An investigation of AC corona in air and sulphur hexafluoride/nitrogen mixtures.

摘要

The aim of this work is to investigate AC corona in air and sulphur hexafluoride/nitrogen (SF₆/N₂), and to increase our knowledge of AC corona processes. The majority of corona studies have been undertaken with direct voltages and point/plane electrode systems since this reduces the complexity of the information gathered.; In this thesis, first of all, the basic theories of gas discharges including the Townsend Criterion, the Streamer Theory and the Paschen's Law are briefly described and the research work reported in the literature on corona discharge measurements and analysis are reviewed, then investigations of breakdown and partial discharge (PD) characteristics for air and SF₆/N₂ mixtures under the AC applied voltage have been made.; A computer-based commercial digital PD detector TE-571 was used to measure, record, display and analyze corona pulses according to the IEC 270 standard. A number of parameters, which characterize the PD behavior, including integrated quantities and statistical moments within the power frequency cycle were derived and shown. The pulse height distribution was measured as a function of phase angle and charge size for a range of voltages and a range of charge sizes. In case of air gap, the onset voltages for both positive and negative half-cycles are the same, but slightly lower than for the DC case, due to space charge effects. In all other respects the patterns for the two half-cycles are different: pulses only occur at the onset voltage (rising) for the positive half-cycle; but continuously from 11 kV (rising) to 14 kV (falling) for the negative half-cycle. Differences between the distributions for avalanches and streamers increase as the breakdown voltage is approached. Qualitative explanations of the patterns and behavior are given. In addition, the strong influence of humidity on the AC corona patterns in air has been also discovered. Results show the presence of humidity in the discharge pressure vessel not only modifies the gas nature but also the surface features of the electrodes like conductivity. The latter property can control the discharge behavior. In case of SF₆/N ₂ mixtures, experimental results show the breakdown voltage can be maximum for certain mixture ratios rather than pure SF₆, and corona onset voltages increase with increasing the point electrode radius, gas pressure and SF₆ content in N₂, moreover, the onset voltage for negative point corona is slightly lower than that for positive point corona. (Abstract shortened by UMI.)