Characterization of the liquefaction properties of fluoronitrile mixtures by a thermodynamic experimental approach

Abstract

Fluoronitrile (C4FN) based mixtures with natural origin gases like dry air are one of the alternatives to replace SF6 in Gas Insulated Substations (GIS) in order to propose a new range of equipment with a lower environmental impact. Today, this solution is already applied in different pilot projects around the world. One of them is RTE “Grimaud” substation in the south of France: a 145 kV GIS switchgear and the first French SF6 free substation in operation since 2018.
To lower the CO2 footprint by reducing the use of SF6 in other GIS is a strategic goal for RTE for the next years. In this direction, better and deeper knowledges on the properties of the fluoronitrile based mixtures are necessary, in particular as far as the liquefaction behaviour is concerned. In this context, a collaboration was conducted between RTE and SuperGrid Institute in order to increase the knowledge on the C4FN mixtures.

One of the drawbacks of the fluoronitrile based mixture is a higher dew point temperature in comparison with the one of SF6. Despite the C4FN being mixed with buffer gases, CO2 and O2 with lower dew points, the liquefaction can occur at relatively high temperatures according to the filling pressures and concentration of the different components. It is known that the liquefaction point of a mixture is not only driven by the component with the higher dew point (here the C4FN), but also by the interaction with the molecules themselves. Hence, the relative concentration in the mixture can lead to some differences with the one considering only the fluoronitrile. In the literature, equations exist to evaluate the thermodynamic properties of gases and mixtures, like the dew point curves. These equations need to be calibrated with experimental data as the knowledge of fluoronitrile is still limited. It is fundamental to perform experimentations in order to determine the liquefaction points and to increase thermodynamic knowledges on these types of mixtures.

Within this scope, experimental tests have been conducted with three types of C4FN mixtures. The tests have been conducted in a climatic chamber cooling down to -40°C and a GIS compartment of about 50 L that has been equipped with different measurement equipment to monitor the temperature and the pressure of the mixture. The tests have been conducted at six different pressures for each mixture, from 5 to 10 bar abs. The liquefaction points at the different pressures have been determined by decreasing the temperature until the emergence of the first drops of liquid, identified with a decreasing of the pressure with a numerical methodology.

The conducted analyses have permitted to increase the knowledge on the liquefaction point of the C4FN mixtures and to investigate the effect of O2 in the liquefaction behaviour. Due to the consistent number of data acquired during the test campaign, the experimental measurements have been fitted to obtain an equation that allows, within a certain validity range, the determination of the liquefaction points of mixtures with different molar ratios of C4FN, O2 and CO2 and at different pressures. The equations of state for the fluoronitrile mixtures, following the Peng-Robinson equation, have been equally determined from the experimental data. The derived equations are very interesting for the practical application of the fluoronitrile mixtures in GIS and allows the evaluation of the liquefaction points at different densities even if the molar quantity of C4FN is changing all along the lifetime, e.g. differential permeation or refilling. It helps for the operational point of view in order to estimate the need or not to intervene to maintain good characteristics of insulation for the GIS.

Caterina TOIGO, Antoine PEREZ, , Frank JACQUIER, Alain GIRODET, Didier LASSERRE (RTE), Michael INVERSIN (RTE), Guilhem BLANCHET (Stattnet).

Presented at CIGRE 2024

Text available on HAL repository: https://hal.science/hal-04676634