Contamination has always been one of the greatest problems of outdoor HV utilities especially near co, industrial and mining areas. Since contamination cannot be avoided the only way to fight it is by performing regular inspections and periodical washing. Nevertheless, it is well known that washing can be an expensive process, tedious & time-consuming. Further-more, in some cases this process even requires de-energizing the transmission or distribution lines, especially if an entire grid or substation needs to be cleaned. The way to perform a cost effective cleaning is by using a corona camera in order to locate precise locations that need to be washed hence, saving the costs of human resources, water, time and ineffective work.
The problems of contamination on electrical insulators take place when the environment that surrounds them contains diverse substances, especially saline which are deposited on insulators forming a polluting layer on their surface. In dry conditions, this layer does not cause great problems, but under the presence of light rain, humidity, dew or fog, the dielectric characteristics of the insulator surface are decreased, allowing the flow of leakage current between the insulator electrodes. The probability and speed of this failure type depends on the type and material of the insulator, the climate of the area, the type and level of contamination, as well as the voltage under which the insulator is working. In order to control the pollution problem, two alternatives have been established: design and maintenance. In the first alternative, the insulator type and material are chosen according to the pollution level in the area in which the insulator is to be used. In the second alternative, efficient maintenance plans are designed. The design of insulators for use under contaminated environments has been based on the properties of the insulating materials as well as on knowledge and experience gained during the development of the electrical industry. In many cases, a good insulator profile design may reduce the number of outages caused by pollution.
Nevertheless, in areas with severe contamination environments or low rain probability, maintenance of the insulators is required. The maintenance usually contains both UV corona camera and IR camera inspections in order to have a complete picture of the grid. In addition this maintenance normally includes periodic washing.
Washing by water jet has shown itself to be the most economic method to remove pollution from the insulator surface. Washing is very effective when the pollutant deposited on the insulator is dust, salt, dirt or acid, because these pollutants do not adhere strongly to the insulating surface. In some areas, the natural washing performed by rain is sufficient in order to prevent flashover on the insulator due to contamination. In regions where rain is not frequent and in areas where rain is not uniform (more contaminated in one side due to no being washed evenly by rain), it is necessary to wash the insulator during the dry seasons.
In areas plagued by contamination related flashovers, it is a challenge to decide when to wash the transmission line or substation insulators. The observed discharge activity is a function of both the contamination severity on the insulator as well as the degree of wetting prevalent at the time. Another challenging decision is the frequency of washing depends on the contamination level, the environmental conditions and the insulator profile.
The insulators must be washed before they accumulate critical contamination levels. This can be determined by means of:
- Experience of pollution flashover occurred over the last years
- Measurements of equivalent salt deposit density on witness insulators installed near transmission line
- Surface discharges observed on the insulator during critical conditions
- Presence of radio interference
- Measurements of leakage current on the insulators installed on the transmission line
With a corona camera, it is very easy to decide where and when to wash since all the condition above will be shown in an easy and typical inspection with a daytime UV camera. Moreover, with a corona camera a trending analysis can be performed and a database can be created so that over time washing plans will become intuitive routine.