It is widely acknowledged that the world needs to reduce the level of greenhouse gas emissions. It is proposed to use potentially cleaner renewable energy sources to replace fossil fuels, and thus reduce greenhouse gas emissions. A significant challenge facing renewable energy sources, however, is that the power generation facilities are often located far from the load centres, meaning that new high capacity long-distance transmission systems would need to be built. This is a particular issue since there are increasing difficulties in obtaining approval to construct new overhead lines (OHL). An alternative is gas-insulated lines (GIL), a system for the transmission of electricity over long distance and is considered as a viable technical solution in places where OHL cannot be constructed. The currently adopted gas medium in GIL, however, is sulphur hexafluoride (SF₆), which is a potent greenhouse gas. Trifluoroiodomethane (CF₃I) has been proposed as an alternative insulation medium to SF6 in GIL, and this thesis investigates the potential of using a CF₃I gas mixture in GIL applications. It is hoped that the research can lead to a new form of environmentally friendly power transmission system that could cope with the increasing power demand in large metropolitan areas, and contribute to the reduction of SF6 usage in the high-voltage industry. The literature survey reviewed the research work on CF₃I gas and its mixtures to date. Several research gaps were identified, and these informed the investigations carried out in this research. Reduced-scale coaxial test systems with the electric field properties of a full-scale 400 kV GIL were designed, developed and fabricated. The designs were simulated using COMSOL to ensure that the highest field would be along the centre of the conductor. The effective ionisation coefficients of various CF3I gas mixtures were calculated using BOLSIG+, which provided estimated values for the critical reduced field strength of each gas mixture. Extensive laboratory tests using a standard lightning impulse (1.2/50) were conducted on the fabricated prototypes using various CF₃I/CO₂ and CF₃I/N₂ gas mixtures to determine the 50% breakdown voltage. The breakdown characteristics of CF3I gas mixtures were examined for pressure, geometric ratio, impulse polarity, buffer gas and mixture content. Based on the measured breakdown voltage and calculated critical reduced field strength of various CF3I gas mixtures, a two-stage streamer/leader mathematical model was developed to evaluate the reduction in field strength at higher pressures. A comparative study was carried out on CF3I gas mixtures in a rod-plane electrode configuration under standard lightning impulse and steep-front square impulse waveforms. This investigation focused on the V-t characteristics of CF3I gas mixtures in this particular configuration. A phase equilibrium experiment was also carried out to determine the boiling point of various CF3I gas mixtures.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:669179 |
| Date | January 2015 |
| Creators | Chen, Lujia |
| Publisher | Cardiff University |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://orca.cf.ac.uk/80744/ |
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