Aging effects of conductor surface conditions on DC corona characteristics

Aging effects are investigated by means of comparing a 7 yr-old conductor of the Takeyama HVDC test line with a new one. Long-term comparative tests were carried out in monopolar and bipolar arrangements. Corona currents of the aged conductor were greater than those of the new one by 1~2 times, ~2 times and 10~30% in the +250 kV monopolar, -250 kV monopolar, and ±250 kV bipolar arrangements, respectively. These results contrast with AC corona currents that decrease due to aging. Artificial pollution experiments, based on the chemical analysis of substances on the aged conductor, reveal that both the corona currents and flow currents at ground level increase as the amount of pollution, especially carbon, increases and that the differences between the monopolar and bipolar arrangements in the comparative tests can be explained by relations between the maximum conductor surface gradient and the corona onset gradient