Introduction

In the early 1960s, the first modern non-ceramic insulator (NCI), also known as a polymeric insulator, was introduced. These insulators offer several advantages over traditional ceramic counterparts, including reduced weight, resistance to vandalism, and hydrophobic surface properties. As a result, their market share has steadily increased. However, the adoption of polymeric insulators as a viable alternative to traditional porcelain and glass insulators faced challenges due to the lack of adequate standards.

In recent years we have seen insulators failure due to corona discharge. Corona discharge is a well-known phenomenon that can lead to the failure of insulation materials.

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When a partial discharge happens between conductors, an electric field (potential gradient) is created in the air, with its highest value at the conductor surfaces. This field makes free electrons in the air move faster. The higher the applied voltage, the stronger the electric field and the faster the electrons move. When the field strehttps://en.wikipedia.org/wiki/Electric_dischargength at the conductor surface reaches about 30 kV per cm, the free electrons move fast enough to knock out electrons from neutral molecules, creating more ions and free electrons. These new free electrons are also accelerated, causing more collisions and creating even more ions. This chain reaction process leads to either a corona discharge or a spark between the conductors.

When alternating current flows through two conductors that are far apart compared to their diameters, the air around them (which contains ions) experiences electrical stress. At low voltage, nothing significant happens because the stress is too low to ionize the air. However, when the voltage increases beyond a critical value of around 30 kV, the electric field strength becomes high enough to ionize the air, making it conductive. This causes an electric discharge around the conductors, creating a faint glow, a hissing sound, and the smell of ozone. This phenomenon is known as the corona effect in power systems. If the voltage continues to increase, the glow becomes brighter, the noise louder, and significant power loss occurs in the system.

In composite insulators, corona discharge can start from hardware, voids within the material, or defects where different materials meet. These voids and defects act as weak points where corona can initiate.

Polymeric materials are particularly vulnerable to the UV light produced by corona discharge. This type of light is more harmful than the UV light from the sun, especially when the corona discharge happens close to the material.

Humidity: Higher humidity levels increase the conductivity of the air surrounding the insulator, making it easier for corona discharge to occur. Moisture in the air can form a conductive layer on the surface of the polymer insulator, lowering the breakdown voltage and facilitating the onset of corona discharge.

Temperature: Elevated temperatures can reduce the density of the air, which in turn reduces the dielectric strength. This makes it easier for corona discharge to initiate. Additionally, high temperatures can accelerate the aging and degradation of the polymer material, affecting its insulating properties.

Pollution and Contamination: Dust, salt, and other contaminants can accumulate on the surface of polymer insulators. These contaminants can form conductive paths on the insulator surface when wet, lowering the inception voltage for corona discharge. Pollution can also lead to localized electric field enhancement, which can initiate partial discharges and corona.

Altitude: At higher altitudes, the air density is lower, reducing the dielectric strength of the air. This increases the likelihood of corona discharge occurring at a given voltage level compared to lower altitudes.

Rain and Fog: Precipitation and fog can create wet surfaces on the insulator, which reduces the surface resistivity and promotes corona discharge. Water droplets can also distort the electric field around the insulator, leading to localized field intensification and partial discharges.

Wind: Wind can blow contaminants and moisture onto the insulator surface, contributing to pollution and increasing the likelihood of corona discharge. It can also cool the insulator surface, potentially reducing the rate of aging and degradation but introducing mechanical stress.

Ultraviolet (UV) Radiation: Prolonged exposure to UV radiation can degrade the surface of polymer insulators, leading to cracks and erosion. This degradation can enhance the electric field on the surface, making corona discharge more likely.

Icing: Ice accumulation on polymer insulators can create irregular surfaces and air gaps, which can enhance the electric field locally and lead to corona discharge. Melting ice can also form water films that reduce surface resistivity.

Corona discharge can cause the polymeric material to become brittle, develop cracks, and lose its insulating properties. This degradation compromises the material’s structural integrity. Long-term exposure to corona discharge leads to progressive material degradation, reducing the insulation’s effectiveness and potentially leading to electrical breakdowns.

Corona discharge can induce chemical changes such as oxidation, chain scission, and the formation of by-products like ozone and nitric acid. These changes alter the material’s chemical composition and degrade its properties.

Research is ongoing to develop new materials and technologies that can better withstand the effects of corona discharge. This includes the use of nanotechnology to enhance material properties and the development of new coating techniques.

Advances in material science are crucial for creating more resilient polymeric insulation. This includes understanding the fundamental mechanisms of corona discharge and developing materials that can resist these effects more effectively.

To reduce the risk of corona discharge failure in polymer insulators, several design precautions can be implemented. Firstly, optimizing the shape and profile of the insulator is crucial; aerodynamic sheds and a smooth surface finish can minimize water film formation and contamination buildup. Using hydrophobic materials helps maintain high surface resistivity, while UV-resistant materials prevent surface degradation over time. Applying silicone coatings enhances hydrophobicity and protects against environmental wear, and ensuring proper sealing of joints prevents moisture and contaminant ingress.

Grading rings or corona rings should be installed at critical points to distribute the electric field more uniformly and reduce field intensity at the conductor-insulator interface. End fittings must be designed to minimize electric field concentration and avoid sharp edges, and their material should be compatible with the polymer material to prevent galvanic corrosion. Sufficient creepage distance and appropriate phase spacing between conductors and insulators are necessary to prevent surface discharges and reduce the risk of corona discharge.

Considering environmental factors, insulators should be designed with self-cleaning properties or composite materials that resist contamination buildup. Weather shields can protect insulators from direct exposure to rain, snow, and ice. Regular inspections and maintenance schedules are essential to identify and address any signs of surface degradation, contamination, or damage. Additionally, online monitoring systems can detect early signs of corona discharge and partial discharges, enabling proactive measures to ensure the insulators’ reliability and performance in various environmental conditions.