• Electrical India
  • Nov 5, 2017

Technological Breakthrough for Insulators

Nano-science and nanotechnology will lead technological breakthroughs in the next few years for innovative solutions for the energy sector. Current concepts of nanotechnology for outdoor insulators, microstructure reinforcement of porcelain body by ceramic nanoparticles and nano-coating for self-cleaning characteristics, will continue under ...

- Dr. Vikramaditya Dave,
Er. Sujit Kumar


 High voltage insulators form an essential part of high voltage electric power transmission systems. Any failure in the satisfactory performance of high voltage insulators will result in considerable loss of capital, as there are numerous industries that depend upon the availability of an uninterrupted power supply. Most electric power transmission and distribution is done via overhead lines with insulators providing mechanical support and electrical protection. The size and specific design of high voltage insulators will vary according to the line voltage, environmental conditions, material of construction and manufacturer. Insulator service life can be affected by electrical, mechanical and environmental factors. The principle dielectrics used for outdoor insulators are ceramics and polymers.

  Decades of in-service performance have demonstrated that ceramic insulators made of porcelain and glass, show good performance and resist environmental aging. In addition to high mechanical strength, they provide excellent resistance to material degradation caused by electrical stress and discharge activities. However, they possess hydrophilic surface due to which water can easily form a continuous conductive film along the creepage path thus, allowing high surface leakage currents to flow on their wetted surfaces. Such currents cause dry bands at areas of high current density and lower wetting rates which eventually cause surface arcing and frequently complete flashover of the insulator shown in figure 1.

  Contamination on outdoor insulators enhances the chances of flashover. Under dry conditions, contaminated surfaces are of little concern. However, under environmental conditions of light rain, fog or dew, surface contamination dissolves. This promotes a conducting layer on an insulator’s surface which facilitates a leakage current. High current density near the electrodes results in the heating and drying of the pollution layer.
An arc is initiated if the voltage stress across the insulator’s dry band exceeds its withstand capability. Extension of the arc across the insulator ultimately results in flashover. The contamination severity determines the frequency and intensity of arcing and thus the probability of flashover.

Figure 1: Flashover of the insulator

Consequences

  As discussed above, surface flashover damages the whole insulator and ultimately results into a complete breakdown of the power system network. This in turn results into a great economic loss to the consumers as well to the owner of the power company. A 250 ms outage can shut down a paper machine, resulting in hours of down time, possible equipment damage and loss of millions of dollars. It is worth mentioning that the failure at any single point of the transmission network can bring down the entire system. Recent reports on grid disturbance in India indicate the loss of 5000 million rupees and 97% of interconnected generation on January 2, 2001. Similar disturbances of lesser magnitudes were also observed during the period of December 2002 and 2005, February and December 2006, January and February 2007, and March 2008. One of the major causes identified was the contamination-induced flashovers. These events have amply portrayed that the performance of overhead transmission insulators and that used in outdoor substations is a critical factor which governs the reliability of power delivery systems.

Figure 2: Lotus Leaf effect

Mitigations

  It was understood that in order to prevent flashover, leakage current must be minimized. The practices used by utilities can be classified into the following groups.

i. Remove accumulation of contamination on insulators by periodic cleaning
ii. Minimize accumulation of contamination on the insulator surface with the use of aerodynamic profiles
iii. Increase the leakage distance by using additional bells or extra long leakage distance units on string insulation.
iv. Keep large area of the insulator dry for a longer time during natural wetting either by the use of a resistive glaze or by use of fog bowl design which has a difficult to wet underskirt area.
v. Prevent water filming on the insulator surface by coating insulators withwater repellent materials(Hydrophobic).

Concept of Hydrophobicity

  The concept of hydrophobic surfaces is originally drawn from the inspiration of lotus leaves in nature. The very high water repellency and the self-cleaning properties exhibited by the lotus leaf have been referred to as “lotus effect”, which has been attributed to a combined effect of the hydrophobicity induced by the epicuticular wax and the surface roughness resulted from the hierarchical structures on the leaf shown in figure 2.

Fig. 3 Glass Insulator RTV Silicone Coating

Conventional Coating

  The fundamental reason that ceramic insulator wets is because they have high surface energy which means that moisture tend to spread forming a low resistance surface rather than bead up into small isolated droplets which results in a high surface resistance. If the tendency of spreading or wetting can be overcome by a protective coating, then contaminated insulator surface would present a higher resistance thus, minimizing leakage current to levels obtained during dry conditions.

1. There are other compounds such as waxes, paints, lacquers and varnishes that have been tried as coatings on ceramic insulators. But the use of these is rather limited due to concerns on their long term performance. They are apparently easily weathered and subjected to loss in electrically high stress area by corona.
2. Grease coating is also a method to mitigate contamination problem. Grease coating was an effective practical count measure for preventing contamination flashover on a large scale. However, the short intervals between re-greasing in highly contaminated areas and the associated high cost of re-greasing necessitated the development of longer term solutions to the insulator contamination problem.
3. The need for a longer lasting coating that has an increased resistance to UV and dry band arcing motivated the development of polymer hydrophobic coatings. Room temperature vulcanized (RTV) coating came out as a solution to this problem. The RTV coating can be applied to ceramic insulator by dipping, painting or spraying shown in figure 3.
The pollution deposit on the surface of the RTV coated insulator leads to surface erosion and the chemical composition of the surface changes and thus, the surface of the coating loses its electrical properties like surface resistance and hydrophobicity.
4. Another way of remediating pollution performance of insulators is to use polymeric insulators. Polymeric insulators offer numerous advantages over porcelain which includes light weight, better pollution performance, and safer flashover because of hollow core housing.

