Sulfur hexafluoride gas has good insulation and arc extinguishing properties and is widely used in transmission and substation equipment such as circuit breakers, gas-insulated metal-enclosed switchgear and gas-insulated metal-enclosed transmission lines, but the global warming potential (GWP) value of sulfur hexafluoride is the highest among known gases and has a strong greenhouse effect. Reducing, limiting or even banning the use of sulfur hexafluoride gas in power systems is an inevitable trend in the development of power grid equipment, and seeking environmentally friendly and high-performing sulfur hexafluoride gas replacement technology is an important research direction and hot issue in this field.
On the whole, after years of research and exploration, some consensus technologies and directions have been basically formed in the exploration and application fields of sulfur hexafluoride gas replacement technology in recent years.
In the medium-voltage field, conventional gases such as nitrogen and dry air are generally used as insulation media, and vacuum circuit breakers are used to open and close the circuit. In equipment with higher rated voltage such as 40.5 kV, a small amount of high insulation strength environmental gas is also added to dry air or nitrogen to achieve the purpose of improving its insulation strength and reducing the volume of equipment.
In the high-voltage field, since conventional gases such as nitrogen and dry air have lower insulation properties and cannot achieve sufficient insulation strength with limited equipment volume and inflation pressure, the main research and application solution is to use environmentally friendly gases with high insulation strength, however, since most of these gases have higher liquefaction temperatures, they need to be mixed with buffer gases with lower liquefaction temperatures.
In terms of the development and application of environmentally friendly power equipment, there are mature products with many years of operating experience at home and abroad in the medium-voltage field, compared to relatively few products in the high-voltage field. However, the research progress still shows the research direction for this field.
The insulation performance of sulfur hexafluoride gas is extremely strong, about three times that of air at the same pressure, so the traditional medium-voltage gas-insulated switchgear with sulfur hexafluoride gas as the insulation medium is small and compact. In order to achieve the goal of “equivalent substitution”.
At present, the medium-voltage power equipment industry is actively studying and applying environmentally friendly insulation alternative technology mainly includes two ways, i.e. using environmentally friendly gas insulation or solid insulation. The existing environmentally friendly insulation gases applied in medium-voltage power switchgear mainly include nitrogen, dry air, perfluoromethylisopropyl ketone insulation gas mixture or heptafluorobutyronitrile gas mixture.
According to the physicochemical and electrical properties of the gases, the advantages and disadvantages of each gas applied as insulation medium can be determined, mainly including
(1) Carbon dioxide, nitrogen and dry air are conventional gases in nature with low cost and no environmental impact, but the insulation performance of these gases is lower than that of sulfur hexafluoride gas, so the application in medium-voltage power equipment requires optimization of electric field distribution and adoption of technologies such as gas-solid composite insulation.
(2) The insulation performance of perfluorinated ketones is more than twice that of sulfur hexafluoride gases, and there is no greenhouse effect, but their liquefaction temperature is very high, and they can only be used in indoor conditions with buffer gases with low liquefaction temperature.
(3) Isobutyronitrile heptafluoride gas has more than twice the insulating property of sulfur hexafluoride gas, has a higher liquefaction temperature but is lower than perfluorone gas, and can be used in general scenarios outdoors when mixed with buffer gas with low liquefaction temperature, but its global warming potential is higher than that of perfluorone gas.
(4) Trifluoroiodomethane gas has slightly higher insulation performance than sulfur hexafluoride gas and has no greenhouse effect problem, but its liquefaction temperature and cost are higher and cannot be used alone. It can be used in general scenarios outdoors when mixed with buffer gas with low liquefaction temperature, but it will lead to a decrease in insulation performance.
(5) Octafluorocyclobutane gas has slightly higher insulation performance than sulfur hexafluoride gas, but its liquefaction temperature is higher and cannot be used alone. It can be applied to general scenes outdoors when mixed with buffer gas with low liquefaction temperature, but the insulation performance of the gas mixture is significantly reduced and its global warming potential is higher.