Lifespan and Environmental Impact of SF6 Gas
SF6 can stay in the atmosphere up to 3200 years. So, the atmospheric life of SF6 gas is immense. You cannot see it, you cannot feel it, you cannot smell it. The global warming potential is currently 24,000 times more than \(CO_2\). What do I mean by global warming potential? Let us take 10 grams of \(SF_6\) and convert it into \(CO_2\) equivalent which is 22,800 times. So, 10 grams of \(SF_6\) is equivalent to 228 kilograms of \(CO_2\) into the atmosphere. That’s what it is termed as \(CO_2\) equivalent.
Now, if I take a very normal vehicle and if that vehicle emits 228 kg of \(CO_2\), and I divide it with the emission rate, it means that if this vehicle travels for 1520 kilometers, then it will emit \(CO_2\) equivalent to just 10 grams of \(SF_6\). That is the impact of \(SF_6\) emission. It is almost equal to traveling from Delhi to Mumbai. That is the impact of a 10-gram SF6 leakage.
Also, when you produce \(SF_6\) gas, there is already a 3 to 8% emission that occurs during the manufacturing process. Additionally, a lot of heat is involved because the formation of \(SF_6\) involves an exothermic reaction. This heat generation itself leads to significant \(CO_2\) emissions.
Eco Friendly Alternatives to SF6 Gas
Alternative gas solutions have started emerging. There are basically two types of alternative gases available now. One of them is called C4, specifically C4-FN, and another is compressed air. C4-FN is commonly referred to as \(G^3\) (g-cubed) by GE, and by Hitachi, it is marketed under a different name. The only difference between them lies in the naming and the mixing ratios or compositions used. The base gas remains the same C4-FN and it is mixed with \(CO_2\) and \(O_2\).
Advantage of using SF6 Alternatives
Comparing Global Warming Potential of \(SF_6\) and C4-FN Gases
The major achievement of these alternatives is a significantly lower global warming potential (GWP). For instance, C4-FN mixtures have a GWP of less than 716, whereas SF6 gas has a GWP of 25,200 times that of CO₂. Siemens, on the other hand, claims to be using compressed air with 20% oxygen and nitrogen, which has a GWP of zero.
Challenges in using SF6 Alternatives
So, unlike \(SF_6\), which is a pure and homogeneous gas, these alternatives are mixtures of gases. The performance of these mixtures depends on the mixing ratio, temperature, and pressure, and as a result, the entire internal design of the breaker chamber must be adapted accordingly.
Various OEMs are now using different gas mixtures and alternative gases across both medium voltage and high voltage levels. You can see different vendors like GE, Hitachi, Siemens, and others using varied gas mixture ratios in their products.
Earlier, SF6 bottles had three phases: a liquid phase, a mid-phase (liquid + gas), and a gaseous phase. It was a homogeneous gas, easy to handle. But with these new gas mixtures, things have changed. Every time you transfer gas from one bottle to another or from a bottle to GIS (Gas Insulated Switchgear), you must ensure that the mixing ratio remains the same throughout. Otherwise, the gas properties can vary, affecting performance and safety.
You cannot use the same \(SF_6\) service cart for handling alternative gases. This is because the alternative gases require an induction system inside the cart to change the temperature of the gas and enable proper mixing. This temperature control is essential to ensure that the gas components blend correctly during handling and filling.
Here also, the problem remains the same – moisture is a villain. Moisture affects the performance of the gas, and now, with compressed gases, you also have to consider the concentration of NOx and SOx (nitrogen oxides and sulfur oxides). These contaminants become relevant again when we use these types of alternative gases. Proper control of moisture and gas purity is therefore critical.