealt with nanofluids was 3707 in comparison with 2642 papers in 2018. Last year, there have been more than 4200 papers published mostly in the categories of thermodynamics, mechanics, and physical chemistry.
3.1. One-Step Method
In a one-step process of preparation of nanofluids, the formation and dispersion of nanoparticles are simultaneous [
32]. This method avoids the process of transportation, storage, drying, and dispersion of nanoparticles to decrease a measure of agglomeration of nanoparticles and to increase the stability of nanofluids [
33,
34,
35]. One of the one-step preparation methods is a vapor deposition. This method was patented by Choi and Eastman in 2001 [
36]. The principle consists in the formation of a thin layer of a base fluid on a vessel wall caused by a centrifugal force of a rotating disk. The material is then heated and evaporated in the vessel filled with an inert gas at a low pressure. The creation of nanofluid is finished when the raw material vapors condense by interacting with the thin film of swirling water and finally settle in the base fluid [
32,
36]. The second possible one-step method is the laser ablation where a highly concentrated laser beam is used for dispersion of nanoparticles from the surface of a material immersed in a base fluid. Important properties are the intensity and the wavelength of the laser beam [
37,
38]. There are more possible preparation methods using the one-step process such as the submerged arc method, precipitation (ion exchange) method, chemical reduction method, emulsion polymerization, sol-gel (hydrolysis) method, or microwave-assisted reaction [
32,
39,
40]. The submerged arc nanoparticle synthesis system is an efficient one-step method to prepare nanofluids based on dielectric liquids containing copper nanoparticles [
41,
42].
3.2. Two-Step Method
In the first phase of the two-step method, nanoparticles (nanorods, nanofibers, or nanotubes) are first prepared by sol-gel method, hydrothermal synthesis, or by other techniques [
32]. The sol-gel method is used for nanoparticles with high surface area and provides effective control over the texture and surface properties of nanoparticles. There are five main steps of this method starting with hydrolysis and then polycondensation, ageing, drying, and thermal decomposition [
32,
40,
43]. According to [
44,
45], hydrothermal synthesis refers to the “heterogeneous reactions for synthesizing inorganic materials in aqueous media above ambient temperature and pressure” and the advantage of this method is the low energy consumption, the low-temperature processes, and the environmental impact. However, the high price of the needed autoclaves may be considered as a potential drawback [
32]. The second step of the two-step method is the preparation of nanofluids using mostly ultrasonication (the bath and probe ultrasonication are of remarkably different effectivity), magnetic stirring, adjusting pH value (addition of a dispersant in
Figure 3), or a combination of these processes [
32]. Ultrasonication enhances performance, stability, thermophysical properties and prevents aggregation and sedimentation of nanoparticles in nanofluid, but the ideal duration of sonication is the point of research of many authors [
46,
47,
48,
49,
50]. Magnetic stirring is used for dispersion of nanoparticles with a low concentration by the stirring action made by a stir bar that spins very quickly because of a rotating magnetic field created by the set of rotating magnets or electromagnets [
32,
51,
52].
Selecting a particular method, additives and timing within the applied methods are very important to make a nanofluid with excellent properties without sedimentation and agglomeration of nanoparticles. A more accurate description of preparation methods of eleven different nanoparticles is in the reference [
53].
To compare the two preparation methods, one can state that the two-step method is more economic in production of nanofluids in a larger scale and it is used most widely. However, without modifying the nanoparticle surface, the tendency to agglomerate before adding to the base liquid is considered as a major disadvantage of the two-step method. On the other hand, the one-step method cannot be effectively used to synthesize nanofluids in a large scale, but this method can yield uniformly and stably dispersed nanoparticles.
3.3. Stability of Nanofluids
The stability of nanofluids is of crucial importance because the sedimentation and agglomeration of nanoparticles in the base fluid cause deterioration of thermophysical properties, mainly a decrease of thermal conductivity and increase of viscosity [
54,
55,
56]. Because of these facts, examination of stability is an important part of research and there are several methods to analyze the stability of synthesized nanofluids. The most applied methods to study the stability are a zeta potential test, a sedimentation method (photograph capturing method), ultraviolet-visible spectrophotometer and dynamic light scattering [
56,
57].
3.3.1. Zeta Potential Method
The Zeta potential (ζ-potential) method is the most used method to examine the stability of nanofluids. It can be defined as the potential difference between the stationary layer of base fluid which is attached to nanoparticles and the surface of nanoparticles [
54]. It indicates the degree of repulsion between charged particles in the fluid and it can be calculated by the Helmholtz–Smoluchowski equation [
58,
59]:
where
U is electrophoretic mobility,
µ is viscosity, and
ε is the dielectric constant of the base liquid. If the value of Zeta potential is over 60 mV, a nanofluid has an excellent stability, between 60 and 40 mV it has a good stability, between 40 and 30 mV the nanofluid is considered as stable, and below 30 mV it is considered as highly agglomerative [
57,
60,
61]. Because of repulsive forces, the Zeta potential can be controlled over pH value [
62,
63]. A change of pH influences the surface charge on nanoparticles and modifies their interaction behavior [
57]. If the pH of the nanofluid has low values, the Zeta potential will be positive. On the other hand, with higher pH values, the Zeta potential will be in negative values. The point when a pH value corresponds to zero Zeta potential is called the isoelectric point, when nanofluids are least stable, so stability rises in the positive or negative direction from that point [
57]. Measurements of the Zeta potential are the most often performed by a Zeta Sizer Nano (ZSN) device [
60,
64,
65].
