3. Rapid Flashover Procedure (to obtain dielectric strength of naturally polluted insulators)
Because relatively few insulators with natural contamination are normally available from service, it would be time consuming to use the standard ‘up-and-down’ testing procedure to obtain flashover voltage. The rapid clean fog test method can then be used instead with the advantage that it is directly applicable to both naturally and artificially polluted insulators. The insulator is tested by consecutive application of voltage until the minimum withstand voltage of the U-shaped curve is obtained. Flashover levels generally decrease before resistance of the insulator starts to increase due to the washing effect. The minimum voltage level obtained is then considered to represent U50%. As a result, only one test is required per insulator, level of pollution and degree of hydrophobicity.
This method is included in CIGRE Brochure 481 (Dec. 2011) and was successfully applied for testing:
• AC porcelain support insulators with industrial pollution;
• DC glass cap & pin insulators with light inland pollution;
• DC composite insulators with light marine pollution;
• DC composite insulators with light inland pollution;
• AC composite circuit breaker housings with heavy industrial/marine pollution.
This method can be applied to create pollution performance curves to be used in the STRI IST program, as was done for insulators replaced from the Skagerrak and Cabora Bassa DC overhead lines. It can also be used to quantify the ability of a composite insulator to recover hydrophobicity. Such methods are presently under review by CIGRE working group D1.44 ‘Testing of naturally polluted insulators’. The test’s excellent repeatability is illustrated by the results from two different naturally polluted insulators (see Fig. 4).
Recently issued IEC 60815-1 requires accurate site severity evaluation performed over a minimum period of one year to identify the maximum stress level and corresponding site pollution severity (SPS) class. Such a process can then be defined as pollution monitoring and is the starting point in the process of selecting and dimensioning outdoor insulators for service in polluted areas. Although a software program called the Insulator Selection Tool follows the dimensioning process of IEC 60815 and has been verified against experience from the field, accurate pollution input data for this program is crucial. Among the latest trends in pollution monitoring to generate such inputs are:
• Expanded application of simple, robust and reliable devices (e.g. dust deposit gauges and single-channel leakage current sensors);
• Novel as yet not standardized methods that take into account specific features of insulators (e.g. hydrophobicity transfer / localized ESDD in the case of composite insulators);
• ‘Smart’ (time and cost-effective) methods to obtain flashover voltage of naturally polluted insulators, e.g. when only a few test objects are available (rapid procedures). These will be standardized in the future’
• Further development of specialized software programs for ultimate application by power companies (e.g. combining technical and economic issues in one program).