Wink of Knowledge: Concentration measurement of SF₆ in insulation gas mixtures for power transformers
What is a Wink of Knowledge?
Do you need to quickly measure, draw or do/build something? The speed with which the result may be achieved counts more than the perfect (scientific) approach. For this reason, we have introduced the Wink of Knowledge. Science in the wink of an eye, so to speak. We don’t want to prove anything scientifically. We simply want to quickly demonstrate something pragmatically. If you are interested, we would be happy to discuss these results in more detail with you and your project.
Why this test?
SF₆ (sulphur hexafluoride) has been the preferred insulating and extinguishing gas in gas-insulated switchgear and high-voltage transformers for decades – due to its excellent dielectric properties. However, SF₆ is increasingly the focus of regulatory attention as a powerful greenhouse gas. To reduce the environmental impact, manufacturers are increasingly relying on gas mixtures, e.g. with nitrogen (N₂) or CF₄ (tetrafluoromethane). It is crucial to precisely measure the concentration of SF₆ to ensure insulation safety and functionality.
When using pure SF6, it is common practice to use the theoretical pressure at 20 °C as a reference value. The “P₂₀ pressure” serves as an industry-wide reference value for filling quantity and alarm thresholds: It translates each measured pressure/temperature pair to 20 °C and thus acts as a temperature-neutral measure for the molar gas density – and therefore for insulation strength, liquefaction reserve and leakage monitoring. However, this value is no longer sufficient in the event of air ingress (e.g. due to leaks in the supply line or insufficient purging processes) or the use of SF₆ mixtures (typically N2 or CF4), because different mixtures have a significantly lower dielectric strength despite identical P₂₀. Modern mixed gas and low-GWP[1] systems must therefore supplement P₂₀ with mixture-specific density algorithms or regular gas analyses.
Figure 1: Relative dielectric strength error (E_rel) and required pressure compensation (P-factor).
Figure 1 illustrates the influence of the SF6 concentration on the dielectric strength. The factor for E_rel describes the expected relative dielectric strength at unchanged P₂₀ filling pressure (1 = pure SF₆). The “P-factor” indicates the factor by which the operating pressure (or the specified P₂₀ value) would have to be increased in order for the mixture to achieve the same strength as pure SF₆.
For example, a pure P₂₀ relay with a nitrogen/SF6 mixing ratio of 50/50 would still report “green”, although the true strength is already 35% below the target. In order to compensate for this and achieve the required dielectric strength, the pressure would have to be increased by a factor of 1.54 – which in turn puts a strain on the seals, pressure design and condensation reserve.
1Global-Warming-Potential. A measure of the global warming potential of a substance.
Which gases were used?
- SF₆ (Sulphur hexafluoride)
- N₂ (Nitrogen)
- CF₄ (Tetrafluoromethane)
Density measurement
The density was measured using the DGF-I1 gas density sensor. For this purpose, the existing calibration process was adapted and extended to include the measurement of pure SF6. The measured values recorded under varying pressure and temperature conditions were then used to optimize the existing physical model for concentration measurement. This model ensures that real gas calibrations with SF6 will no longer be necessary in the future, so that each sensor can be subsequently and easily configured for this application.
Nitrogen (N2)
1.2503 kg/m³
at 0 °C, 1.01325 bar abs
Sulphur hexafluoride (SF₆)
6.616 kg/m³
at 0 °C, 1.01325 bar abs
Tetrafluoromethane(CF₄)
3.947 kg/m³
at 0 °C, 1.01325 bar abs
The TrueDyne sensor
With a diameter of 33.5 mm and a length of 63 mm, the DGF-I1 density sensor has a very compact design and fits into even the smallest of spaces. It is screwed with the integrated connection directly into the gas line or the control cabinet to be insulated; a filter protects against contamination. The measured values are transmitted to the higher-level system via an RS485 interface. The low response time and power consumption of the sensor enable continuous monitoring of the desired SF6 concentration directly in the process – the measurement does not have to be interrupted.
DGF-I1 Density meter for gases
Max. measuring error:
Density: <0.1 kg/m³
Temperature: <0.8 °C
Pressure: <0.04 bar
In-field adjustment Density: <0.05 kg/m³
Repeatability:
Density: <0.015 kg/m³
Temperature: <0.06 °C
Pressure: <0.005 bar
Permissible density measuring range:
0,2 … 19 kg/m³
Permissible pressure range:
Max. Measuring range:
1…10 bar (absolute)
Only use gas mixtures with argon (Ar) up to
max 9 bar (abs).
Burst pressure 30 bar
Results
Due to the high precision of the density measurement (max. measurement error <0.1 kg/m3), both the concentration of the insulating gas and the progress of the purging process can be easily monitored. This ensures that the desired technical requirements are met and that a high level of safety is traceable during all process steps (e.g. during filling or maintenance of the enclosures).
With our compact density sensor, various gas mixtures with SF₆ can be monitored precisely and continuously under operating conditions. Real-time monitoring is essential to ensure insulation safety and process quality, especially when gradually replacing SF₆ with less climate-damaging gases – existing systems are often not designed for gas mixtures.
Our technology simultaneously enables:
- Continuous inline monitoring during filling, operation and maintenance
- Traceability in quality management
- Cost reduction through reduction of laboratory analyses
- Correction of the operating pressure to ensure dielectric strength
Precise density measurement is therefore a powerful tool for determining the concentration of SF₆ in insulating gas mixtures. This in turn serves to ensure the desired dielectric strength, depending on the selected gas mixture and the measured concentration.
In a regulatory and ecologically sensitive environment, it creates transparency and safety – all in the interests of intelligent, sustainable transformer operation.
Sensors that might interest you
Gases
Viscosity
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