For many years, sulphur hexafluoride – more commonly known as SF6 – has been widely used as a switching and insulating medium in MV and HV switchgear. However, it’s now clear that SF6 has many drawbacks, including an unacceptably high environmental impact.

Since viable alternatives are now available in most applications, it must surely be time to think again about the wisdom of using SF6, says Alan Birks of Eaton’s Electrical Sector.

It’s not difficult to see why SF6 was, in the past, an attractive choice for quenching switching arcs and to providing insulation in medium- and high-voltage switchgear. It is, in most circumstances, chemically inert and in itself it is non-toxic. It’s also non-flammable, thermally stable and relatively inexpensive. Furthermore, it has excellent electrical properties. It is a good insulating agent and its short dielectric recovery time means that it’s good at quenching the arcs that are produced during switching operations.

What is the Problem with SF6?
In addition, when SF6 first came into use in switchgear applications in the mid-1970s, it allowed the development of equipment that was physically much smaller than the air- and oil-insulated equipment that was being used at the time. In short, when SF6 was first adopted for electrical applications, it seemed almost too good to be true. Unfortunately it was.

What are the problems with SF6? The first and probably the most important is that it is a greenhouse gas with a very high global warming potential. It is well known that the high level of carbon dioxide (CO2) emissions that the industrialised world is currently producing is leading to the so-called “greenhouse effect”, where excessive levels of CO2 in the atmosphere are trapping heat from the sun leading to global warming and climate change. CO2 is, however, not the only gas involved. Other gases that are being released into our atmosphere also contribute to the greenhouse effect, and many of these are more potent at trapping the sun’s heat than CO2 itself.

The ratio of the effectiveness of a particular gas in trapping the sun’s heat to the effectiveness of CO2 is designated the global warming potential (GWP) of the gas. For example, a gas with a GWP of 100 has 100 times the potential of CO2 to produce global warming. SF6 has an enormous GWP of 23,900, which has led to it being listed by the International Panel on Climate Change (IPCC) as the most potent greenhouse gas.

And its gets worse – SF6 has an atmospheric lifetime of up to 3,200 years, so gas released today is going to affect the climate for a very long time into the future. In order to control these problems, SF6 has been put on the Kyoto list of substances, the use and emission of which must be minimised. In fact, the use of SF6 is now prohibited in most of its former applications, which included sports shoes, tennis balls, car tyres and double-glazing.

It is still permissible, however, to use SF6 in medium-voltage (up to 52 kV) and high-voltage (above 52 kV) switchgear. As a result, 80% of the SF6 produced in the world today is destined for electrical applications and most, if not all, of this will ultimately find its way into the atmosphere.

Legislation and SF6
It can be confidently expected, therefore, that legislation will ultimately be introduced controlling the use of SF6 in switchgear. In fact, 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 both expensive and difficult.

At this point, it is worth mentioning that its very poor environmental characteristics are not the only shortcomings associated with SF6 – its use also gives rise to potential health and safety issues. While SF6 itself is usually considered to be harmless in the concentrations in which it is normally encountered, 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 all 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 will be released into the surrounding area.

There are Practical Alternatives
We have established that there is a very 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 very 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 any practical alternatives?

In answering this question, it’s necessary to make a very clear distinction between HV and MV switchgear. When it comes to HV switchgear that operates above 52 kV, there are, at present, not very many viable alternatives to SF6 in its insulating 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 (the type of equipment most likely to be of interest to electrical contractors) 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 in MV switchgear 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.

Reduced Risk
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 between the phases and between the phases and earth is distributed in a manner that is far from uniform. This means that 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 for the overall assembly.

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 using arc absorbers. These not only guide the gasses and smoke produced by the arc out of the panel, they also have a large absorbing surface that significantly 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 to ensure its continued reliable and safe operation; 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.

The Benefits fo Solidinsulated Switchgear
It is now clear that 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. Electrical contractors and their customers 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 minded 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 that the relevant regulations will soon be changed to reflect this development.

In short, SF6 is definitely on the way out, so don’t wait to be pushed, switch now to modern green technology!

The Author:
Alan Birks is commercial and marketing manager for Eaton’s Electrical Sector in the UK, with responsibilities that cover power distribution, industrial automation and industrial control as well as systems and solutions. He started his career in the electrical industry in the 1970s and, in 1997, he joined MEM 250, which subsequently became part of Eaton, as Sales & Marketing Director. Since then he has held a number of senior sales and marketing posts in the organisation, covering both the UK and Europe.

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