*5.1. Transformer Insulation*

Epoxy resins, polyester, silicone, and imides are the most commonly used polymers for electric machines and dry transformers (see Figure 12) [6]. Dry-type transformers, known as epoxy-insulated transformers, are applied in areas requiring high fire protection, such as oil depots, high-rise buildings, and airports. Reinforced polyester with glass filaments is primarily used as a support for the protective system in oil transformers. These films increase the load due to breaking and provide strong resistance to infringement [118]. These tapes are suitable for leaded insulation wires, cable winding, and external protection. On the other hand, silicone transformer oil is primarily used as a coolant in high voltage power transformers. It has good heat capacity values, low viscosity, and high dielectric strength. Polyesteramideimides have long time endurance to 180–210 ◦C and have high heat resistance [119]. They are used as wire insulations in oil transformers.

**Figure 12.** Epoxy-resin cast dry type transformer.

#### *5.2. Insulated Power Cables*

In most power cables ranging from high to medium level, polymers are extensively applied as insulation materials. Such polymeric materials include polyethylene (PE), HDPE, LDPE, ethylene-propylene rubber (EPR), etc. Nevertheless, LDPE is more versatile and, until the 1960s, it was the most widely used polymeric material. Nowadays, cross-linked polyethylene (XLPE) has been preferred over paper because of its enhanced properties, such as efficiency and the ability to withstand high temperatures [120]. However, recently, XLPE cables have started to be replaced by HDPE in more advanced distribution systems because of their higher resistance to lightning strikes and water (see Figure 13) [121]. In some medium cables that require greater flexibility, EPR is used [120]. However, for low voltage applications, PVC is much more preferred because of its low manufacturing and durability cost. In new technologies such as the HVDC, polymeric cables are not used, and

instead, oil impregnated paper is widely utilized. For such applications, polymeric cables are highly prone to partial discharges. Novel techniques such as the modifying the thermal resistivity and reducing the space have been proposed for the development of the polymerinsulated HVDC cables [122]. Other techniques being developed to improve polymeric cables under the HVDC include the development of nanocomposites, addition of inorganic particles [7,123,124]. All these additions can improve mechanical strength, thermal stability, and stringent dielectric breakdown. The research now focuses on applying nanoparticles to enhance the dielectric properties of nanocomposite [125]. For polyethylene and poly-vinyl chloride insulation, their rated voltages can be up to 275 kV and 3.3 kV respectively.

**Figure 13.** XPLE insulated power cable.

Nowadays, the focus is on developing insulating cables with unique properties capable of operating at high temperatures and electric stress levels. A recent research demonstrates the existence of a byproduct-free cross-linked copolymer blend that is seen as a potential solution to the widely used XLPE commonly employed for high voltage DC cable insulation [126]. The result show that the copolymer blend's loss tangent is three to four times lower than that of XLPE, with magnitudes of 0.12 at 70 ◦C and 0.01 at 50 ◦C [126]. The copolymer has shown good electrical properties and is free from cross-linking byproducts. As a result, this material is a promising option for HV components, such as HVDC cables, that require clean insulation materials.

#### *5.3. Electrical Encapsulation Materials*

Electrical encapsulation materials are required to properly operate transformers, motor coils, sensors, and solenoids [127]. Among the first electrical encapsulation materials were the resins, which had many advantages for protecting encapsulated electrical components [128], such as high insulation, low relative permittivity, low cost, and easy synthesis [129]. Thermoplastic resins have shown a better performance than thermosets by requiring thinner walls for them to be stronger than thermosets [127]. In addition, thermoplastics are produced in faster cycles, generate lesser scrap, and lack the environmental issues related to thermosets [127]. Nevertheless, when submitted to specific temperatures, resins' behavior could cause shrinkage or expansion of the material, generating focused stress and leading to premature failure [128]. A test is proposed to study this impulsive failure factor where cured resins samples are subjected to a wide range of temperatures. The embedment pressure is calculated; this study shows that the embedment pressure is higher for lower temperatures [128].

To manufacture most high-performance coils, thermoplastics such as PA 66, PBT polyester, and PET polyester polymer compositions are used for the encapsulation [127]. PA 612 could be applied as the encapsulation material for encapsulated sensors because it can withstand repeated thermal cycling more than polyesters [127]. Nowadays, to encapsulate high voltage multichip power assemblies, usually silicone gels are implemented; because of the high electrical insulation and softness, these are mainly used if there are bonding wires [130]. However, for temperatures above 250 ◦C, these materials have shown breaking symptoms. In cases requiring a higher temperature operation, silicone elastomer has proven to be a proper replacement [130].

