Application of C3F7CN/CO2 Gas Mixtures for Retro-filling SF6-designed Gas Insulated Equipment Rated at Transmission Voltages

Abstract

The most significant drivers behind climate change are the greenhouse gases mainly caused by human activities. Environmental agreements, such as the Kyoto Protocol and the F-gas legislation, have been put in place to control the emission of greenhouse gases. Sulphur hexafluoride (SF6), the most potent man-made greenhouse gas in existence, is widely used in the power industry as a dielectric medium in gas insulated equipment. Hence, the power industry has been looking for replacements to phase out the use of SF6 in the power equipment. This thesis investigates the possibility of replacing SF6 in existing gas insulated lines (GILs) and busbars (GIBs) within the power network with the more environmentally friendly C3F7CN/CO2 gas mixtures. Two types of electrical characterisations were carried out in this study, namely breakdown voltage and partial discharge (PD) tests. Coaxial and hemispherical rod-plane electrode configurations, with electric fields as found in practical GIL/GIB equipment, were used for AC and lightning impulse (LI) breakdown tests. For the PD tests under AC voltage, needles were attached to the high voltage and grounded electrode of plane-plane and hemispherical rod-plane configurations to mimic protrusion defects that can occur in practical GIL/GIB equipment. SF6 was tested as a benchmark and compared to the performance of C3F7CN/CO2 gas mixtures. Finally, a full-scale, 420/550 kV rated GIB demonstrator, filled with SF6 first and then with the 20% C3F7CN / 80% CO2 gas mixture, was subjected to type tests of various voltage waveforms in accordance to IEC standards. The results showed that the 20% C3F7CN / 80% CO2 gas mixture has comparable LI and AC breakdown performance to SF6 under quasi-uniform fields. However, as the fields become more non-uniform, the 20% C3F7CN / 80% CO2 gas mixture has lower LI breakdown voltages than SF6 especially under positive polarity. The PD tests showed that the 20% C3F7CN / 80% CO2 gas mixture has a poorer performance than SF6 under highly divergent fields but can exceed the inception and extinction values of SF6 when more uniform fields are used. The type tests using the full scale GIB demonstrator showed that the 20% C3F7CN / 80% CO2 gas mixture has passed all the required voltage levels as SF6. This could lead to at least 190 t of SF6 being replaced with the 20% C3F7CN / 80% CO2 gas mixture in the UK power network, where a reduced GWP can result to the CO2 equivalent emissions being decreased by 95% of the current annual leakages. The findings in this thesis are an encouraging step towards a technically viable SF6-free retro-fill solution for existing GIL/GIB installed for the 400 kV transmission network in the UK.

Date of Award	28 Sept 2020
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Qiang Liu (Main Supervisor) & Tony Chen (Co Supervisor)