|Title:||An investigation of AC corona in air and sulphur hexafluoride/nitrogen mixtures|
Electric insulators and insulation -- Sulphur-hexafluoride
Hong Kong Polytechnic University -- Dissertations
|Department:||Department of Electrical Engineering|
|Pages:||x, 99 leaves : ill. ; 30 cm|
|Abstract:||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. AC corona studies are much fewer in number and mostly ignore the variation in voltage with time. Some reports on AC corona investigations again demonstrate the differences between the positive and negative half-cycles in their statistical analyses of the pulses, but the results are averages taken over the range of voltages from onset to maximum and down to the extinction voltage. Consequently, the interpretations are inherently limited in the early work. 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 SP6/N2 mixtures under the AC applied voltage have been made. Also, the AC breakdown behavior for SF6 mixed with nitrogen has been studied and recorded for this non-uniform field system, the concentric-hemisphere point/cup electrodes configuration. A computer-based commercial digital PD detector TE-571 was used to measure, record, display and analyze corona pulses according to the [EC 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/N2 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. Finally, some useful conclusions obtained from this study are summarized, and also some suggestion for future research regarding the corona discharge in air and SF6/N2 mixtures are made.|
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