Decomposition Characteristics Of SF₆ Under Partial Discharge And Sensitivity Of Single-wall Carbon Nanotube To SF₆ Decomposition Products

Gas Insulated Switchgear (GIS) is widely used in electrical industry for its stability and compactness. However, there might be insulation defects formed in the processes of manufacture, transportation, installation and long term of operation. Those defects will develop and may finally lead to serious insulation faults. It has been shown by lots of research that most insulation defects lead to partial discharge (PD). Detecting the characteristics of PD can reflect the insulation condition. The traditional methods to detect PD include pulse current method, ultrasonic method, UHF method, and so on. Each of them has its own features, and has been applied at different degree. However, the traditional measurement method is difficult to get rid of the electromagnetic disturbance. Because of what, the online detection veracity is doubtable. Since the non-electric methods don't affected by electromagnetic disturbance, SF₆ decomposition components detection has drawn lots of attention recently. Preliminary study has shown that SF₆ decomposition characteristics are different in various insulation defects. And the SF₆ decomposition components are related to the insulation defect types, which make it possible to detect the insulation condition by analyzing the components. Study on this method has great value in both theory and engineering. In this article, based on the reaches have been done, SF₆ decomposition characteristics were studied, and a carbon nanotube gas sensor for trace gas online detection is developed. In the study, the characteristic components of SF₆ decomposition were selected preliminary, and the micro-sensing mechanisms and macro-sensing characteristics of carbon nanotube gas sensor have been investigated through theoretical simulation and experiments. The specific work is as follows.1) Four kinds of physical models of typical insulation defects were constructed and the distribution of electric field of these models was analyzed by Finite Element Simulation. A PD decomposition chamber was built. A gas chromatography detection system was built to separate and analyze the components. In these four models the decomposition experiments were carried out, SOF₂, SO₂F₂, CF₄ and CO₂ from the decomposition components were detected. The result shows that PDs happen in all four models at the certain positions and expected voltage ranges. The N-type defect has large discharge energy and high values in decomposition quantities and speed. In the P-type defect, every PD pulse has a high altitude. And the value of SOF₂, SO₂F₂ is high. The M-type defect produces more and more CF₄ as experiment time goes on. The G-type defect shows a high PD level but produce little decomposition.2) Different defects make the decomposition characteristics quite different. It was suggested preliminary to use SOF₂/SO₂F₂, CO₂/CF₄ and (SOF₂+SO₂F₂)/(CO₂+CF₄) three values as the characteristic quantities to identify these four kinds of defects. SOF₂/SO₂F₂ was explained to represent the PD intensity. And the content of CF₄ can reflects the level of insulation ageing.3) Using interdigitated electrodes printed circuit board as the base SWNT-OH and SWNT sensors were made. SOF₂, SO₂F₂, CF₄ and SO₂ four components were experimented on the sensors. After comparing their performances, we paid special attention to SWNT-OH. Its sensitivity, stability and recovery characteristics were studied. The result shows SWNT-OH sensor has high reacting speed and high sensitivity to SO₂ in those four components. It satisfied certain linear relations between the resistance change rate of SWNT-OH and SO₂ gas concentration. And the SWNT-OH sensor can self-recovery in the air costing a short time. To a certain extent, the SWNT-OH sensor meets the stability requirements.4) Adsorption models, between four types of gas components (SOF₂, SO₂F₂, SO₂, CF₄) and SWNT, SWNT-OH, have been established by Materials Studio. Based on density functional theory (DFT) theoretical calculations were carried on. The micro-sensing mechanism of SWNT-OH was analyzed. First, the absorption capacity of SWNT-OH on the four types of gas molecules was judged according to the calculation results. Second, the difficulty levels of interaction between SWNT-OH and four types of molecules have been analyzed according to the molecular frontier orbital theory. Third, how gas molecules adsorbed on the SWNT-OH to influence electron transfer ability was investigated according to the results of energy gaps. Combined with the results of density of states, the hybridization of atomic orbitals was also investigated during the adsorption process between SWNT-OH and SO₂. Finally, the calculation results of SWNT-OH were compared with that of SWNT. Simulation results show that SO₂ was the most easily adsorbed on SWNT-OH, and the adsorption capacity of SWNT-OH on SO₂ was the largest among the molecules; the energy gap of SWNT-OH was reduced after adsorbing SO₂, which improved its ability of electron transfer; during the adsorption process, atom partial orbits tend to hybridization between SO₂ and SWNT-OH, which could help form electron transport channel, conducive to electron transfer between SWNT-OH and SO₂. Simulation results also indicated that the adsorption capacity of SWNT-OH on gas components was larger than that of SWNT.