At first investigations defined the contamination as one of the most frequent causes of failure on overhead insulators. It is now known that contamination is one of the main failure causes on the electrical systems of most countries. The contamination mechanically degrades the insulators and affects some of the electrical and mechanical characteristics of the insulating material.
When the accumulated pollution layer on the outer surface of electrical equipment becomes wet, surface leakage current increases. The areas dried from the heat generated by the leakage current are known as dry-bands. These bands withstand higher E-field and localized corona and arcing activity begins. The region of such corona will extend over the surface of the insulator become an arcing until the entire surface of the insulator is bridged which is the process leading to flashover. Leakage current, pollution levels, climate, visual surface condition and discharge development have a close relationship with one another.
- The connection between leakage current and pollution level is obtained by the creepage distance of the insulator, the lower the creepage distance and the higher the pollution level is, the leakage current is higher as well
- Leakage current and ambient humidity are related as well, the higher the humidity, the easier the surface becomes wetted, the deposit upon the insulator becomes conductive and leakage current appears
- The relationship between flashover and type of structure, given the same pollution environment the level of arcing along the outer insulation depends on its shape, size and type of structure. Arcing activity is higher on equipment with lower diameters. Shorter diameter porcelain means shorter sheds which result in less creepage. By contrast, wider diameter equipment such as bushing does not arc easily. Such leakage current density tends to be greater along the shed of equipment with smaller diameter
In the world of transmission and distribution there are a number of sites where pollution severity is of such a scale of challenges when it comes to performance of electrical insulation. Coastal, mining and industrial pollution has for years challenged some of the most advanced insulators designs. High voltage insulators exposed to coastal or industrial environments collect particles, some of which are conducting when dissolved in water. When wetting occurs due to rain, fog or mist, leakage current therefore flows over the insulator surface to ground. The conducting layer dries out at certain locations limiting the flow of leakage current. As a result, high voltages appear across the dry bands, causing corona and sparking across the bands, often accompanied by higher leakage current pulses. The electrical discharge activity on the insulator, such as corona, streamers, sparking and arcing, may lead to material degradation.
- Water drop corona: water droplets increase the electric field strength, which may produce corona around the water drop
- Spot corona/discharges: corona around a pollution patch, or discharges between water droplets
- Dry-band corona: corona within the dry-band zone
- Dry-band discharges: streamer, spark and arc discharges over the dry-band zone
Koeberg Insulator Pollution Test Station (KIPTS) tested 5 different types of insulator for pollution resistance. KIPTS is situated along the Cape West Coast, 50m from the sea. A number of new 22 kV insulators having the same shapes but made of different materials were energized at the site in order to test their resistance to severe pollution. Porcelain was used as the reference material as its surface characteristics remain stable. Visual surface inspection and discharge observation were done on a weekly basis for the first six weeks and thereafter every sixth week.
- Water drop corona was mainly observed on the sheds (bottom or tips), from the cold to the live end. It was seen more often on silicone rubber insulators and RTV silicone rubber coated porcelain insulators than on the others insulators
- Spot corona/discharges were present on the shank and sheds, from the live to the cold end. Corona spots were seen most often on porcelain insulator (65% of observation days) nevertheless it was visible in all the rest of types as well
- Dry-band corona was present mainly on the shank, ranging from the live to the dead end. It was seen most often on insulators EPDM rubber and silicone rubber. On the other insulators dry-bands were formed but in much less severity
- Dry-band discharges were mainly present in the areas of the dry band corona, and discharges were seen on the shank, shank-to-shed or shed-to-shed, from the live to the dead end. They were seen most often on EPDM rubber insulator and seldom on the other types
All discharge types were present:
- during 1st week on the EPDM rubber insulator
- during 2nd week on the silicone rubber insulator
- during 3rd week on the porcelain and RTV silicone rubber coated insulator
- during 4th week on the cycloaliphatic epoxide insulator
The leakage current monitored during the observation period was highest on the cycloaliphatic epoxide insulator and the lowest on RTV silicone rubber coated insulator.
The RTV Silicone rubber coated porcelain insulator had the best discharge and leakage current performance, despite some irreversible surface damage. Insulator 5A was the worst performer of all the test insulators.
Degradation on the silicone rubber insulator was found to be minimal and restricted to the pollution layer. The EPDM insulator showed signs of crazing on the sheds and light erosion in the shank area. The porcelain insulator, as expected, showed no degradation. Material erosion and tracking were present on the RTV silicone rubber coated porcelain insulator, exposing the porcelain at the shank areas close to the end fittings. However, the larger part of the area at the end fittings was still coated and functional.
The cycloaliphatic epoxide insulator showed clear signs of severe material degradation, chalking, tracking and erosion. From a visual perspective, this insulator had failed due to severe material degradation. It should be noted that a non-energized cycloaliphatic insulator, identical to the one in the test showed minimal signs of material degradation (only light discoloration in the mold line).
Water drop corona was found to be present on all the test insulators. It was found to be towards the shed tips and anywhere from the live to the dead end. Field simulations of these insulators confirmed that the presence of a light uniform pollution layer causes stress intensification at the shed tips
Dry-band corona and discharges were found to be the most dominant form of electrical discharge activity on all the insulators. The electrical field confirmed that the stresses in the dry-band regions were very high, and that a large percentage of the supply voltage was transferred to the dry band.
In conclusion, contamination is one of the most sever sources of corona and arcing activity and in order to avoid this effect, UV corona camera can be used as an effective technique to inspect and analyze insulation of electrical equipment so as to provide guidance for cleaning and preventing both pollution and flashovers. Corona camera can show all of the phenomena related to contamination such as water drop corona, spot corona/discharges, dry-band corona and dry-band discharges moreover a corona camera can reveal all these phenomena in each type of the mentioned insulator. Hence, predictive and regular maintenance and inspection can prevent flashover and failures caused by pollution.