corona forms when the voltage gradient at the surface of a conducting material exceeds a critical value and ionises the surrounding air，corona becomes a great concern at extra high voltages where precautionary measures are necessary，effects of corona include radio and television interference，voltage loss and audible noise on the line，grading rings OEM manufacturing to suit EHV transmission lines for both suspension and tension configuration，typical designs are shown below for demonstation of factory's ability to manufacture to customer requirements，please contact us for further information and design suited to your string configuration.