Experimental investigation of CF3I-CO2 gas mixtures on the breakdown characteristics in uniform and nonuniform field configurations

This thesis is concerned with the investigation of trifluoroiodomethane (CF3I) gas mixtures as an alternative for an insulation medium in high voltage applications. The work has involved a broad review of literature, followed by developing a test rig for carrying out experimental investigations, extensive computational modelling and simulation studies as well as extensive laboratory tests on CF3I gas and its gas mixtures. The literature survey reviewed the current trend of efforts taken by researchers to find solutions for minimizing the usage of sulphur hexafluoride (SF6) as a gas insulator, focusing on CF3I and its mixtures. The physical properties of CF3I are investigated, along with thermal and electrical properties. A new test rig has been designed and constructed specifically to be used for gas insulation research. The test rig is integrated with wireless temperature and humidity sensors, as well as an electrode gap length control system. The test rig is completed with a gas recovery system to ensure proper gas handling is carried out after each test. Extensive laboratory experimental investigations on CF3I mixtures have been completed, focusing on the mixture of CF3I-CO2 gas with a ratio of 30%-70%. Standard lightning impulse of 1.2/50 has been used, with both positive and negative polarity. The effects of electrode configuration, impulse polarity, electrode gap length, gas pressure, and CF3I content have been investigated. Insulation properties such as 50% breakdown voltage (U50) and V-t characteristics for each test condition are investigated and presented, as well as the electric field behaviour. Finite element method (FEM) has been used to determine the electric field behaviour of a given test condition. This study revealed that CF3I gas mixtures perform better under more uniform field condition. It was also found that an increase in gas pressure will increase the insulation strength and an increase in CF3I content is more likely to give benefit in conditions with a more uniform field when compared to less uniform field conditions. Also, relation between liquefaction temperatures of a CF3I-CO2 mixture with varying CF3I content has been developed for various pressures based on literature. Observations on solid by-products of CF3I have also been carried out. It has been found that iodine particles are deposited on both high voltage and ground electrodes, which can affect the insulation properties of CF3I and its mixtures