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.