Gas density monitoring of alternative insulating gases in SF6-free switchgear
Alternative insulating gases, or alternative gases for short, are replacing sulphur hexafluoride (SF6), which has been commonly used in gas-insulated switchgear for many years because SF6 is considered to be a potent greenhouse gas. The Trafag product range for gas density monitoring in high-voltage switchgear offers full compatibility with the more environmentally friendly alternative insulating gases.
Gas-insulated switchgear is used in high and medium voltage networks. Although the physical properties of SF6, the insulating gas commonly used today, make it very suitable for use in gas-insulated switchgear, it is also a powerful greenhouse gas with a Global Warming Potential (GWP) of 22,800. The GWP expresses how much more a gas contributes to global warming compared to CO2. In the case of SF6, this means that 1 kilogram of SF6 contributes as much to global warming as 22.8 tonnes of CO2. For this reason, the use of SF6 is already severely restricted today and will also be largely banned for use in high-voltage switchgear from around 2030 under the F-Gas Regulation in the EU. SF6 is increasingly being replaced by more environmentally friendly alternative insulating gases. The alternative gases can be divided into two main groups: C4FN-based insulating gases such as EconiQ or g3 and air-based mixtures (technical air) such as CleanAir*. The gases in both groups are lighter than SF6 and have a lower insulating capacity at the same pressure. The compartments of gas-insulated switchgear are therefore filled with significantly more pressure in order to achieve the same insulating properties.
How are alternative insulating gases monitored in switchgear?
As with sulphur hexafluoride, alternative insulating gases are also filled into the sealed compartment of the switchgear with a quantity of gas tailored to the required insulating properties. The pressure at 20°C is usually used as a reference. While the amount of gas in the sealed, tight compartment remains the same, the pressure changes significantly if the temperature changes, e.g. during the course of the day or over the seasons. For this reason, the density, i.e. the constant amount of gas in the volume, is monitored for alternative insulating gases, and not the pressure. For the same insulating properties, however, the alternative gases require a higher pressure than SF6. Their temperature behaviour, expressed in isochores (change of state at constant volume), also differs from that of SF6. Gas density monitors must therefore be specially designed for the higher pressures and the individual properties of the gases. As the density of alternative gases with the same insulating properties is massively lower than that of SF6, the electronic gas density sensors must also be specially designed with regard to the measuring range.
What are the differences between gas density monitors for alternative gases and for SF6?
The proven reference chamber measuring principle of the Trafag gas density monitors is used unchanged for alternative gases. Due to the higher pressures required for alternative gases, the entire measuring system has now been qualified for a measuring range of up to 13bar (absolute pressure). The main difference to gas density monitors for SF6 is the filling of the reference chamber. This is filled with a gas mixture that represents the isochoric gradient of the alternative gas, i.e. the gradient of the line of the same specific volume (i.e. density) in the pressure-temperature diagram. The switching points and the scaling of the display are adjusted to the effective values of the alternative gas.
How do gas density sensors for alternative insulating gases differ from gas density sensors for SF6?
In addition to being suitable for the higher pressures, the sensor system must also be calibrated for the lower density of the gases. The unique measuring principle of Trafag gas density sensors, which measures the density directly — without detour via pressure and temperature — is suitable for measuring the density of all types of gases. In order to be able to measure lower densities accurately, the resolution of the evaluation electronics has been increased. In the medium term, the heavier gases such as SF6 will also benefit from the higher accuracy. For each gas mixture, the parameters required for precise linearisation of the output signal (and for conversion to pressure at 20°C, in the case of digital sensors RS485 Modbus), must be determined once by measurement. These parameters are uploaded into the internal chip during the manufacture of the density sensors before calibration.
Can the integrated test, service and filling valves also be used with alternative insulating gases?
The patented, directly in the gas density monitor integrated valves for periodically checking the switching points, for gas sampling for off-line analysis and for filling or refilling the insulating gas tank are also suitable for operation with alternative gases. The valves have been designed to withstand the higher pressures of the alternative insulating gases during development. The sealing materials are suitable for use with the alternative insulating gases commonly used today. The cover caps of the valves integrated in the Trafag gas density monitors are marked in the usual colours: Green for C4FN-based alternative insulating gases, blue for air mixtures and orange for SF6.
The measuring ranges for alternative insulating gases integrate seamlessly into the existing and future Trafag range of gas density monitors, gas density sensors and hybrid gas density monitors (combined gas density monitors and sensors). This means that new systems can be designed directly for alternative gases and existing systems can be retrofitted with SF6-free insulating gases as part of a refurbishment with minimal adaptation and training effort. Using a technology that has been tried and tested for many years, the environmentally friendly systems of the future can be reliably monitored to ensure safe and trouble-free operation.
* EconiQ is a trademark of Hitachi Energy
g3 is a trademark of General Electric Company
CleanAir (Clean Air) is a term used by Siemens Energy
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About the author
Andreas Koch
Head of Marketing and Product Management
Dipl. Ing. FH (Mechanical Engineering), EMBA
At Trafag since 2011
