g3-filling: a mastered process

g3 filling process capitalizes on decades of SF6 handling experience

10/29/2019 - 5.18 pm

Climate change
g3
g³
SF6-free solutions
SF₆

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 gas mixture, so what?

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.

The fluoronitrile provides dielectric strength to the gas mixture thanks to the nitrile triple bond combined with fluorine.
CO₂ handles the arc interruption process.
Oxygen plays a major role in the gas chemical decomposition especially in case of heavy arc interruption.

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

1_A cylinder of g3 please

On site handling of g³ gas can be done in three different ways:

1. delivered as three different pure products in separate packages; prepared & mixed on site
2. delivered on site as a pre-mixed gaseous mixture
3. delivered on site as a liquefied gas mixture prepared in a single package (cylinder or container).

The first way implies the handling, tools and supply chain management of three different products with their associated package types and suppliers.

The second method compared to the third requires 10 times more packages of the same size.

Therefore, the best industrial way to manage gas-handling on site is to deliver the mixture as liquefied gas already prepared.

As a result, the gas handling is done in the same way as SF₆.

The slight difference in the handling process at site is a warming step of the liquefied mixture so that g³ reaches its homogeneous gaseous state at the expected concentration before the emptying of the cylinder. Homogeneity of the different components is maintained throughout the filling process thanks to the automation applied on the cylinder and the filling cart.

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

2_Achieving a homogeneous g3 gas

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.

g3 gas mixture in its liquid form as delivered to the customers in cylinders g3 homogeneous liquid-gas mixture obtained above supercritical phase
Picture 4: g³ gas mixture in its liquid form as delivered to the customers in cylinders
Picture 5: g³ homogeneous liquid-gas mixture obtained above supercritical phase

^{(1) Source: Wikipedia}

3_A dedicated service cart for g3

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.”

Air Liquide g3 liquid-to-gas Service cart Air Liquide g3 Service cart for gas tank
Picture 6: Air Liquide / Liquid to Gas cart (L2G)
Picture 7: Air Liquide / Cart for Gas Tank (C4GT)

4_9 basic questions from users of g3 gas handling cart to Frédéric Loray, Air Liquide

1 – Can we use the same cart for SF₆ and g³? This would improve the investment during the first years of deployment of the g³ solution. In addition, are the SF₆ and g³ cylinders the same?

“To optimize the investment of the g³ handling cart, the possibility to handle SF₆ with the same system was recently added. However, I strongly advise against the use the same equipment for two types of insulating gas: the risks of cross contaminations and human error might be high. This recommendation will also be considered in a forthcoming Cigré publication on SF₆-free mixtures.”

2 – Can I operate it myself or do I have to ask for the support of g³ experts?

“To operate the gas handling cart, operators are recommended to pass a one-day training course to understand the differences between SF₆ and g³ and to become familiar with the equipment. Thanks to the fully automatic mode and continuous monitoring, the equipment is easy to use”.

3 – How can I secure g³ maintenance operations on my GIS or GIL?

“GE proceeds to the first GIS filling with g³. Once the GIS (or other equipment) is in operation, the operator has three alternatives:

1. be fully autonomous and to buy the gas handling cart,
2. contract GE to help ensure the GIS maintenance,
3. lease the cart for short periods when needed”.

4 – Are there specific precautions to take regarding gas leaks in GIS or GIL?

“As with SF₆, GIS compartments are engineered to be fully gas-tight. The latest generation of GIS compatible with g³ have implemented features to increase the gas-tightness.

5 – Is the GIS connection hose part of the equipment? Do we have the same fitting diameters as for SF₆ GIS?

“The flexible pipe from the compartment to the handling cart is considered an additional accessory to the cart, as is the case with SF₆. Moreover, this pipe should be dedicated to g³ in order to avoid cross contamination with SF₆-GIS (and vice-versa). To avoid human error in filling SF₆ into g³ compartments, compartments' gas fittings are specific to g³-GIS. The standard diameter for g³ is M48 instead of M45 for SF₆.”

6 - Can we rent equipment for a specific use instead of purchasing it?

“Yes, GE has adapted our offering to the different needs users may have: long-term leasing over a few years and short-term leasing for a few weeks are possible.”

7 – Which training is available for g3 handling?

“Air Liquide trained GE staff for cart operations and now GE offers complete training for g³-GIS operations. Air Liquide is also happy to provide complementary training on g³ gas cart.”

8 – Do I have to mix before the connection of the hose to the GIS compartment?

“The mixture homogenization is done directly within the cylinder as well as the monitoring so there is no need to 'waste' gas to purge pipes before filling. You can connect the hose before or after the homogenization, you can fill the hose at filling pressure before connection and then connect it to the GIS.”

9 - Do I risk a harmful manipulation error?

"This is possible with manual or semi-automatic equipment. But with fully automatic equipment from g³, the user follows a “guided dialog” with the HMI to enter the tare of the cylinder, the filling pressure, confirm the mixture and answer questions… Let it guide you to have some peace of mind! Step by step, you will have filled your first compartment in g³ as you did it with SF₆!”

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g3-filling: a mastered process