Grading rings

Grading rings reshape the electric field, reducing corona discharge on the insulator, high RIV levels, and power losses. If corona persists, it may damage the polymer material and lead to insulator failure.


Corona discharges can form on the metallic ends of an insulator when the electric-field intensity exceeds the dielectric strength of the surrounding air. Several factors that affect the onset of corona are air pressure, humidity, end fitting material, and voltage level. If the end fitting has sharp edges or is irregular in shape, corona can result from a concentration of the electric field.
The elevation at which insulator is applied is an important consideration when specifying grading rings. Tests have shown that corona extinction and inception levels decrease with increasing altitude; approximately a 1% change for every 1000 feet above sea level. However, the effect is not as significant below 3000 feet since the bulk of corona tests occur near this level. Therefore, corona extinction levels must be adjusted for applications above 3000 feet.
Grading rings are selected according to the corona extinction level. A variety of ring shapes and sizes are necessary to meet the requirements of the different voltage levels and insulator configurations. Generally, by increasing a corona rings diameter or thickness the inception level increases.

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