Impulse breakdown characteristics of SF6 and its mixtures in highly non-uniform field gaps
The work reported in this thesis was undertaken in the Centre for Electrical Power Engineering, Department of Electronic and Electrical Engineering, University of Strathclyde to study the impulse breakdown characteristics of SF₆ and its mixtures in highly non-uniform field gaps. Of particular interest were the effects of space charge, artificial irradiation, different gas mixtures, different additive gases and different wave fronts on the impulse breakdown characteristics. High divergent fields can exist in GIS under certain conditions as, for example, when a needle-like free metallic particle is attracted to the inner conductor or is deposited on the surface of an insulator. Such defects can result in very low breakdown levels and, with large defects (e.g. particles several mm long), failure can occur even at the working stress of the equipment. The breakdown characteristics of gases in nonuniform fields, however, are much more complicated than in uniform fields and are not fully understood. This is probably due to the complex effect of space charge on breakdown process  and the space charge effect on positive impulse breakdown characteristics of SF6 and its mixtures in highly nonuniform field gaps has become an increasingly important subject on the gaseous dielectrics with high electric strength in high voltage apparatus. The main purpose of the present work is to acquire a better understanding of the corona stabilised breakdown mechanism in SF₆ gas under impulse voltages and to supply a physical base to choose an efficient additive for improving the insulating strength of SF₆ gas. A general introduction is first given, based on a renew of experimental and theoretical work on the subject to date. Descriptions of apparatus and experimental techniques are then given. Two newly developed space charge injection methods, namely corona pin arrangement and direct injection method were used throughout the work. In the case of positive lightning impulse voltage, injected positive space charge has little effect on minimum impulse breakdown, whereas a decrease in breakdown voltage is observed when negative space charges are injected into the gap. Artificial irradiation also decreases the minimum impulse breakdown voltage, though the reduction rate is lower. From these and other observations, it is generally concluded that the major source of initiatory electrons for positive impulse breakdown in an enclosed gap is from the electron detachment from unstable negative Ions and initiatory electrons make an important contribution to the breakdown process. The conclusion is confirmed to a large extent by using a photomultiplier to observe the light emission during the discharge process. The study of the effect of space charges and artificial irradiation shows that although initiatory electrons make an important contribution to the breakdown process, it appears that there is a limit beyond which the breakdown strength cannot be further decreased by increasing the electron or negative-ion population. The study of the addition to SF₆ of 5% R20 or R12 has shown that although space charges have a great effect on impulse breakdown strength in SF₆ there is little, if any, effect on breakdown strength in SF₆/R12 and SF₆/R20 mixtures. The result implies those additives containing chlorine preferentially produce very stable negative ions which do not readily detach. The impulse strength is increased in mixtures containing these additives because there is then a reduced likelihood of successful development of the discharge channel through a scarcity of initiating electrons in the gap. The initiating electrons will be produced mainly by the detachment from negative ions so that the rate of production in the critical volume will itself depend upon the applied waveform . It has been found that the wavetail has little influence on impulse breakdown process [141,158] and only the effect of wavefront is studied. It had been found that the longer the wavefront, the higher the minimum impulse breakdown voltage, the reason is believed to be the sweeping off action which negative ions in the gap are swept out the effectively. Suggestions for further research work are offered in Chapter 9.