Corona test of grading ring for UHV insulators using gradient equivalent method

Corona test is necessary for verifying the validity of grading ring design for insulators on ac transmission lines. But there seems a lack of standards about simulation test method for ultra-high voltage (UHV) power fittings, such as grading ring for insulators. This paper describes a test approach that specifically applies to grading ring for UHV insulators. First, based on finite-element (FE) calculation, a gradient equivalent coefficient which reflects difference of electric stresses acting on surface of grading rings mounted on different sites is determined. Next, refer to IEC and national standard, an available test voltage is corrected by previous coefficient considering interactions of three-dimensional environment. Finally, the corona test of a type of grading ring for UHV suspension composite insulators is performed using above method as an example. This method shows good effect by field observation on tangent towers with an ultraviolet camera and the tested grading rings have been applied in the UHV pilot project in China successfully.