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In 2014 at CIGRE in Paris, Utilities and T&D experts discovered g3 (g cubed), an alternative SF6-free solution to replace SF6 in high voltage equipment. Since then, many grid operators across the world who are concerned about the environmental impact of their grid, have implemented diverse g3 equipment. The filling of g3 into gas-insulated equipment is part of a fully-mastered handling process, like SF6. The g3 offering is delivered as a ready-to-use cylinder together with a dedicated, fully-automated service cart. Overall, a very simple process.
g3 is a mixture compound of three gases, said fluoronitrile ((CF3)2-CF-CN), carbone dioxide (CO2) and oxygen (O2).
Each component has its own purpose.
Though it is a gas mixture, in its gaseous form, it behaves exactly like a single gas, similar to how SF6 behaves.
Picture 1: Fluorinated Nitrile molecule used in the g3-gas mixture
Picture 2: g3 cylinders delivered at customer site in the UK
Picture 3: g3 cylinder being warmed-up using DILO's g3 dedicated service cart
g3 is ideally delivered in cylinders in its liquid form. To reach its homogeneous gas state, the liquid mixture must be warmed up so that it reaches a so-called ‘super-critical’ phase. What a terrific word for a simple thermodynamic process that was discovered in 1822 by Baron Charles Cagniard de la Tour, a French engineer and physicist(1).
This thermodynamic process has been applied in many industries. Supercritical CO2, for instance, is becoming an important commercial and industrial solvent due to its role in chemical chemistry in addition to its low toxicity and environmental impact. It is used by coffee manufacturers looking to move away from classic decaffeinating solvent.
So, how does it work? When warmed, a liquid-vapor equilibrium does not continue indefinitely. At specific temperature and pressure it terminates at a point on the phase diagram called the critical point, where the liquid and gaseous phases become indistinguishable, in what is known as a supercritical fluid. At this state, the mixture occupies the entire physical volume and behaves like a single gas having the density of the liquid. The transition to this homogeneous state is done by heating a liquid mixture under pressure. The temperature and pressure to reach will depend on the composition of the mixture.
As an example, the temperature to achieve a homogeneous state of a g3 mixture consisting of 6% vol. of C4F7N, 5% vol. of oxygen and the complement of CO2 is 42 °C.
Picture 4: g3 gas mixture in its liquid form as delivered to the customers in cylinders
Picture 5: g3homogeneous liquid-gas mixture obtained above supercritical phase
(1) Source: Wikipedia
Special gas-handling carts have been developped in order to bring the g3 gas to the correct conditions described in previous section, before transfering it to the high voltage equipment.
The correct conditions to have a homogeneous mixture as described in the previous section are characterized by the temperature, the pressure and the density in the cylinder. As these three characteristics are linked together, managing a successful filling can be done by controlling only two of these three parameters, in our case pressure and density.
The detailed physical characteristics of g3 linking the three parameters are known. To bring the fluid to supercritical state, the gas handling cart heats the g3 cylinder with a heating belt. The cart can, therefore, manage the filling of the high voltage equipment by controlling only the g3 pressure and density inside the cylinder. Knowing the cylinder inner volume, the density is obtained by controlling continuously the cylinder weight with a weighing scale. By doing so, it is helping to ensure that the fluid is kept in the right domain at any time, assuring a homogeneous fluid before it is extracted from the cylinder.
The cart has been developed with a fully automated warming process that regulates the temperature at the right level during the entire filling process. In actuality, the entire g3 handling cart is automated, thus preventing human errors and creating a more comfortable experience.
This principle of homogenization is transposable and applicable to higher volume containers, such as the already available 500-liter containers used for SF6 or even larger containers in the future.
For smooth adoption, the g3 gas cart needed to operate outdoor down to -20°C and allow similar HV compartment filling time durations as with SF6. By ‘operate’, we mean it ‘shall fill the g3 gas mixture at the constant specified concentration, at any time’. The g3 handling cart was configured to help meet all of these requirements.
Frédéric Loray, Industrial Markets and Design Manager at Air Liquide, explains, “we chose the components, established the architecture and integrated functionalities based both on my previous experience in HV substations and on Air Liquide’s experience in specialty gas handling systems. The cart had to be sturdy to be used on construction sites under all weather conditions, easy to move and to use, and especially safe.” g3 handling carts were already used on several g3 projects, such as Sellindge in England, Kilmarnock in Scotland, Grimaud in France, ... and demonstrated that they work reliably even under adverse weather conditions. Frédéric Loray notes “the time to heat a full cylinder takes from 10 minutes in warm ambient conditions to 35 minutes at -10°C, then the filling can be performed at a steady pace. A good productivity level was reached. At the end of these projects, the transferred gases were always reached as required: no issues were detected on the quality of the g3 transferred.”
Picture 6: Air Liquide / Liquid to Gas cart (L2G)
Picture 7: Air Liquide / Cart for Gas Tank (C4GT)
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