NCI Aging - Environmental Factors
Ceramic and glass insulators are being substituted by composite insulators due to their low weight, high resistance to pollution and vandalism and for their lower installation costs. Yet their in-service lifetime is shorter due to aging processes. Aging is a natural process that although cannot be stopped can still be controlled. Corona cameras are being utilized by electrical engineers to identify those processes, control them and prevent major damage. In particular, daytime corona cameras are very much efficient because they let clearly see and document indications of an existing problem and/or a developing problem.
Design of NCI
A typical composite insulator consists of 2 main parts:
- Rod – the principal load bearing component made of glass-fiber reinforced resin (GFR) onto which two metal end-fittings are attached
Housing – improves the insulator resistance to environmental stresses. Common housing materials are EPDM, SIR and the mixture of these two.
Insulators' shapes vary by their sheds' design, number and size that correspond to the required leakage distance and pollution performance <.
Definition and factors of aging
NCI aging refers to the degradation of the insulator's material caused by electrical and environmental factors. Degradation is the breakdown of macro molecules causing reduction in the molecular weight. While electrical stress can be controlled, the uncontrollable environmental effects must be taken into consideration.
- Electrical stresses - corona, dry bands, arcing, roughness & erosion of surface
- Environmental effects - heat, light, UV radiation, moisture, atmosphere pressure, biological degradation caused by microorganism
Polymer insulators are made out of organic materials and are susceptible to microorganism activity. There are 5 major effects of microorganisms on polymeric insulators:
- Contamination - which may interfere with the functionality and properties of the housing material increasing surface conductivity
- Degradation of the housing material
- Errosion leading to rapid degradation of materials
- Hydration or penetration of water in the material reducing mechanical stability and causing short circuits that may lead to a complete failure
- Discoloration that cannot be removed through cleaning
Chemical pollutant (sulfur-dioxide, ozone & NO2)
Sulfur dioxide, a contaminant frequent in industrial zones with gaseous wastes, forms a filament on the surface of insulators leading to flash overs. Ozone is an unstable allotrope of oxygen and is considered an air pollutant with destructive effects to all materials. NO2 is a prominent air pollutant, an intermediate in the synthesis of nitric acid induced by corona in high voltage lines. Nitric acid dissolves the composite material leading to its degradation.
Environmental stresses (heat, light, moisture, wind, dust, precipitation and UV from corona)
Ultraviolet light is one of the major factors responsible to degradation of polymer insulators. The main sources of UV are the sun and corona phenomenon during which UV is emitted. UV affects the dielectric and weathering properties of the housing material and leads to chemical degradation, surface deterioration and electrical stresses that will continue until the insulator is unusable. The effect of UV on polymer material can be seen as crazing, chalking, cracking, discoloration.
Rain, mist & fog can cause high enough local electrical e-field for corona partial discharge to develop due to non-uniform wetting. Heat, light & moisture produce surface cracking, erosion and corrosion. Wind and dust can change physically the insulating material by roughening or cracking it due to the loss of soluble components.
Rain & salt turn surfaces into hydrophilic letting water penetrate into cracks, causing material breakdown and to flash under along the rod.
The environmental effects are commonly classified as either affordable or unaffordable:
- Loss of gloss and discoloration > – normally this is the first sign of aging of an insulator
- Chalking – appearance of a rough and white powder. The factors responsible for chalking are UV radiation and electrical activity. During chalking a small amount of rubber is removed from the surface and the filler material (a white powdery material) is exposed giving the insulator a chalky appearance.
- Crazing – shallow cracks (less than 0.1mm) on the insulator surface due to electrical stress
- Loss of hydrophobicity– the resistance to water film formation is lost allowing corona and arcing activity to appear
- Alligatoring – a more severe form of crazing. It reduces contamination performance and may lead to rod exposure.
- Corona cutting – cutting induced by corona discharges. These discharges expose the insulators to sever electrical and chemical degradation. The severity of the damage increases with increasing voltage.
- Holes at the junction of shed and housing – Electrical discharges trough the rubber cause punctures resulting in loss of dielectric strength.
- Tracking – An irreversible deterioration due to formation of a conducting path on the surface of the insulator allowing leakage current. These tracks have the appearance of carbon tracks which cannot be easily removed and are conducting even in dry conditions allowing corona to be formed on the insulator's surface
- Erosion – irreversible degradation of the insulator's surface that occurs by major (more than 1 mm) loss of material. It significantly reduces the thickness of the polymer sheath that prevents ingress to the core rod.
- Seal damage – includes lifting of seals, seal erosion and tracking at the end fitting. It can promote tracking of the rod which is not visible from outside.
Composite insulators are usually used in severe natural environment and are constantly subjected to environmental effects leading to electrical stress such as corona. Most of these effects are involved with UV and corona either as an outcome or as a triggering factor. As a result, using corona cameras, also called UV imagers, to inspect NCI insulators is imperative during routine maintenance. Corona cameras can certainly assist to identify aging processes that lead to failures.