Remedies are called for, evidently, when the flashover frequency rises above acceptable levels. What is 'acceptable' depends on the importance of the line or substation and on the required quality of supply, in terms of outage time per year. Standards thus vary widely: for lines, a rate of about one flashover per 150 km per year is general for industrialised countries in Europe, while much higher rates are tolerated elsewhere, e.g. in rural parts of the USA. Flashovers in substations often have serious consequences, and rates lower than one per year per station would normally be called for. The causes of flashover are part systematic and part random. In service, an insulator will carry a resident layer of contamination, accumulated since in stallation or the last cleaning operation, which fluctuates as the resultant of depositing and purging events, but is quasi-stable. The insulator is also challen ged by random occurrences like condensation, frost and onshore gales. These add water, ionisable material or both, which, depending on the design of the insulator, either will or will not carry the surface conductivity into a range where flashover can develop.

Inspec keywords: substations; insulator contamination; flashover

Other keywords: condensation; surface conductivity; ionisable material; industrialised country; insulation contamination; flashover frequency; onshore gales; substation; frost gales

Subjects: Power line supports, insulators and connectors