NGK 115 kV & 138 kV Corona Ring Statement

NGK recommends an 8 inches (204 mm) corona ring for polymer suspension insulators at 115 kV and 138 kV as a default, unless the customer, NGK, or a reputable 3 ^rd party performs a 3D modeling analysis of the polymer insulators, without a corona ring, or with a smaller corona ring, in the intended applications(s), and the results pass EPRI criteria.

All NGK 115 kV and 138 kV polymer insulators pass ANSI C29 standards for electric stress control without corona rings. The ANSI standards for polymers, originally published in 1997, are based on electric stress control on the metal end-fittings at 15% above nominal voltage.

However, EPRI's and STRI's published research findings, beginning in 1999, have found that electric stress control on the rubber is also important to prevent damaging water drop corona. They recommend performing 3D modeling to demonstrate that the electric field gradient is below 0.45 kV/mm on the rubber. This recommendation is more stringent than ANSI requirements. Subsequent inspections of in-service polymers and failures of polymer insulators due to corona damage on all manufacture's polymers at 115 kV and 138 kV have validated the EPRI and STRI recommendations. Recently EPRI has recommended the installation of 8 inches (204 mm) diameter corona rings at voltages as low as 115 kV in some applications.

Please contact NGK engineering department for additional explanation.

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