Search

The accumulation of charge on insulator surface is a key problem that restricts the development of high voltage direct current (HVDC) gas-insulated transmission lines (GILs) and gas-insulated switchgears (GIS), the ionization of insulating gas is an important source of hetero-polar charges accumulated on insulator surface. The value of gas conduction current is one of the key factors that determine the surface charge accumulation characteristics of insulators. In this article, the conduction current characteristic of C4F7N/CO2 gas mixture is experimentally studied with a coaxial electrode system like GIL structure, and the source of gas ions under different electric fields is analyzed. The results show that the average ion pair generation rate caused by natural radiation ionization of C4F7N/CO2 gas mixture during saturation current regime increases with the increasing C4F7N content, and the growth rate of the average ion pair generation rate of gas mixture decreases with the increasing C4F7N content. The increase of gas pressure shows a significant positive correlation with the average ion pair generation rate, but the growth rate decreases gradually. The critical electric field when the C4F7N conduction current increases exponentially is significantly higher than that of SF6 and CO2, and the critical electric field of C4F7N/CO2 gas mixture increases with the increasing C4F7N content. In addition, with an increase in the gas pressure, the higher C4F7N content in C4F7N/CO2 gas mixture is, the more obvious the inhibition effect on gas ionization is within the range of the electric field in surge regime. The critical electric field with exponential growth of C4F7N/CO2, CO2, and SF6 conduction current decreases with the increase of temperature, and the current in surge regime increases rapidly with the increasing electric field. The results of this article provide a reference for the theoretical study of surface charge accumulation at the solid-gas interface.

Suppressing the activity of metal particles is a key issue in the development of dc gas-insulated power transmission lines (GILs). This study investigates the effects of five different dielectric coatings of the electrodes—namely surface oxidation treatment and four different coating materials (i.e., 1032K high-resistance paint (1032K), teflon, epoxy marlin orange primer (EM primer), and iron red primer (IR primer) spray)—on the movement characteristics of ball and block metal particles under dc voltage and polarity reversal voltage in an SF6 gas environment at an absolute pressure of 0.3 MPa. The results showed that the surface oxidation treatment of the electrodes had little effect on suppressing the particle activity. The coating in the high-voltage electrode did not significantly change the particle jumping voltage, and once the particles jumped, they were adsorbed onto the coating. Coating the ground electrode significantly increased the jumping voltage of the particles. Under dc voltage, the lifting field strength of the particles exceeds 5 kV/mm which is much higher than the maximum design field strength at the ground electrode of dc GIL. The increase in coating thickness did not significantly improve the jumping field strength. Under polarity reversal voltage, the jumping field strength of the particles was lower than under dc. However, the coating of the ground electrode also had a significant effect on suppressing the particles during the reversal process. Simulation results showed that the coating hindered the charge exchange between the particles and the electrode, increased the field strength at the contact position between the particles and the electrode, and reduced the field strength on the side of the particles away from the electrode.

In this article, a 3-D simulation model of surface charge accumulation for 200-kV dc gas-insulated transmission line (GIL) basin insulator is constructed. Meanwhile, the whole process of electrostatic probe online measurement is established, and the motion process of electrostatic probe is simulated by the dynamic grid module in finite element software. The key factors in electrostatic probe online measurement are considered comprehensively, including the probe initial position, central conductor potential, probe measuring time, and measuring position. The variation of surface charge density and surface normal electric field after online measurement is calculated and analyzed. The results show that the surface charge density near the probe center is obviously affected after online measurement, while the surface charge density in the region far away from the probe is less affected. When the distance ${D}$ between the probe initial position and the insulator surface is less than 10 mm, the polarity of the surface charge at the measured point will be reversed. When ${D}$ reaches 100 mm, the change rate of the surface charge density is less than 5%. In addition, with the increase of the central conductor potential and probe measuring time, the surface charge density at the measured point decreases more after online measurement. When the probe measures at the same position for more than 3 min, the surface charge density decreased by more than 30%. Moreover, the closer the probe is to the central conductor, the more the surface charge density is reduced after online measurement. This study can provide reference for the online measurement technology of dc GIL insulator’s surface charge density.