Compatibility of C4F7N/CO2 and Epoxy & Optical PD characteristics of the Gas

MSc thesis of Uwai Abdulla Arman

SF6 has been the primary dielectric medium in electrical power applications for many years, notably in high voltage gas-insulated equipment. Governments have pursued emission reductions from Gas insulated equipment, recognizing that SF6 has an extremely long atmospheric lifetime and significant global warming potential. The electrical power industry has responded to this environmental concern by incorporating SF6-free technologies to an expanding range of applications that formerly used SF6, such as in gas-insulated switchgear and gas-insulated lines or bus bars.

Because of its lower impact on global warming and excellent dielectric strength, fluorinated nitrile (C4F7N)/CO2 mixture is a potential SF6-alternative gas in HV Gas-Insulated Systems. Laboratory accelerated thermal ageing experiments on material compatibility were performed to assure long-term physical and chemical stability at the interface between C4F7N/CO2 gas mixture and epoxy resin with different fillers. As there are no standards for accelerated ageing for this novel Gas, the ageing test is performed based on the recommendations of CIGRE. The compatibility of Gas and solid insulating materials in contact was assessed based on the changes in insulation performance after ageing. Before and after ageing experiments, such as AC and LI flashover tests, visual inspection, FTIR of epoxy samples and breakdown strength of the Gas were analysed and compared.

The optical detection of partial discharge (PD) is a critical method for determining the insulating quality of equipment. Moreover, the PD of different gases correspond to distinct multispectral characteristic distributions, but the current investigation is limited to SF6. As a result, this research offers a solution to identify the optical PD spectrum of the novel alternative gas C4F7N/CO2. Based on the finding, the research is extended to use it in fluorescent fibres for PD detection in Gas Insulated Systems. The selection of fluorescent fibres and sensors is analysed, and tests are performed to produce PRPD patterns from the optical sensor output. Investigations on the Correlation between electrical and optical PD detection methods were also performed.

Finally, the Industrial perspective on eliminating SF6 is presented based on the author's work at Prysmian. The different HV cable network components that use SF6 for testing, its test voltages and waveforms are identified and based on the electrical field calculations, suitable alternatives are proposed. Functionality tests with synthetic air in real network components are carried out to validate the calculations. Further, an experimental study for using compressed air in network components testing is carried out.