Electricity generated is transmitted to the end-users through overhead transmission lines operating at a very High Voltage (HV) level. Most of the transmission towers carrying these HV lines are made up of metallic materials. Ever wondered why one doesn’t get electrocuted despite touching the metal poles? Why is there no short circuit between different wires having physical contact at the poles? You might have seen a series of circular discs often in grey or brown color hanging between the pole and the line. These are the insulators that impede the flow of current from the line to the tower and ultimately to the ground or another phase traveling in parallel. Thus, insulators not only prevent the line to line faults but also the electrocution of living organisms coming in contact with transmission towers. So, let’s have a detailed discussion about the insulators used in HV transmission.
Insulators are majorly differentiated based on the materials they are made up of. More than 90% of the insulators used in the transmission of electricity are made up of Glass, Porcelain, or polymers. Glass and porcelain insulators have been used throughout the world for over more than a hundred years. But in the last 20 years, composite insulators have been quite popular in the market due to their unique properties. Every material has certain advantages and some disadvantages as well. A brief overview of each of them is given below and it will help decide which insulators should be deployed in different circumstances.
Glass insulators are made up of glass that is heated at a very high temperature. These insulators also contain small fractions of lime and quartz in them to enhance the strength of the insulator. Despite the addition of these materials, there are still chances of leakage current on the surface of the insulator. In a polluted environment, the contamination of dust and other pollutants badly impacts their dielectric strength leading to flashover. Recently, RTV coating of high voltage insulators has been quite beneficial in mitigating these problems. RTV Coated insulators yield better results in foggy, rainy, and humid environments as high voltage insulator coating (HVIC) is highly water repellent.
Owing to the crystalline properties of the glass, the fault can be easily detected in these insulators because any kind of crack is visible on them. They have a very small value of thermal expansion coefficient. Therefore, normal temperature variations don’t impact the performance of these insulators significantly. The electrical resistance offered by this insulator type is almost 500 to 1000 kV per centimeter. The crystalline nature of the glass negatively impacts the insulator when a strong and heavy object falls on its surface. As a result, the whole insulator may become brittle resulting in complete failure of insulation.
Porcelain insulators are the most commonly used insulator in high voltage lines. As the name indicates, such types of insulators are composed of porcelain which is obtained by heating kaolinite materials or a mixture of clay, quartz, aluminum silicate, and feldspar at a very high temperature. Porcelain is a porous material; therefore, its surface is glazed with a specialised coating to prevent the accumulation of contaminants like moisture and dust, on the insulator’s surface causing the breakdown of insulation strength. But is this enough for enhancing its insulation strength? Can we get rid of flashovers by just glazing its surface? Unfortunately, flashovers still happen in polluted areas. To avoid these pollution-induced flashovers, High voltage insulator RTV coatings have been designed and they have yielded excellent results in such circumstances. They suppress the leakage current and provide high resistance to air. Besides this, they are excellent repellent to water and hence are quite favorable during thunderstorms and rain to avoid power interruption as a result of current leakage. Porcelain insulators are ideal for transmission lines having a medium-voltage level. The Electrical resistance of this insulator is about 250+kV per centimeter. Another important characteristic of this insulator is that, unlike glass insulators, it can be designed and molded in any shape depending upon the area of application and voltage level of the transmission lines.
A composite insulator is the latest type of high voltage line insulator. It is also known as a polymer or rubber insulator. This insulator type has many advantages over glass and porcelain insulators. They are lighter in weight and smaller in size than any other type of insulator applying less burden over the cross arms. Unlike glass and porcelain insulators, the accumulation of dust over their surface doesn’t impact their performance. Their mechanical strength is much higher compared to glass or porcelain insulators and they can easily withstand the severity of winds, storms, and extreme weather conditions. Also, the tensile strength of this insulator is very high and it doesn’t break easily. One of the major disadvantages of using composite insulators is that they may deteriorate when exposed to overheating either due to overcurrent or any type of fault.
As discussed on previous the previous sections, flashovers are one of the most common problems these insulators are facing. Different methods to prevent flashovers were introduced, such as washing, silicone greasing and RTV coating. Among which, RTV coating is considered as the most effective measures. With this advanced coating, the flashover voltage of contaminated insulators will improve, that will eventually yield a better performance under a polluted environment, reduce economical losses, and offer a low carbon economy. Midsun IKM offers RTV Silicone Coating Service on a global-scale especially on highly contaminated areas.