**1. Introduction**

Various types of composite and polymeric materials are suitable as insulators for electrical systems. The dielectric strength of polymers depends on the application and other external factors such as electrode size, shape, and nature of the outer surface, among others, and test conditions [1]. The studies suggested materials permittivity behaviors can play an essential factor and available polarizable aromatic rings like bromine and iodine that can enhance its dielectric constant. The applications and materials listed below (Table 1) with different values are used for capacitors insulation [2]. According to findings in [3], higher voltage stress, thinner insulation, and higher working temperature are the primary requirements for electrical equipment as insulating to possess higher dielectric

**Citation:** Haque, S.M.; Ardila-Rey, J.A.; Umar, Y.; Mas'ud, A.A.; Muhammad-Sukki, F.; Jume, B.H.; Rahman, H.; Bani, N.A. Application and Suitability of Polymeric Materials as Insulators in Electrical Equipment. *Energies* **2021**, *14*, 2758. https://doi.org/10.3390/ en14102758

Academic Editors: Issouf Fofana, Stephan Brettschneider and Laurentiu Marius Dumitran

Received: 8 March 2021 Accepted: 7 May 2021 Published: 11 May 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

strength and higher temperature ratings. Müller et al. [3] stated that composite materials are becoming an essential part of today's industry due to low weight advantages. An additional benefit of polymeric materials is the ease of processing, low relative permittivity, adhesive properties [4–6], corrosion resistance, high fatigue strength, outstanding performance, faster assembly, and favorable cost to traditional materials [7,8]. Consequently, composite materials offer the opportunity to provide the suitable product with the final application's required performance, thereby optimizing the price-performance ratio [9]. Additionally, nanocomposites are also characterized by distinctive advantages, including homogenous structure, no fiber rupture, optical transparency, improved or unchanged processability [7].


**Table 1.** Dielectric materials for capacitor insulations. Adapted from [2].

Polymeric materials and processes used for manufacturing cables at different voltage ranges are described, and their application to HVDC is discussed. The evolution and limitations of polymeric insulated power cables for various applications and their advantages and future trends are mentioned. These are extensively applicable with high-voltage (HV) systems as insulation material. Polymers can work at high temperatures under electrical strain due to their high breakdown toughness. This study contributes to the breakdown field as well as the electric strength of polymeric materials. Composite materials are conveniently used as insulators, with dielectric strength in the range of 106–109 V/m at room temperature [10]. The composite materials are convenient as insulators. It can be prepared by mingling two or more materials with distinct properties that simultaneously display unique properties. Natural fibers (cellulose, cotton, silk, wool) with sand, quartz, and natural resins extracted from plants with petroleum deposits (shellac, pitch, or linseed oil) were used to prepare the first composite material for the insulation system. The early stage of development for composite material looks on only at new materials with fewer design criteria. With time for action, the early development stage needs to focus on the mechanical, electrical properties and operating temperatures [11]. Inorganic materials are incorporated with micro and nanoscale to form a composite that procures awareness in power and voltage [7,12–17]. The alumina (Al2O3), silica (SiO2), titanium oxide (TiO2), glass and carbon fibers, carbon nanotube, graphene sheet combined as filler received particular attention concerning traditional composite material as it shows improved and enhanced electrical and mechanical properties than the single polymer [18–40]. Microcomposite polymer requires a large amount of filler than the nanocomposite and looks like the original polymer, and hence the nanocomposite behavior remains unchanged to density. Additionally, the length between two adjacent fillers is minimal compared to typical micro composites. The interaction between filler and polymer matrices depends on the fixed surface area, which is more with nanocomposite due to the ratio between nano and micro is 3:1 [7]. Nowadays, smart polymeric materials are a well-known composite for the new generation in biochemical sciences. It can enclose unique potential by using different compounds with distinct properties for biological and medicinal applications. These polymers are more sensitive to the environment, and as such, a small difference in the background will drastically produce changes in their physical properties.

The advantages of materials performance in the polymer composite field give rise signals to basic research and development unit to produce low-cost synthetic route with new composite materials that can pursue efficient energy. The HV insulation system's critical parameters are excellent adhesion to the substrate, grater glass transition temperature, lower ability for moisture absorption, and the polymer material's excessive thermal stability. The problem of polymer's high-dielectric properties can be solved by introducing voids into polymers, which help decrease their dielectric strength. The performance of polymers depends on size as well as the distribution of gaps in the structure.

Therefore, the present review article is adapting attention to developing the polymers' electrical features concerning the polymeric materials' thermal and electrical properties as insulators. Most commonly, epoxy resin and polyethylene are cross-linked with different sizes of particles used for insulating systems. These polymeric materials are applicable with HV systems power generators, transformers, cables, or in general, for electrical equipment. Additionally, this paper covers polymers' electrical behavior, mainly their dielectric strength, partial discharge in polymeric insulation, and high current arc resistance to ignition. Partial discharge has been recognized as a suitable technique to assess polymeric materials for insulation applications. Partial discharge refers to electrical discharges appearing in HV equipment insulation subjected to high voltage stress.

In Section 2, the types of polymers are described. Properties of polymers are depicted in Section 3. Section 4 describes specific techniques to determine the suitability of polymeric insulations for high voltage apparatus applications, while Section 5 demonstrates some real life examples of this type of insulation system for power equipment. Afterwards, Section 6 discusses the challenges and future directions. Finally, in Section 7, the conclusions from this review are presented.
