Dielectric insulation media in liquid or gaseous state are conventionally applied for the insulation of an electrical active part in a wide variety of electrical apparatuses, such as switchgears or transformers.
In medium or high voltage metal-encapsulated switchgears, for example, the electrical active part is arranged in a gas-tight housing, which defines an insulating space, said insulation space comprising an insulation gas usually with several bar pressure and separating the housing from the electrical active part without letting electrical current to pass through. Thus, metal-encapsulated switchgears allow for a much more space-saving construction than switchgears which are mounted outdoors and insulated solely by ambient air. For interrupting the current in a high voltage switchgear, the insulating gas further functions as an arc extinction gas.
Conventionally used insulation gases with high insulation and switching performance have some environmental impact when released to the atmosphere. So far, the high global warming potential (GWP) of these insulation gases has been coped with by strict gas leakage control in gas-insulated apparatuses and by very careful gas handling. Conventional environmental-friendly insulation gases like e.g. dry air or CO2 have a quite low insulation performance, so that gas pressure and/or insulation distances would have to be increased.
For the reasons mentioned above, efforts have been made in the past to replace these conventional insulation gases by suitable substitutes.
For example, WO 2008/073790 discloses a dielectric gaseous compound which—among other characteristics—has a boiling point in the range between about −20° C. to about −273° C., which is low, preferably non-ozone depleting and which has a GWP less than about 22,200. Specifically, WO 2008/073790 discloses a number of different compounds which do not fall within a generic chemical definition.
Further, U.S. Pat. No. 4,175,048 relates to a gaseous insulator comprising a compound selected from the group of perfluorocyclohexene and hexafluoroazomethane, and EP-A-0670294 discloses the use of perfluoropropane as a dielectric gas.
EP-A-1933432 refers to trifluoroiodomethane (CF3I) and its use as an insulating gas in a gas-insulated switchgear. In this regard, the document mentions both the dielectric strength and the interrupting performance to be important requirements for an insulating gas. CF3I has according to EP-A-1933432 a GWP of 5 and is thus considered to cause relatively low environmental load. However, because of the relative high boiling point of CF3I (−22° C.) gas mixtures with CO2 are taught. Additionally, pure CF3I-gas has about the same insulation performance as conventional insulation media having a high insulation and switching performance, so that the proposed gas mixtures have around 80% of the specific insulation performance of a pure conventional insulation medium which would have to become compensated by increased filling pressure and/or larger insulation distance.
Therefore there is an ongoing need for an insulation medium which causes even less environmental load than CF3I and does not require an increase of the gas pressure and/or the insulation distances above today usual values.