Nitrogen constitutes a natural part of air and is a non-global warming gas. There have been recent attempts to use it as a working medium in high voltage circuit breakers to replace SF6, which is an excellent insulating and arc quenching gas but also a strong greenhouse gas with a Global Warming Potential (GWP) of 23,900 and a life time of 3,200 years. To employ the full potential of nitrogen for use in high voltage circuit breakers, the properties and radiation characteristics of its mixture with Polytetrafluoroethylene (PTFE) vapour, an electrical and thermal insulation material, need to be known, forming the objectives of the present work. This research has investigated the capability of nitrogen and SF6 on insulating and quenching an electric arc in a high voltage circuit breaker. Depending on the current of the electric arc range from 100 A to 330 A, the arc can be extinguished in 25-50 ms with nitrogen gas flow, while it takes 20-45 ms with SF6. The peak voltage of the electric arc with SF6 is 10% higher than it is with nitrogen. According to the preliminary data obtained in the experimental investigation, it seems possible that SF6 can be replaced by nitrogen as the operation gas in high voltage circuit breakers. Research on the properties and radiation characteristics of a gas mixture of nitrogen and PTFE is required to allow modelling of a nitrogen filled circuit breaker. Net Emission Coefficient (NEC) describes the radiation characteristics of the hottest area in an electric arc. The NEC of a nitrogen and PTFE mixture is computed by using the most advanced method considering both continuum and line radiation. The result shows that the influence of PTFE on the NEC cannot be neglected. The influence of uncertainty in atomic data on the calculated NEC is studied. A 50% adjustment in the uncertainty of the atomic data on high energy levels results in a 0.001% variation of the NEC because of the low population density of the particles with high energy levels involved. Due to the lack of experimental results of the mixture, the validity of the model is confirmed by a good agreement between the results calculated and those obtained by experiments or those predicted by the other research groups for pure nitrogen gas and pure PTFE vapour. Results show that influence of PTFE vapour on the NEC of the mixture is substantial. It has been determined that the NEC of the mixture cannot be estimated by NEC of the pure gases multiplied by their proportions in the mixture. Re-absorption has to be considered when the temperature is lower than 25,000 K. To solve the Radiation Transfer Equation (RTE), the P1 and Discrete Ordinate Method (DOM) have been used to calculate the radiation with re-absorption. The P1 approximation is computationally cheaper but it has poor accuracy. The accuracy of DOM is determined by the number of calculation bands. It is obvious that the more bands used, the higher the accuracy. A comparison of the DOM 8-5 and 5-3 methods indicates a balanced compromise between accuracy and cost of calculation with DOM 8-5. A two-temperature model was applied to solve the properties of the plasma in non-LTE condition when the temperature gradient is sufficiently high. A number of existing calculation methods are discussed. A modification of Godin's method has been proposed to satisfy the two-temperature model. Results of a gas mixture composition, obtained by different methods, are analysed. The modified method leads to results that agree very well with those from the existing methods but with better convergence that when the Newton-Raphson algorithm is used to obtain the solution. Results with different molar percentages of PTFE vapour have been presented and discussed. The results for pure nitrogen and pure PTFE vapour have been compared with existing data to verify the validity of the method.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:724464 |
| Date | January 2016 |
| Creators | Yang, F. |
| Publisher | University of Liverpool |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://livrepository.liverpool.ac.uk/3004928/ |
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