In the 1970s when SF6 (sulphur hexafluoride) was first used in MV switchgear, it seemed to be an almost ideal insulating and switching medium. Since then, the environmental and other hazards associated with SF6 have become increasingly apparent, leading to a shift towards alternative types of switchgear that eliminate its use with no cost or performance penalty. This means, says Alan Birks of Eaton’s Electrical Sector, that SF6 is now yesterday’s technology
When the search was on in the 1960s to find a viable alternative to the potentially flammable, always messy and sometimes carcinogenic oils used in the MV switchgear of the era, SF6 must have seemed like a godsend. It combines excellent electrical properties with chemical stability and low toxicity. It’s non-flammable and low in cost. Unsurprisingly, these very desirable characteristics lead to its widespread and enthusiastic adoption in MV switchgear.
Unfortunately the picture was not quite as rosy as it at first appeared. In particular, as concerns about the environment and, in particular, global warming started to grow, it became all too clear that SF6 had significant potential for causing environmental damage.
Global warming is the consequence of the greenhouse effect and this is usually associated with elevated levels of CO2 (carbon dioxide) in the atmosphere, which trap more of the sun’s heat. CO2 is not, however, the only culprit; there are many gases that are much more potent in trapping heat than CO2 and, unfortunately SF6 is one of them. In fact, SF6 is currently listed by the International Panel on Climate Change (IPCC) as the most potent greenhouse gas, with a global warming potential 23,900 times that of CO2. That’s not all – SF6 has an atmospheric lifetime of up to 3,200 years, so gas released today will affect the climate for a very long time.
Clearly the release of SF6 into the atmosphere – which is virtually impossible to avoid when the gas is used, no matter how carefully it is handled – is highly undesirable. As a result, SF6 is on the Kyoto list of substances, the use and emission of which must be minimised. In fact, SF6 is now banned in most of applications, but it is still permitted in medium-voltage (up to 52 kV) and high-voltage (above 52 kV) switchgear. As a consequence 80% of the SF6 produced in the world today is destined for electrical applications.
It can be confidently expected legislation will ultimately be introduced controlling the use of SF6 in switchgear. Some measures are already in place, including the voluntary programme of the Environmental Protection Agency in the USA and the F-gas regulations that were introduced in Europe in 2007. These legislative changes are already increasing the cost of maintaining switchgear that uses SF6 as well as starting to make its end-of-life disposal expensive and difficult.
It is worth mentioning poor environmental characteristics are not the only shortcoming of SF6 – its use also gives rise to potential health and safety issues. While SF6 itself is usually considered to be harmless in normal concentrations, the derivatives that are inevitably formed by the arcs created during switching operations are another matter entirely.
These by-products, which include HF, SOF2, SF4 and S2F10, are toxic. Granted they are produced in relatively small quantities during the normal operation of the switchgear, but they are likely to be present when switchgear is dismantled for maintenance or at the end of its life. Further, should a fault occur that causes an explosion in the switchgear, these toxic by-products are released into the surrounding area.
We have established there is a strong case for avoiding the use of SF6 switchgear for new installations. Not only is it harmful to the environment, it is also likely to have a high lifetime cost, as the inevitable legislative changes make the maintenance and disposal of equipment that uses SF6 more and more expensive. But are there practical alternatives?
In answering this question, it’s necessary to distinguish between HV and MV switchgear. When it comes to HV switchgear that operates above 52 kV, there are, at present, few viable alternatives to SF6 in its switching role. However, development is proceeding rapidly in this field and this situation can be expected to change in the not too distant future.
However, for switchgear operating at below 52 kV, it’s a completely different story. Practical and affordable alternatives are readily available that make the use of SF6 completely unnecessary. The best of this new generation of SF6-free MV switchgear is based on vacuum interrupter technology used in conjunction with solid insulation.
In addition to their almost negligible environmental impact, vacuum interrupters have many other characteristics to recommend them. Because of the way arcs behave in a vacuum – they constantly move from point to point on the electrodes rather than establishing themselves at a single location, and they are always extinguished at the first current zero – contact erosion in vacuum switching elements is almost non-existent. This has two important consequences. The first is that the switching elements require no maintenance, and the second is that they have very long working lives. The latest types are, for example, certified for 30,000 operating cycles.
Modern vacuum interrupters are ideally complemented by solid insulation produced using cast epoxy resin technology. This approach allows the parts to be shaped specifically for the best possible insulation performance, with components such as busbars and vacuum interrupters integrated directly into the mouldings.
The use of solid insulation also allows excellent control over electric fields in the switchgear. With conventional shapes for the primary components like busbars and other conductors in MV switchgear, the electric field is distributed in a manner that is far from uniform. This means there are areas with high field concentrations and, in these areas, there is risk of partial breakthrough. This can trigger avalanches leading to flashovers.
With solid insulation, however, engineers with experience of breakthrough phenomena and field-steering techniques can arrange for the components and insulation used in the switchgear to be shaped in such a way that flashovers are eliminated entirely, while still achieving a very compact design.
While the risk of internal arcs is very small with solid-insulated switchgear, it is impossible to say, as with any kind of switchgear, that there is no risk at all. However, solid-insulated switchgear has the additional important benefit that careful design can ensure that, if an internal arc event does occur, its environmental impact is minimised. This can be achieved by adopting single-pole construction, which means that the only conceivable type of internal fault is a single-phase short circuit, rather than a potentially more damaging phase-to-phase short circuit.
In the best examples of solid-insulated switchgear, the impact of internal arc events is reduced still further by arc absorbers. These guide the gasses and smoke produced by the arc out of the panel and they also have a large absorbing surface that breaks up and filters the gases, greatly reducing their potential for causing damage and injury.
Further benefits of solid-insulated switchgear over its SF6 counterpart include elimination of the costly and inconvenient routine pressure checks that are always needed with SF6 equipment; and low end-of-life disposal costs. In fact, the newest types of solid-insulated switchgear have been designed specifically to make re-cycling of the components used in them straightforward and inexpensive.
It is now clear there is an alternative to SF6 switchgear in MV applications that not only eliminates the need to use this environmentally unfriendly gas, but also offers very significant benefits in its own right. Solid-insulated switchgear is safe, compact and very cost-effective, especially when lifecycle costs are considered. It offers dependable performance, it needs minimal maintenance and it has a very long service life. What possible reason can there be, therefore, for the continued use of MV SF6 switchgear?
In truth, there is no reason. Specifiers and users of MV equipment would be well advised, therefore, to avoid SF6 equipment for all new installations. In addition, end users may wish to consider the benefits of replacing their existing SF6 equipment sooner rather than later, before the regulatory regime relating to greenhouse gasses tightens still further and pushes the costs associated with dismantling and disposing of such equipment sky high.
A final thought for those who may be tempted to ignore this call to action – your option to do that may not last much longer! The use of SF6 in MV electrical equipment is still tolerated only because it is currently considered a special case, where there are no reasonable alternatives available. As we’ve seen, that’s no longer true, and it’s not hard to predict the relevant regulations will soon be changed to reflect this development.
In short, SF6 is yesterday’s technology; it’s served its purpose but now it’s obsolete. SF6 offers no technical or financial benefits – in fact quite the opposite – so let’s confine SF6 MV switchgear to the one place where it still belongs. And that, of course, is a museum!