Radio noise due to insulator corona

Radio noise due to corona discharges on suspension insulators is studied as influenced by atmospheric humidity, insulator-surface pollution and harmonic contents in the alternating-voltage wave applied across the insulator. The radio-noise level is also measured under direct-voltage conditions. The tests are carried out in humidities between 10 and 38 g/m3 at temperatures between 15 and 38°C. Alternating voltages up to 45 kV are used, with harmonic contents up to 40%, and direct voltages up to 70kV are applied. The radio-noise level is found to increase with the air humidity and decrease if the insulator is kept in a humid atmosphere for some time. The noise level with alternating voltages is found to be considerably higher than that with direct voltages of the same amplitude. It is found to be still higher if the alternating-voltage wave contains third harmonic so as to be double-humped. The cement binding the metal cap and pin to the porcelain is found to be a main source of radio noise when the insulator is under corona discharge