Retro-Filling Existing SF6 Insulated High Voltage Equipment with Alternative Gases: National Grid UK & Manchester University Perspective

Application of Alternative Gases in Retro-Filling Existing SF6 Insulated High Voltage Equipment: National Grid UK & Manchester University Perspective

The most prominent current strategy to reduce SF6 emissions from the power industry is to upgrade and replace SF6-filled equipment in the network with state-of-the-art equipment specifically designed for suitable SF6 alternatives (g3 of GE, DryAir of Siemens and AirPlus of HitachABB etc). Replacing or modernizing all or substantial portion of existing SF6-filled assets is costly and time consuming. So, investigating the feasibility of retro-filling existing SF6-filled equipment with alternative gases makes perfect sense.

In 2016, National Grid UK and Manchester University came together with a research project to explore “Application of Alternative Gases In Retro-Filling Existing SF6 Insulated High Voltage Equipment”. Recently they concluded the project and published the outcomes in IEEE. The key research steps and findings are:

a) The selection process of suitable gas candidate based on Dielectric Strength, Global Warming Potential (GWP), Toxicity and Boiling Point at 4.5 bar (abs). Finally they selected a mixture of C3F7CN and CO2 as most suitable alternative for SF6

b) Development of a reduced scale coaxial prototype that mimics the electric field found in a full-scale GIL/GIB

c) Breakdown characteristics obtained using the coaxial prototype to determine a C3F7CN/CO2 gas mixture with comparable dielectric performance to SF6

d) Assembly of a unique demonstrator made of a gas-insulated bushing and bus bars rated at 420/550 kV for experimental validation

e) Type test results of the demonstrator using SF6 and a pre-determined C3F7CN gas mixture. Type tests were conducted as per IEC 62271-204

f) Material compatibility investigation of C3F7CN with a common gasket material (EPDM) used within the gas insulated equipment. There was no degradation to the EPDM elastomer gasket or to the C3F7CN when they were exposed to elevated temperatures over an extended period of time.

N.B. The contents of this article are extracted from the paper by Loizos Loizou, Lujia Chen, Qiang Liu, Ian Cotton, Mark Waldron, John G. Owens, "Technical Viability of Retro-Filling C3F7CN/CO2 Gas Mixtures in SF6-Designed Gas Insulated Lines and Busbars at Transmission Voltages", IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 35, NO. 5, OCTOBER 2020