Correct Answer - Option 4 : once
Insulator pollution in transmission lines:
- The level and the type of pollution of a region are associated with the sources of pollution, as well as with weather factors of the place.
- The pollution is placed on the surface of the insulator and a contaminant layer appears.
- The pollution can be caused by a great variety of sources, (sea salt, industries, ashes…).
- The wind is the main bearer of the particles, having a secondary role the gravity and the electric field.
- By the action of rain, fog, etc. ... the layer on the surface is dampened and enlarges the conductivity. The contaminant layer dries. Thus, there is an increase of conductivity and leakage current.
- Dry bands are formed as a consequence of the warming-up of the layer on the insulator surface.
- Partial arches appear through the dry bands. Partial discharges are produced, these discharges produce audible noise. Finally, the total discharge is produced.
- The insulators exposed to coastal or marine environments, can become to be conductors due to the formation of a conductive layer on its surface.
- This layer will be formed on account of the salted dew of the mornings in these zones close to the coasts.
- When dried with the heat produced in the same insulator or with the environment temperature, is going to deposit in the insulator the evaporated salt that had absorbed before.
- The particles placed in the insulators are not dangerous in dry weather but, the problem arises when the environmental weather is humid, rains, there is dew, fog, etc. then the layer can be come conductor.
- The conductivity of this layer will depend on the kind of salt that form it.
- The weather conditions vary considerably from the coastal areas to the interior areas and they play a very important role in the contaminants deposition rate and in the operation of the insulator.
The maintenance of the transmission line passing through polluted or coastal areas should be completed once before the onset of winter.