Abstract
Sulphur hexafluoride (SF6), the most popular dielectric medium adopted in compressed gas insulated equipment, has been identified as a highly potent greenhouse gas. This has led to increased interest in finding a more environmentally friendly replacement candidate. In this paper, the technical viability of C3F7CN/CO2 gas mixtures was assessed as a potential retro-fill solution for existing SF6-filled gas insulated lines (GIL) and busbars (GIB). A reduced-scale coaxial prototype was developed to establish the breakdown strength of 20% C3F7CN / 80% CO2 and 16% C3F7CN / 84% CO2 gas mixtures in direct comparison with pure SF6 under the standard lightning impulse (1.2/50 μs). Breakdown results demonstrate that a mixture of 20% C3F7CN / 80% CO2 exhibits comparable insulation capability to pure SF6 in coaxial geometries with similar field uniformity to GIL/GIB. This initial finding has led to the construction of a full-scale GIB demonstrator rated for 420/550 kV. Type tests according to IEC 62271–204 showed that the 20% C3F7CN / 80% CO2 gas mixture has passed all the required voltage levels as SF6. The research findings in this paper 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.
Highlights
S ULPHUR hexafluoride (SF6) is a colorless, odorless, nonflammable and chemically inert gas which has been used in gas insulated equipment for decades owing to its exceptional arc quenching and dielectric insulation capabilities
This paper presents findings from a retro-fill investigation of C3F7CN/CO2 gas mixtures
These tests have effectively established that the 20% C3F7CN / 80% CO2 gas mixture can be retro-filled with a significant safety margin for the 400 kV transmission network
Summary
S ULPHUR hexafluoride (SF6) is a colorless, odorless, nonflammable and chemically inert gas which has been used in gas insulated equipment for decades owing to its exceptional arc quenching and dielectric insulation capabilities. One current strategy, which targets reducing SF6 emissions from the power industry, is upgrading and replacing SF6-filled equipment in the network with state-of-the-art equipment designed for suitable SF6 alternatives [2], [3]. While this is intended to reduce overall SF6 emissions, it is costly and replacing all existing SF6-filled assets worldwide with new-builds is time consuming. Most current research focuses on developing new high voltage equipment filled with alternative gases. The proposed C3F7CN gas mixture can provide most of the aforementioned properties that are required to replace SF6 in high voltage insulation applications [3]. Busbars rated at 420/550 kV for experimental validation; e) type test results of the demonstrator using SF6 and a pre-determined C3F7CN gas mixture; and f) material compatibility investigation of C3F7CN with a common gasket material used within the gasinsulated equipment
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