3.3.1. Dielectric Strength

In certain cases, a material's dielectric strength is the determining factor in the design of the device in which it would be used. The common way to know the dielectric strength of a material is by applying a voltage until the material or insulator reaches a point where the electrical properties breakdown. The breakdown normally manifests itself as an electrical arc between the electrodes, resulting in a drastic reduction in resistance.

It is challenging to know the exact value of any material's dielectric strength because it may vary due to external factors and depending on the application [93]. Practically, it is challenging to realize the intrinsic dielectric strength of polymers because of external factors, which may lower polymers' dielectric strength. Most pure polymeric plastics such as PVC, PMM, PET, polycarbonate, polyethylene, and polypropylene have a dielectric strength range of 100 to 300 kV/cm. Some halogenated polymers such as Teflon show up to 700 kV/cm [94]. Table 5 provides the approximate values of dielectric strength of few commercial polymeric materials used in electrical equipment [94].

**Table 5.** Dielectric constant and dielectric strength of some polymers [94].


Although it is difficult to understand the actual dielectric strength of polymeric material due to the above factors involved, many researchers have endeavored to increase the dielectric strength of polymeric materials proportional to the voltage for a specific thickness by adopting three primary methodologies are and filling nanoparticles in polymers, engineering filler-polymer interfaces, and coating film surfaces [95].

Factors affecting the dielectric strength of polymers include:


#### 3.3.2. Dielectric Constant and Dissipation Factor

Generally, polymeric materials are used in two ways i.e., to isolate electrical network devices from one another and from the ground, and to act as a capacitor dielectric component. For polymeric materials to be used as insulating materials, the insulating material's capacitance should be as low as possible while retaining sufficient chemical and mechanical, chemical properties. As a consequence, it's preferable to use a material with a low relative permittivity. For the other application, a high dielectric constant value is desirable such that the capacitor dimensions should be reduced to a minimum value.

Polymeric materials used to provide insulation as well as capacitor dielectrics should have minimal dissipation factor to minimize the material heating. Since the dielectric loss increases linearly with frequency for a given loss index value, a low loss index value is recommended for high-frequency applications. When comparing materials with nearly the same dielectric constant or when using some material under such conditions that the dielectric constant remains basically constant, the dissipation factor is a quantity to consider [98].

#### 3.3.3. Volume or Surface Resistivity

Volume resistivity may be used to help design a polymeric material for a specific application. Humidity or temperature resistivity will vary dramatically, and this must be taken into account when planning for operating environments of the polymeric material. Volume resistivity is commonly used in the study of an insulating material's uniformity, either in terms of processing or to detect conductive impurities that impair the material's durability but are difficult to detect using other methods. Ref [99] defines a commonly used research technique for determining electrical insulating materials' insulation resistance, volume resistance, and surface resistivity, as well as their conductivity. This technique only considers measurements taken when the DC voltage is applied.

In summary, the electrical breakdown strength of polymers decreases with increasing electrode area, the frequency of the voltage, the thickness of the sample, absorbed moisture, temperature, and reducing the rising rate of ramp voltage.

#### **4. Assessment of Polymeric Insulations for HV Applications**

To determine the suitability of polymeric insulations for high voltage apparatus applications, specific techniques can be deployed and discussed in this section.
