| SF6 is one of the most widely used insulating gases in power system,but it will be restricted gradually in the future beacause of its greenhouse effect.CF3 I and its gas mixtures with N2 are considered to be one of promising alternative insulating gases to replace SF6.In consideration of the effect of liquefaction temperature,discharge by-products and the sensitivity to inhomogeneity of electric field,it is usually considered that the CF3 I content in CF3I/N2 mixture should not be more than 30% and it is applicaple to equipments with medium and low pressure.Investigation of insulation performance of CF3I/N2 mixture in extremely non-uniform field in this case is not thorough enough.In this paper,the insulation performance of CF3I/N2 mixture with CF3 I content k=10,20,30 is investigated by means of theoretical calculation of electron swarm parameters and experimental measurement of breakdown characteristics and streamer inception characteristics.Two-terms Boltzmann equation is used to analyse electron swarm parameters of CF3I/N2 mixtures in the range of E/N over 100-1000 Td.Electron energy distribution function and electron drift velocity,ionization coefficient,attachment coefficient,effective ionization coefficient and reduced critical electric field are obtained.The results show that the reduced critical electric field of gas mixtures increases linearly with CF3 I content,the reduced critical electric field of 30%CF3I/70%N2 is 65kV/cm,nearly 2/3 of SF6 gas.Streamer inception time delay model taking into account of the effect of gas pressure and electric field distribution is founded based on the volume-time method.To simplify the calculation,it is assumed that:(1)the ion density profile does not change during the impulse voltage application,(2)the ion density profile is constant in the whole gap space.A test tank for gas discharge is designed,together with rod-plane electrodes of different rod diameters which is used to simulate extremely inhomogeneous electric field.The experimental platform used for synchronous observation of streamer discharge in strong electronegative gases has been founded based on the photomultiplier tube and ICCD camera.Streamer inception voltage and time delay of 30%CF3I/70%N2 gas mixtures together with their variations under lightning impulse voltage are investigated.The influence of gas pressure and electric field distribution on streamer initiation is studied.The semi-empirical parameters in the model is obtained through the experimental results,the theoretical results agree well with the experimental results.Based on the mechanism of leader breakdown in electronegative gases in extremely non-uniform fields,the processes of streamer corona and leader are taken into account.The calculation formula of streamer radius,corona length and corona charge is deduced and the corona charge criterion of breakdown voltage is given.The lightning impulse breakdown characteristics of CF3I/N2 gas mixtures under different gas pressure and different ratios of CF3 I content is studied and 50% impulse breakdown voltage and V-t characteristics are obtained.The test results show that: U50% and time delay for breakdown as well as their scatter increse slightly with the CF3 I content.U50% increases with gas pressure while the increasing tendency reduces.When radius of curvature of electrodes increases,U50% rises rapidly while the time delay for breakdown also increases.Negative U50% is much higher than positive U50% under different electrode configurations while there is no significant difference between the time delay for both polarity. |
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