By Allen Austin, ABB Inc.
Sulfur hexafluoride, a human-engineered gas known as SF6, is one of the most inert substances known, which makes it incredibly good at insulating electrical components from one another. It’s also the world’s most potent greenhouse gas with a global warming potential (GWP) 23,500 times that of CO2. For that reason, the utility industry has been searching for an alternative to SF6 for nearly 20 years.
While SF6 leakage is low, (around 0.1% in new equipment, as high as 3% in old equipment) once in the atmosphere, it has a lifespan of 3,200 years. Since 2002, the SF6 concentration in the atmosphere has doubled.
ABB has been committed to exploring design opportunities to exclude the use of SF6 in compact gas-insulated switchgear for secondary distribution since 2002 when the electrical energy industry in Norway signed an agreement with the Norwegian Minister of Environment with the intent to reduce SF6 emissions by 50%.
Looking for an alternative
When designing gas-insulated switchgear (GIS) with SF6 alternatives, one important task is to compensate for the lower insulation performance and ensure no flashovers occur during operation and dielectric tests. But SF6 can also serve as switching medium, today frequently applied in ring main units (RMUs).
Up to now facing the ‘switching challenge’ has been circumvented by using vacuum interrupters, where switching happens outside the insulation gas. The use of vacuum interrupters facilitated the first generation of eco-efficient RMUs, balancing the poorer arc-quenching performance of climate-friendly insulation gases.
However, vacuum technology comes with downsides compared to load break switch (LBS) designs with puffer-interrupters and knife-type switches used in SF6 today. Vacuum interrupters are more costly, larger in size, and require an additional disconnector/earthing switch along with changes to the operation scheme. With puffer-interrupters optimized for the use with eco-efficient gases, the downsides of vacuum switching can be overcome.
In 2013, ABB introduced a solution for 12 kV needing only dry air to meet the IEC requirements. The company followed up with the low-GWP gas mixture, AirPlus, which was suitable for use in RMUs with rated voltage higher than 12 kV.
AirPlus combines dry air with up to 15 percent 3M’s Novec insulating gas and reduces global warming potential by almost 100 percent compared to SF6. AirPlus equipment has similar dielectric insulation benefits, footprint, and reliability as SF6 switchgear and is well suited for use across the whole medium-voltage (MV) range up to 40 kV, both for primary and secondary distribution switchgear and ring main units (RMU).
Some of the main technical challenges with replacing SF6 include reduced dielectric strength, thermal conductivity, and arc-quenching properties of suitable alternatives. To ensure that degradation of the gas is negligible and does not affect switchgear performance, ABB evaluated the long-term chemical stability and compatibility of AirPlus with switchgear materials.
The Liander test
In November 2015, ABB conducted a joint field experiment with Dutch utility Liander to monitor SF6-free switchgear under normal service conditions in a windfarm network. Four SafeRing AirPlus switchgear units were installed at wind turbines in Liander’s MV grid–two equipped with data logging equipment and a quick connect valve for collecting gas samples during site visits. After three years, the two unmonitored units were returned to ABB for analysis including visual inspection of internal components, mechanical operation tests (3,000 operations), dielectric testing, and IR thermography.
Data collected from the monitored units as well as testing on the unmonitored units showed AirPlus performs well. There were no signs of wear during visual inspection, no evidence of material degradation of internal components, and no indications of thermal abnormality.
Taking the research to market
Since 2015, ABB has sold more than 5,000 dry air and AirPlus switchgear units. ABB developed a low-pressure design for maximized safety and highest reliability with an SF6 alternative. In case of a leakage and sudden loss of overpressure, operating personnel stay safe. ABB’s ecoGIS AirPlus units with low-pressure design can still handle operational voltage ratings avoiding prolonged outages, the switchgear can remain in service until the problem is identified and fixed. ABB GIS solutions are sealed for life and do not require on-site gas handling, additional trainings, or certifications for facility staff.
As the transition to renewable energy continues, more attention is being paid to supporting systems and equipment. Despite its outstanding performance as an insulating medium, SF6 gas carries an outsized environmental cost. Utilities are already looking for suitable alternatives and it seems likely that regulation will eventually force a shift away from the use of SF6.
In light of this reality, and to expedite the transition away from SF6, ABB and 3M elected to open the basic technology patents for the gas mixture to all manufacturers. ABB is also easing the transition with PrimeGear ZX0, the first switchgear compatible with both SF6 and AirPlus. In this way, the company hopes to provide a clear roadmap for equipment manufacturers and their utility customers to work towards a sustainable future.