3.3.2. Sedimentation Method (Photograph Capturing Method)
The sedimentation method is the simplest option of measuring the stability of nanofluids qualitatively, by observing photographs taken in different periods [
60,
66]. Owing to external forces (gravitation), the nanoparticles settle on the bottom of the fluid in a clear glass test tubes that can be observed by comparing photographs taken at different times [
67]. In an unstable nanofluid there are three ways of sedimentation. The first one is a dispersed sedimentation where the height of a sediment rises from the bottom. The second one is a flocculated sedimentation where the sedimentation is lowered with time, and the last one is a mixed sedimentation where the behavior of both previous cases is observed at the same time [
60,
67,
68]. The most significant impact on sedimentation has the concentration of nanoparticles and properties of the base fluid [
69,
70].
3.3.3. Ultraviolet-Visible Spectrophotometer
The ultraviolet-visible spectrophotometer is commonly used for the quantitative characterization of the colloidal stability of the dispersions [
56]. One of the major advantages of this method is its suitability for all base fluids because its functioning is about the intensity of the light that becomes different because of lights scattering and absorption when passing through the fluid [
57,
61,
71]. According to [
57,
71] and [
72], the range of UV–visible spectrophotometer is from 200 to 900 nm wavelengths and basically, it measures various dispersions in the fluid. The stability is determined by the dispersion of nanoparticles in different time results [
73].
3.3.4. Dynamic Light Scattering
Dynamic light scattering is a suitable method for measuring mainly spherical particles and the most significant advantage is that this method does not need drying of the dispersion (some dispersants are difficult to remove) [
74,
75]. A simple description of this method is that a source of monochromatic light shines on the sample and a detector collects the scattered light signals [
72,
76]. There is a need to know the refractive index and viscosity of a measured base liquid, and the measurement output is a signal that shows random changes due to the randomly changing relative position of the particles due to the random Brownian motion. Size as the final output is calculated by the Stokes–Einstein equation [
61,
62].
3.4. Structural Characterization of Nanofluids
The essential parameters determining the physical properties of nanofluids are the nanoparticle size distribution, morphology, crystal structure, and elemental composition. Several techniques can be used to characterize nanoparticles from these points of view. In this chapter we mention just a few techniques.
3.4.1. Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM)
These methods have evolved over many years into a highly sophisticated instrument and have found different applications across many scientific disciplines, because of their excellent ability to distinguish the shape, size, and distribution of nanoparticles [
77,
78,
79]. In [
57], the methodology of transmission electron microscopy is described as: “the electrons shoot through the sample and measures how the electron beam changes as it is scattered in the sample. Scanning electron microscope images the sample surface by scanning it with electron beams in a raster scan pattern. The electrons interact with the sample atoms producing signals that contain information about the sample’s surface topography, composition and other properties”. The disadvantage of this method is that it does not capture the real situation of nanoparticles in nanofluids because there is a need for dried samples prepared in a vacuum oven [
61,
80].
3.4.2. X-ray-Based Techniques
One of the most extensively applied methods for nanoparticle characterization is X-ray diffraction (XRD) [
81]. It provides information on the crystal structure, nature of the phase, crystalline grain size and lattice parameters. For this method, the nanoparticles in powder form are commonly used after drying the colloidal solution. XRD provides statistically representative, volume averaged values. For instance, this method has been applied to determine the average crystallite size of magnetite nanoparticles [
82]. Another X-ray-based analytical method to determine the structure of nanoparticles in terms of averaged size or shapes is small angle X-ray scattering (SAXS) [
83]. Normally, a transmission mode is used, when the X-ray beam is sent through the nanofluid sample and the average structure of all illuminated particles is measured.
3.4.3. Neutron Scattering Techniques
Analogously to the SAXS method, small angle neutron scattering (SANS) is often used to study the structure of nanofluids, in terms of nanoparticle size and shape distribution, but also to study assembly and alignment of nanoparticles [
84]. Among the advantages of neutrons, one can highlight their larger penetration depth and an option of using contrast variation. In this way, different parts of a sample can be selectively viewed via isotopic labelling. This method has been found especially useful in structure research of magnetic nanofluids [
85].
3.4.4. Atomic Force Microscopy
Atomic force microscopy (AFM) is a technique capable of providing three-dimensional images of surfaces. It measures the interacting forces between a fine probe and the sample. In this way, individual particles and groups of particles can be resolved and shape and size distribution of nanoparticles can be obtained [
86]. AFM can scan the sample under different modes depending on the degree of proximity between the probe and the sample (contact, non-contact, and tapping mode). The tapping mode is the most common when characterizing nanoparticles [
87].
Clearly, in order to get the complete picture of an unknown sample, which allows extraction of proper correlations between the structure and the improvement of the base liquid properties, one needs to employ various methods, because their results are complementary.