Early Morning Outages
A power generating company in New Zealand complained having outages occurring in early hours of mornings with thick fog & light rain. The fault was on one of the 110kv lines, with EPDM polymer insulators, where previous foot patrols by its maintenance crew showed nothing. The line protection indicated an existing possible insulation fault. "…I flew the line with the DayCor camera and found intense corona on the line at some pole locations and on the particular phase that was indicating the transient fault…" (P.D)
The whole line had varying amounts of corona activity on the insulation. Analysis of the insulation indicated contamination of geothermal origin (geothermal discharges of steam etc… to the atmosphere) and a reduced creepage distance from the normal 20mm/kv to 15mm/kV on these items under moist conditions. Design engineers are now looking for a suitable replacement for the insulation on this line in the designated places. "It was decided that I will do a yearly condition inspection on the remaining lines…" (P.D)
So what happened?
All outdoor insulators are subjected to deposits of contamination and as long as their surface is dry their performance is common. But, when exposed to fog, mist, dew or light rain that do not wash away the dirt a conducting film is created. In general, when the surface of an insulator is deteriorated the degradation of its electrical characteristics may have been caused by an increase in the passage of leakage current between its terminals. This increase is due mainly to the degree of contamination by pollutants that change the electrical resistivity of the surface and humidity. These contaminants may be deposits of dust, inert conductive minerals, such as carbon and metal oxides, and solutions of water and salt, which lead to behavior similar to a highly variable and nonlinear resistor, and, in many cases, instabilities in the presence of electric fields. Ultimately this leakage current causes heat, products from electrolysis, and corona discharges.
The above case from New Zealand is a typical example of coastal conditions where salt and sea-brine are present in the atmosphere and determine the performance of the installed insulators. Most often other existing contaminants such as cement/lime, dust, bird excrement, chemicals (fertilizers, insecticides), smog (automobile emissions), aircraft emissions, cooling tower effluents, desert soil/sand, volcanic ash, smoke, mold, algae, fungi, lichen, ice/snow are also present and their mixture affects the insulators.
Pictures 1 and 2 clearly show that the passage of leakage current between the insulator terminals has been changed. Apparently, the pollutants and the humidity changed the electrical resistivity of the surface. When the electric field parallel to the surface insulator exceeded a certain critical value (4.5 kV/cm) the process of electric discharges started and corona was emitted followed by electrical tracking. Ongoing exposure to the harsh contaminated environmental conditions and to corona discharge turned the insulator surface into a conductive layer with low resistance and a changed leakage current path. High heat generated by the leakage current formed non-uniform conductive tracking areas. These conductive areas turned into dry bands. The presence of dry bands let arcing bridge the non-conductive areas dissipating heat causing the formation of conductive carbonized residues ending in dielectric breakdown or flashover.
To summarize the chronology of this flashover process:
- Wetting and contamination of the insulator surface formed a conductive layer with low surface resistance
- Flow of leakage current on the surface with high heat dissipation and non-uniform loss of the conductive layer triggered formation of dry bands
- Leakage current flew due to the presence of dry bands
- Dry bands arcs raised surface temperature causing the formation of carbon residues and erosion of the insulation
Awareness to the geographical conditions, to the local range of contaminants and to the degrading process of the used materials is needed. Solid contaminants migrate and get deposited on insulators because of various reasons and are influenced by factors such as wind, gravity, electrostatic attractions, evaporation of solutions etc. Insulators' shapes must be adapted to their installed location and to the deposition of those solids because their shape enables trapping of those solid particles and unless removed naturally by rain or wind should be washed routinely as often as locally needed. At the same time selecting the correct material is important. EPDM was mentioned in our case.
EPDM unlike silicone rubber or PDMS become hydrophilic after ~2 to ~8 years in service. One of the reasons for their degradation is their being susceptible to UV. The process of EPDM oxidation starts with UV absorption originating both from the sun and from corona activity. UV that is being absorbed by impurities produce highly reactive free radicals. These radical attack the polymer backbone and lead to surface cracking. As a result polymer substance is lost exposing the chalky filler material. The chalky dust is wettable and therefore the surface of the insulator loses its hydrophobicity. The following processes involve formation of alcohols and acids, in particular nitric acids that result from corona discharge which lead to increase in the roughness and hardness of the insulator and to brittle fracture.
The New Zealand Company decided to replace the insulators using SIR with corona rings. As a result the faults stopped. According to the survey service company, using the corona camera in airborne configuration saved the company lost revenues due to lost generation & profits costs of approximately $500,000NZ while the cost to identify & repairs (Corona Camera, Helicopter, Live line Crew) was about $13000.