Surge arresters are the primary defense for electrical equipment against over-voltages. These essential devices house metal oxide varistors (MOVs), which act as rapid-response protectors against voltage spikes. To optimize the performance and ensure the safety of MOVs, effective insulation is required around their outer surfaces.
This article will explain the value of glass as a uniquely suitable insulation material for this purpose, exploring its advantages as well as some considerations to bear in mind when implementing glass coatings in surge arresters.
Understanding the Need for Insulation in Metal Oxide Varistors
Metal oxide varistors are discs composed primarily of zinc oxide crystals, a type of polycrystalline semiconductor ceramic that exhibits nonlinear volt-ampere characteristics. This property enables MOVs to conduct strongly when subjected to high voltages, thereby diverting excessive currents away from sensitive equipment. However, to prevent unintended electrical pathways and ensure reliable operation, proper insulation is indispensable. Herein lies the significance of glass coatings in MOVs.1,2,3
Glass, known for its excellent insulating properties, emerges as an ideal candidate for encapsulating MOVs within surge arresters. Its intrinsic physical and chemical qualities provide a combination of electrical insulation, thermal stability, and mechanical robustness, essential for the demanding operating environments in which it must perform.4
The Electrical and Dielectric Advantages of Glass Coatings
One of the primary advantages of glass in this application is its exceptional dielectric strength, which is greater than certain ceramics. With the ability to withstand strong electric fields without degradation, glass coatings effectively isolate MOVs from external electrical interference, minimizing the risk of short circuits or leakage currents. This dielectric resilience ensures the longevity and reliability of surge arresters, crucial for safeguarding sensitive electrical equipment.5,6
In situations where manufacturing conditions are not well-controlled, there may be insufficient adhesion between the glass coating and the MOV disk. Consequently, the disk surface may become exposed and deteriorate. In the instance of porcelain-housed arresters, transverse fields resulting from significant external pollution may induce internal partial discharges. These discharges can deteriorate certain polymeric coatings and trigger the production of corrosive gases. It has been observed that polymeric coatings are less adept at preventing damage to the disk surface from these phenomena compared to glass coatings.7
Thermal Stability: Enhancing Performance with Glass Insulation
Glass offers exceptional thermal stability, capable of withstanding elevated temperatures without degradation. In surge arresters, where rapid dissipation of heat is essential to prevent overheating and maintain performance, the thermal resilience of glass coatings becomes invaluable. By dissipating heat efficiently, glass insulation enhances the operational lifespan of MOVs, ensuring sustained protection against voltage surges over extended periods.
Furthermore, the thermal expansion coefficient of glass insulators is designed to be matched to that of MOV substrates, resulting in minimal relative deformation even with temperature fluctuations. Yet, in response to thermal shock, glass insulators are prone to shattering rather than cracking. This underappreciated benefit of glass makes it easier to detect defects when compared with materials that fail without any obvious physical change.5
Mechanical Durability: Protecting MOVs in Challenging Environments
The mechanical durability of glass reinforces the structural integrity of surge arresters, providing resistance against environmental factors such as moisture, vibration, and mechanical stress. Compared to ceramics, glass exhibits greater resistance to breakage and boasts a mechanical compressive strength that is 1.5 times higher. This robustness is particularly important in outdoor applications, where surge arresters are exposed to harsh weather conditions and mechanical impacts.5
Versatility in Design and Manufacturing Processes
In addition to its electrical, thermal, and mechanical properties, glass offers versatility in manufacturing processes, enabling precise customization to suit various surge arrester designs. For example, glass powders can be tailored for wet application through dipping, rolling, or spraying, as well as for dry electrostatic techniques. Powders can also be precisely milled to meet specific particle size requirements and can be combined with additives to enhance surface roughness for ease of handling. Moreover, glass insulation can be provided containing lead or be lead-free and is easy to dye in a diverse array of colors to suit application requirements.8
Considerations for Implementing Glass Coatings in Surge Arresters
Despite its undeniable advantages, the adoption of glass coatings in MOVs for surge arresters necessitates careful consideration of material compatibility, manufacturing processes, and cost-effectiveness. While glass can offer superior performance, its implementation requires expertise in materials engineering and specialized fabrication techniques. Additionally, cost considerations and supply chain dynamics may influence the feasibility of integrating glass insulation into surge protection systems.
Advancing Surge Protection Technology with Glass Insulation
Glass is a highly effective insulator for metal oxide varistors within surge arresters, offering excellent electrical, thermal, and mechanical properties. Its exceptional dielectric strength, thermal stability, and mechanical durability make it an indispensable component in safeguarding electrical systems against voltage surges. While challenges exist in terms of manufacturing complexity and cost, the benefits of glass insulation typically outweigh alternative materials, reaffirming its status as a cornerstone in surge protection technology.
To learn more about the suitability of glass as a coating and explore the variety of specialty glass coatings available, reach out to a member of the Mo-Sci team today.
References and Further Reading
- Meshkatoddini, M.R. (2011). Metal Oxide ZnO-Based Varistor Ceramics. Advances in Ceramics – Electric and Magnetic Ceramics, Bioceramics, Ceramics and Environment. doi.org/10.5772/23601
- Yutthagowith, P., et al. (2021). A Simplified Model of a Surge Arrester and Its Application in Residual Voltage Tests. Energies. doi.org/10.3390/en14113132
- Meng, P., et al. (2020). Excellent electrical properties of zinc-oxide varistors by tailoring sintering process for optimizing line-arrester configuration. ICHVE. doi.org/10.1109/ICHVE49031.2020.9279439
- Saleem, M.Z., et al. (2022). Review of the Performance of High-Voltage Composite Insulators. Polymers. doi.org/10.3390/polym14030431
- Taherian, R. (2019). Advantages and Disadvantages of Glass Insulators. In: Electrical Conductivity in Polymer-Based Composites.
- Su, T.Y., et al. (2023). Modelling and analysis of electrical performance outdoor glass insulator under various services and lightning impulse. Journal of Physics: Conference Series. doi.org/10.1088/1742-6596/2550/1/0120201
- INMR. (2018). [Online] Quality of Metal Oxide Disks Impacts Surge Arrester Performance. Available at: https://www.inmr.com/quality-metal-oxide-disks-impacts-arrester-performanc/ (Accessed on 29 March 2024).
- 3M Advanced Materials Division. (2015). 3M™ Specialty Glass for Metal Oxide Varistors. Available at: https://studylib.net/doc/18476855/3m%E2%84%A2-specialty-glass-for-metal-oxide-varistors-data-sheet (Accessed on 29 March 2024).