| SF6 has a strong greenhouse effect,which is widely used in traditional gas insulated electrical equipment.It is imperative to find another eco-friendly insulating gas.C3F7CN is regarded as a potentially replacement to SF6 due to its merits in stable chemical properties and high dielectric strength,which is applied in AC gas insulated transmission line(GIL).Surface charge accumulation of spacers is the key problem restricting the research and development of DC GILs.Charge accumulation distorts the local electric field,which is a potential threat to the reliability of DC gas insulated equipment,When the environment-friendly insulating gas C3F7CN is applied to DC GIL,the charge characteristics and mechanism of gas-solid interface are unknown.Therefore,the surface charge accumulation characteristics and and influencing factors of spacers in C3F7CN/CO2 gas mixtures were studied.From the aspects of gas,gas-electrode interface and gas-solid interface,the charge accumulation mechanism was excavated.Then,the gas-solid interface charge calculation model and the gas-solid interface charge control method based on gas side were discussed based on the above mechanism.The following researches are carried out in this paper:1)A automatic surface potential measuring platform for GIL spacers in high-pressure environment was built,which realized the surface potential measurement of spacers in C3F7CN/CO2 gas mixtures at 0-0.7 MPa.The results were compared with surface potential on spacers in SF6.It is found that with the increase of the number of C3F7CN gas molecules,the amount of hetero-polar charge on the spacer surface increases.Meanwhile the distribution range,which starts from the outbreak of "hetero-polar charge band",gradually expands to the whole spacer surface.With the increase of the C3F7CN ratio,the hetero-polar charge shows a "root like" segmented growth law.While with the increase of the gas pressure,the hetero-polar charge shows a saturated growth trend,and the growth rate decreases significantly at 0.5 MPa.It is proposed that under the premise of equivalent dielectric strength with SF6,4%C3F7CN/96%CO2 gas mixture at 0.7 MPa can make the the least surface charge accumulation on the spacer.2)Aiming at the generation of charge in high-pressure gas,a high-pressure gas conduction current and weak light joint measurement platform was built.The conduction current characteristics of C3F7CN/CO2 gas mixtures with different ratios and gas pressure were obtained.It was found that there was a negative correlation between the saturated ion current and the breakdown voltage of the gas mixtures.Through weak light synchronous detection,two patterns were proposed,which were’micro pulse cluster’ and "intermittent pulse rush peak".The field zoning dominant mechanism of gas natural ionization and electrode surface micro ionization in current formation were verifided and revealed.In the low field,natural ionization is dominant.With the increase of C3F7CN gas ratio and pressure,the conduction current increased,and the optical signal showed the pattern of ’micro pulse cluster’.In the middle field,the contribution of the two kinds of ionization to the current was similar.Both ’micro pulse cluster’ and ’intermittent pulse rush peak’ coexisted.In the high field,with the increase of C3F7CN gas ratio and the gas pressure,the conduction current decreased,and the optical signal was dominated by ’intermittent pulse rush peak’.In particular,for the micro ionization of conductor surface,the correlation among the conductor surface micro morphology,the surface local field strength and the gas conduction current was studied.A DC GIL conductor surface evaluation method considering conductor surface arithmetic average height(Sa)and kurtosis coefficient(Sku)was proposed.3)For the capture of gas side carriers on the insulator surface,the existence of solid gas interface transition layer was verified by experiments.A gas surface energy test platform was built.It was found that the bonding ability between C3F7CN and CO2 and epoxy surface was greater than that of SF6.Based on density functional theory(DFT),a bimolecular model of gas-solid interface transition layer between C3F7CN,CO2,SF6 and epoxy resin under DC electric field was established.Through the calculation of surface electrostatic potential distribution of the transition layer molecules,it was found that the average deviation of surface electrostatic potential of C3F7CN epoxy interface transition layer was 1.20 times that of the SF6 epoxy interface transition layer.And the average deviation of surface electrostatic potential of CO2 epoxy interface transition layer was 1.09 times that of the SF6 epoxy interface transition layer.C3F7CN epoxy interface transition layer showed high local polarity.It may have high selectivity and affinity for the deposition of hetero-polar charges.4)Based on the mechanism above,an new numerical calculation model of gas-solid interface charge dynamics based on the field enhancement factor of arithmetic average height Sa and kurtosis coefficient Sku,micro ionization current and gas affinity coefficient aaffinity was established.And the accurate simulation of potential test results in different gas was realized.The results showed that both gas electrode interface and gas epoxy interface could affect the polarity and potential distribution of surface charge.The increase of the conductor surface field enhancement factor β and coefficient aaffinity will promote the change of charge mode from homo-polar dominance to hetero-polar dominance.5)A multi-performance cooperative control model of eco-friendly gas mixtures considering insulator surface charge was established.And a C3F7CN/CO2 gas formula suitable for eco-friendly DC GIL was proposed.Based on the new numerical calculation model of gas solid interface charge and the AC GIL design standard,the design method of DC C3F7CN/CO2 GIL insulator was proposed.When C3F7CN/CO2 gas mixture is adopted,the conductivity of the insulator was in the order of 10-17 S/m.The 100 kV DC spacer was prepared.It passed the DC withstanding voltage test and DC superimposed lightning impulse voltage test,which verified the feasibility of the design method. |
PDF Full Text Request