  However, polymer insulators are also suffering from some drawbacks.

a. Weathering degradation: Polymer materials have weaker bonds than porcelain which means they can be aged and changed by the multiple stresses encountered in service.
b. High raw material costs
c. Low mechanical strength: Polymer insulation is typically not rigid nor self-supporting.

  From the prior discussion, it is clear that each technique has some drawbacks. Hence, new effective method is required to mitigate the problem of contamination. A ray of hope arises with the advent of nanotechnology.

Figure 4: Nanoparticles alternatives for mechanical improvement of ceramic materials.

Nanotechnology

  Nanotechnology describe “creation, analysis and application of structure, molecular materials, inner interfaces and surfaces with at least one critical dimension or with manufacturing tolerances below 100 nanometers”. The decisive factors is that new functionalities and properties resulting from the nanoscalability of system components are used for the improvement of existing products or the development of new products and application options. Such new effects and possibilities are predominantly based on the ratio of surface to volume atoms and on the quantum mechanical behavior of the elements of the material.

  Nanotechnology is rapidly growing as a new technology alternative to create advance materials with unique characteristics and performance for different applications in several industrial sectors. In recent years, many nanotechnology-based products have appeared in our everyday life. On the other hand, industries have also considered nano-concepts to produce high-added value products with superior capacity, reliability and efficiency. Electric insulators are components with high importance in the electricity network system; reliability and high performance are essential characteristics demanded by actual markets. Recent studies have demonstrated the technical feasibility of innovative nano-concepts to improve the final properties of these electrical components.

  Nanotechnology has been also considered as technological alternative to enhance the final properties of outdoor porcelain insulators, based on the extraordinary and interesting results of investigations carried out in several ceramic materials (Figure. 4)

  Some authors have reported that the nanotechnology can be successfully applied to electrical engineering applications. These include:

• Improvement in the conductivity of metals used as conductors.
• Improvement in the properties of insulators
• Miniaturizing of design and thus, reduction of used material.

Figure 5: Synthesis/Fabrication techniques of nanomaterials

Material Selection for Coating

  The protection of insulation by hydrophobic coating is one of the most important and versatile means of improving ceramic insulator performance. We have the following choice in this respect

i. Metal
ii. Semiconductor
iii. Dielectric

  The metal choice is rejected as it will aid in the process of flashover. The second choice of semiconductor is also rejected as due to thermal effect semiconductor will also become conductor and thus aid in flashover. The last choice of dielectric is the most suitable for coating material

Figure 6: Characterization techniques of nanomaterials

Dielectric Materials

  The dielectric material chosen should possess following properties.

• The Dielectric constant of the material should be high
• The Dielectric material should be thermodynamically stable in contact with ceramic materials
• The band gap should be large
• The refractive index should be fairly good

  Presently, many researchers are searching for inorganic dielectric materials to replace RTV. Since a higher dielectric constant means we can grow thicker films to reduce leakage current. Many metal oxides and ferroelectric materials have been investigated as candidate materials, but most of them are not stable in contact with ceramic. Furthermore, the dielectric constant of materials generally tends to increase as the band gap decreases, making it difficult to select a material with a large band gap and dielectric constant. Transition metal oxides, their oxynitrides and their composites are attractive materials for industrial and engineering applications due to their remarkable physical, dielectric and mechanical properties including high hardness, high melting point, chemical inertness and good thermodynamic stability

Synthesis Technique

  The preparation of these hydrophobic dielectric coatings will be carried out by two methods shown in figure 5 that is either by physical method or chemical method depending upon the application where they are used.

  Deposition of thin films by Physical Vapor Deposition (PVD) techniques has found widespread use in many industrial sectors and there is an increasing demand for such coatings with enhanced properties. These coatings will be deposited on various substrates such as glass and quartz to study its influence on the structural evolution of coatings.

Characterization

  The physical, electrical, optical etc. properties of the coating/film will be investigated using different equipment like X-ray Diffractometer, atomic force microscopy etc. Characterization techniques of nanomaterials are shown in figure 6.

Conclusion and Future Trends

  Nano-science and nanotechnology will lead technological breakthroughs in the next few years for innovative solutions for the energy sector. Current concepts of nanotechnology for outdoor insulators, (1) microstructure reinforcement of porcelain body by ceramic nanoparticles and (2) nano-coating for self-cleaning characteristics, will continue under investigation through new types, concentration, morphology and distribution of nanoparticles, alternative techniques to incorporate them into ceramic mixtures and new nano-films depositions techniques will be also included.

  Authors suggest that nano-concepts, which have been already demonstrated in other ceramic materials, will be adapted to outdoor insulators, besides new research topics will appear. Future trends regarding nanotechnology applications for electric insulators will be focused on reducing the weight and size, increasing the insulation capacity, having a self-repair surface behavior, reducing the sintering temperatures, etc. On the other hand, there will be significant challenges to successfully adapt these nano-concepts into the conventional industrial manufacturing processes. It will be essential to have a strong technological link between academic researchers and industrial research engineers to transfer the knowledge of new properties, performance and mechanisms of nanomaterials into industrial procedures to produce high-added value and innovative nanostructured outdoor insulators.


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