The effect of humidity and temperature on the total electric field of unipolar positive and negative HVDC lines in the natural environment

It is of great significance to study the influence of temperature and humidity on total electric field of HVDC lines for improving the refinement of line design and environmental protection. In this paper, an outdoor experimental platform is built to study the effects of humidity on ground total electric field under unipolar positive and negative HVDC test lines in natural environment. The experimental results show that in natural environment, in the range of 35%-70% humidity, with the increase in the relative humidity, the ground total electric field under positive and negative HVDC test line increases at first. But when the humidity reaches 70%-85%, for the negative HVDC test line, the increase trend of ground total electric field obviously slows down, and the electric field begins to decrease when the humidity is higher than 85%. In contrast, the ground total electric field under positive HVDC test line keeps increasing all the time. According to the theoretical analysis, the increase in the relative humidity in natural environment is accompanied by the decrease in the temperature, which is the main reason for these experimental phenomena. According to the research conclusion, stricter field strength control measures should be taken in the design of unipolar HVDC transmission lines in areas where the annual average humidity is relatively high than in the area with low annual average humidity. At the same time, in areas where annual humidity is relatively low, for the bipolar HVDC transmission line, it is merely of need to assess the ground total electric field under negative conductor, and the asymmetric corridor scheme can be considered.