The most common material for integrated circuits (IC) encapsulation is epoxy resins [131]. These properties differ among them because of the different formulations made to improve specific properties that enhance individual performances. These are related to thermal resistance, electric insulation, reliability in moisture, reliability temperature, and pressure [132]. Incorporating fillers such as alumina, boron nitride, alumina nitride, or other ceramic powder increases the thermal conductivity of the encapsulation's electrical insulation [129]. The inorganic filler reaches 65% to 90% of the total weight [131]. In recent years the incorporation of microscale and nanoscale insulating fillers in epoxy resins has been studied to increase the mechanical stress resistance and increase the dielectric strength [133–135]. Those investigations had shown that increasing the filler content increased the current conduction and volume conductivity, a conclusion that was not expected given the nature of the inorganic fillers [131]. This hint that the epoxy/filler intermolecular interaction could be directly related to the bulk transportation capabilities [131]. The issue with the epoxy/filler encapsulation is related to generating charge propagation over the surface of the ICs, due to the significant injection of electronic charge from the embedded bond wires [136]. The amount of filler in an epoxy molding compound varies the dielectric strength because at high temperatures for higher quantities of filler, the conductivity and electric field dependence increases [137]. This could be attributed to the consequent increment in the filler particles and the epoxy matrix [137].

#### *5.4. Electrical and Electronic Plastics*

Many electrical applications are requiring high graded polymer. At present, electrical apparatus commonly use plastic as an insulation system. Plastics have good dielectric strength, heat performance, and water resistance, making them ideal for electrical components. Several computer parts are made up of polymers. In general, conductive polymers are utilized in every computer element to ensure conductivity so that the device works well. On the other hand, plastics are used in making external parts of home appliances such as TV, toasters, juicers, and blenders. Currently, most electrical tools are made of plastics. Other plastic materials applications are relays, circuit breakers, transformer comments, cabling, and wires [138].

#### *5.5. High Voltage Transmission Line Insulators*

In the past, porcelain ceramic insulators were used in both transmission and distribution lines. In high-voltage transmission systems, polymer or composite insulators are becoming more common. Polymer Insulators, which are distinguished by their compact size, lightweight, high mechanical power, ease of installation, and low maintenance, have emerged as a new generation of high voltage transmission line insulators. The protective ribbed mold on polymer insulators is made of silicon organic rubber, which makes it different from other insulators [139].

Since its introduction in the early 1970s, electric utilities have gradually embraced polymer insulators as suitable substitutes for porcelain and glass insulators. Ethylene propylene polymers are used to make the insulators. EPR and Silicon Rubber are some of the polymers that are used as insulators. Among them, the EPR are one of the most weather-resistant synthetic polymers available [140]. It has superior ageing and color quality as well as excellent electrical, chemical, and mechanical properties. Heat, oxygen, ozone, and sunlight resistance is exceptional in all EPRs. Figure 14 shows a polymer suspension insulator used for high voltage line. Most polymer insulators are designed with a rated voltage ranging from 7.5 to 765 kV. For bushings in high voltage lines, certain type of polymers are used. These include polyoxymethylene, polyphenylene sulfide and ultra-high molecular weight polyethylene.

Nowadays, the focus is to develop new stress control techniques using advanced materials. Many researchers have recently looked at using field grading material to minimize electric field enhancement on high-voltage insulators in order to improve the design of the equipment [141]. Two main types of grading exist, i.e., capacitive grading and resistive grading. In the capacitive grading, a number of fillers may be added to the host matrix in order to boost the dielectric materials permittivity. In this situation, the electric field on the overhead insulators is redistributed [142]. In resistive grading, the idea is to

have the electric field varying with the conductivity so as to have a non-linear conducting behavior. In this case, the base polymer is filled with an inorganic filler to achieve nonlinear characteristics. When the electric field strength reaches a withstand level, the nonlinear grading material turns out to be conductive, which tends to homogenize the electric field propagation within the bulk of the insulation, thereby eliminating the field enhancement effect [143].

**Figure 14.** Polymer suspension insulators.

#### **6. Challenges and Future Directions**

For over a century, insulating materials have been developed. There have been ongoing advances in insulating materials, from pure polymers to nanocomposites, and the mechanical, thermal, electrical properties of such materials have significantly improved. For instance, there have been increase in the electrical strength of materials at low temperature by nine folds, the withstand temperature of materials has increased 15 times. Furthermore, the thermal conductivity and the breakdown strength of materials increased by 30 and 3000 times respectively [144].

Despite all the aforementioned developments, there are still problems and challenges. For instance, the fundamental chemistry and physics underlying improved dielectric properties of polymer nanocomposites is not well understood, interface modifications is not well developed, insufficient nanoparticle dispersion and the repeatability of related experiments has been poor. As a result, there is refine individual materials while simultaneously improving composites' overall performance. Another challenge has to do with the biodegradable nature of conductive polymers, severely limiting their applications.

This research suggests new directions for polymers used as insulating materials: improving interface modification and manufacturing technology, investigating novel dispersion and surface modification techniques for nano materials, analyzing the formation of interfaces using computer calculations and analog simulations, designing multilayer technologies and components for application in a simpler and smaller items [7]. Furthermore, adding practical nanofillers to a polymer matrix, such as graphene, will significantly improve the conductivity of biodegradable polymers.

In terms of electrical applications, the focus should be on developing nanostructured materials for ultra-capacitors, electro-optic, discharge-resistant high-voltage equipment insulation, sensors, and actuators.
