One of the most important parameters for composite insulators is hydrophobicity, allowing superior insulation performance compared to ceramic cap & pin strings. However, these initial hydrophobic properties can deteriorate in service if there is continuous corona activity either directly on the housing or from nearby metal fittings. In fact, this can prove to be a specific mechanism leading to premature ageing and therefore must always be taken into account, even at the design stage. Electric field must always be controlled in the most vulnerable areas of these types of insulators.

Composite insulators have seen growing application in relatively clean environments due to their comparative ease of handling and attractive acquisition costs. More recently, voltage upgrading as well as compact design of new AC lines have become additional niche areas where composite insulators are being applied in clean environments. In the case of the latter applications, insulator arrangements are often designed relatively short to fit into the reduced space window of towers. Therefore, limiting maximum E-field becomes even more critical. Another growing area of application are composite station post insulators, especially those having a solid core since these do not differ much in flange design from composite line insulators. Three criteria have to be taken into account to assure optimal dimensioning of composite insulators equipped with grading rings:

1. Limiting electric field on grading ring & end fitting;

2. Limiting electric field along surface of insulator housing;

3. Limiting electric field at ‘triple point’ (where air & housing meet metal fitting).

All three are normally verified by E-field calculations, the first by the standard RIV test described in IEC 60437 2nd Edition (1997-09). The third criterion cannot be verified by a test while the second is as yet not verifiable by any test. Power companies, however, are now increasingly interested in having such verification.

Criteria for limitation of E-field in sensitive parts of a composite insulator have been established after comprehensive research performed separately by STRI and EPRI, but leading to basically the same numerical results. It is possible to verify the results of E-field calculations using the newly developed Water Drop Corona Induced (WDCI) test procedure. This test almost fulfils all typical IEC requirements with final reproducibility verifications conducted in another test laboratory. The test method is cost-effective if performed in conjunction with a standard RIV and/or corona test and can also be considered an additional type test in any user specification for composite insulators. A combination of E-field calculations and their verification by laboratory testing creates a solid basis for optimal dimensioning and positioning of grading rings and arcing horns on composite insulators.

From : http://www.inmr.com/optimal-dimensioning-of-grading-rings-for-composite-insulators/2/