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Title: The effect of humidity and pressure on space charge growth in high voltage air discharges
Author: Turri, R.
Awarding Body: University College of Swansea
Current Institution: Swansea University
Date of Award: 1988
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In this thesis the effect of humidity and gas density on discharge development and breakdown characteristics in a short sphere/plane gap subject to positive switching impulses has been investigated. The experiment was carried out in a sealed ionization chamber using laboratory air for pressures in the range 0.7 to 1.4 bar and absolute humidity levels from 2 to 15 g m-3. Both pressure and humidity could be closely controlled and measured. The impulse shape was = 100/2500 μs and the test gap consisted of a 5 cm diameter sphere 20 cm above an earthed plane. An improved version of biased field-filter and a newly designed composite field/charge probes have been developed, which enabled time-resolved measurements of the electric field distribution at the cathode in the presence of space charge to be made together with a quantitative measurement of the conducted charge in the central region of the cathode. Statistical flashover tests and measurements of apparent charge injection were made and both sets of data were analyzed using a Generalized Liklihood Method, which has been developed during this work and is recommended to be considered for possible adoption as a standard procedure. The breakdown probabilities were found to satisfy a cumulative normal distribution and the charge measurements provided significant additional information at low probability levels. The measured values of V_50 were found to depend linearly upon absolute humidity with a slope of 1.5 kV per g m^-3 at all pressures. The withstand voltages, on the other hand, were only weakly dependent upon absolute humidity. The humidity correction factors found in the present work agree well with the IEC Standards at 1.2 and 1.4 bar, but they depart appreciably at low pressures. In a second series of measurements, simultaneous time-resolved measurements of the light output from the discharge, the apparent charge injection at the HV electrode, the electric field distribution and transfer charge at the cathode, have been made. The results have greatly clarified the mechanism of the space-charge growth in the transition regime from uniform-field to non-uniform-field breakdown, and should enable predictive models of the discharge development and breakdown voltage to be made.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available