| Popis: |
The knowledge of the external field distribution around High-Voltage Direct Current (HVDC) apparatus and lines is important, and the importance of accurate knowledge of the fields in air is expected to increase with increasing voltages. Consequently, the study of the non-linear relation between the electric field and the electric current in air is necessary as a way to understand how charges in this gaseous medium affect the DC electric field. In order to identify the main factors that influence the charge generation process, the ionic current in air was measured under uniform electric field in a plate-plate setup. Different environmental conditions and electrode configurations were also studied to understand what parameters affected the charge generation process. The specific parameters investigated were electric field magnitude, surface roughness, humidity, temperature and distance between electrodes. The test results showed that the electric field magnitude, humidity and surface roughness had the largest influence on the charge generation process. The ionic current presents an exponential behavior with the applied electric field and the humidity. The measured ion current reached higher magnitudes compared to what can be expected from only cosmic rays and background radiation production of charge carriers. The experimental results show that there is a strong charge injection process close to the surface already in the pre-corona region. The exponential increase of current with humidity and electric field due to charge injection is not well described by available models. In particular, there is insufficient information about strongly field dependent charge generation processes in air under DC-electric fields in the pre-corona region. |
