Sulfur hexafluoride (SF6) is widely used as an electrical insulator and arc-quenching gas in power transmission and distribution equipment. However, since 1995, a steady increase in atmospheric SF6 concentrations has been observed, with emissions even rising. Furthermore, SF6 has a long atmospheric lifetime, making its climate effects almost irreversible. While the atmospheric lifetime of SF6 is difficult to estimate precisely, it is approximately 850 years. Therefore, the use of SF6 raises environmental concerns.
Despite these concerns, SF6 cannot be immediately banned in the power industry in the absence of alternative technologies. Instead, measures to reduce SF6 emissions have been implemented. However, with the emergence of more and more SF6-free alternatives for various applications, it is foreseeable that SF6 bans for specific applications in the power industry will be implemented in the near future.
Medium Voltage
Medium Voltage Switchgear is classified into primary and secondary switchgear. Primary switchgear, installed at the interface between high-voltage and medium-voltage networks, is primarily equipped with circuit breakers. Secondary switchgear, installed at the interface between medium-voltage and low-voltage networks, is typically equipped with load switches rather than circuit breakers. Traditionally, air-insulated switchgear (AIS) and gas-insulated switchgear (GIS) exist in medium-voltage networks; the latter is more compact and independent of external conditions (sealed with ambient air). Where space permits, AIS with vacuum circuit breakers (VCBs) can be selected. GIS alternatives for medium-voltage distribution can be found not only with alternative gases but also with solid (e.g.) and liquid insulation (e.g.) as well as VCBs. Some of these products have been on the market for over a decade. Regarding alternative gases, artificial air at elevated pressures up to 1.4 bar (up to 12 kV) and air with C5-PFK at 1.4 bar (up to 36 kV) are available as products with vacuum circuit breakers as primary equipment. In secondary switchgear, LBS functionality is now implemented as a simple contact system embedded in SF6, capable of interrupting the typically rated 630 A nominal current. This, without special assistance, could not have been achieved with non-SF6 gases until now, but can be addressed through vacuum interruption technology. For certain market segments of medium-voltage primary and secondary switchgear, SF6-free solutions have been developed and are offered as products. While extending some solutions to higher rated voltage or current levels is technically more difficult (e.g., the thermal cooling challenges of solid insulation), there is no technical reason why SF6-free solutions cannot be developed for all applications. Of course, offering new products in smaller market segments is an economic challenge, but this cannot serve as a solid argument for replacing SF6 in MV equipment. Users should be flexible in their substation layout and compare the benefits of purchasing SF6-free equipment with those of accepting only traditional layouts and configurations. Regulations can support the transition on an economic argument, as technology from decades ago is likely cheaper.
High Voltage
AIS in high voltage is (almost) exclusively for outdoor applications due to extreme differences in size requirements. "CO2"-based alternative circuit breakers are available on the market up to 145 kV and 40 kA. Using non-gas insulation and switching media in high-voltage switchgear is more challenging than in medium-voltage switchgear. Solid-insulated high-voltage cables exist, but extrusion processes cannot be used in high-voltage switchgear design. Paper-oil insulation is used in HV cables, power lines, and transformers, but is unlikely to be used in HV switchgear. Therefore, insulation is considered only in gas-insulated designs, and switching considers gas and vacuum technologies. The main market segment for HV GIS is voltage levels up to 145/170 kV, which is where major manufacturers are addressing it first. Several technical solutions are commercially available: compressed air with VCB, air combined with C5-PFK, and CO2/O2 combined with C4-PFN. For all these solutions, there are initial installations, but development continues. Development of C4-PFN-based solutions has been announced, reaching 420 kV by 2022. A two-step retrofit of 380 kV SF6 substations with C5-PFK-based solutions has been announced, with plans to make the substations completely SF6-free by 2026. There are no known technical reasons suggesting that it is theoretically impossible to develop SF6-free solutions for all high-voltage levels. Developing SF6-free substations is far more difficult than developing medium-voltage switchgear, but reliable regulations here can economically support and accelerate the transition to SF6-free substations. From the user's perspective, a rethinking is needed; not all manufacturers prefer a single optimal solution (unlike SF6). Depending on the weighting of different selection criteria (size, minimum ambient temperature, global warming potential, absence of F-gas, ease of gas handling, etc.), different technological solutions will be preferred, and they may even operate in parallel.
