Reliable operation of transmission and distribution systems relies greatly on insulator performance. Yet proper dimensioning is usually not easy since it requires knowing site contamination as well as withstand ability of the insulators to that specific contamination. Contamination depends on a number of site dependent variables. Wind, rain, humidity and the chemical-physical characteristics of contaminants all play a key role while other parameters such as dominant wind direction and rain intensity also have an influence. These parameters are site specific and evaluating them is difficult and even more complicated since there is usually some uncertainty when estimating these parameters. Specific statistical methods are then necessary to reduce such uncertainty.
Contamination measurements represent another approach to classify a site and can be performed both on energized and un-energized insulators or using specific ‘dust catchers’. Both the a priori characterization of a site based on meteorological parameters and measurement with dust catchers needs a further step: evaluating the contamination actually depositing on insulator surfaces. The methods mentioned above also need feedback of actual operating experience to validate them and to help refine assessment of the contamination level of a site.
Insulator withstand ability to a specific contamination is related to insulator characteristics and numerous tests have been performed on different types and profiles at laboratories the world over. This data have then been assembled to yield mostly 2-parameter models that give the Unified Specific Creepage Distance (USCD) for specific contamination levels. The final aim in choosing the most suitable insulator/string for a specific site is evaluating risk of failure in order to ensure that this value is lower than or equal to some acceptable level of risk. This acceptable risk usually varies with voltage level as well as type of installation, whether line or substation.
Attend the 2017 INMR WORLD CONGRESS in Barcelona-Sitges this November to hear a paper by Eng. Massimo Marzinotto of Italian grid operator, Terna. In his paper, separate analyses are performed on line and station insulators since acceptable risk for the two is in general different. There is also an extensive report on field experience gathered after many years of operation on tends of thousands of kilometers of high voltage overhead lines and hundreds of high voltage stations.