Insulating gases protect air-insulated and gas-insulated switchgear from failure. Not just pure sulphur hexafluoride (SF6) but also SF6 mixtures, N2, g3 and others can be used for this purpose. WIKA is a leader in sensing solutions that help power transmission and distribution companies to remotely monitor the quality and quantity of these gases.
Medium- and high-voltage switchgear can be insulated using different media, for example oil, “Dry Air” and other gases such as SF6 and new gas mixtures. The traditional and most effective insulator is SF6. Its use, however, comes with several hazards, especially as this synthetic fluorinated compound has a global warming potential (GWP) of around 24,000. This is the highest value of any known gases. For reference, carbon dioxide has a GWP of 1.
Challenges when using SF6
Power transmission and distribution companies that use SF6 must monitor density to detect leakages. They also need to periodically analyse the gas to detect contamination. Even as many companies still rely on this gas to insulate switchgear, they do realise its role in climate change and are looking for SF6 alternatives.
Reasons for failure in switchgear
Most power plants and substations use air or a gas to insulate their switchgear. While air is abundant, non-toxic and has a GWP of 0, it is not as effective in quenching arcs. As a result, the gas compartment in air-insulated switchgear (AIS) has to be about three times as large as those in gas-insulated switchgear (GIS). Both AIS and GIS are susceptible to issues that reduce their effectiveness and lead to switchgear failures.
Challenges with air insulation:
- Dirt and dust
- Contact resistance
- Overload
Challenges with gas insulation:
- Gas leakage
- Humidity, a potent impurity for SF6-insulated switchgear
- Decomposition products, many of which are toxic and will corrode GIS
Measures to protect switchgear
Monitoring the condition of the insulator is a key step in preventing potentially dangerous situations. AIS operators should monitor the air’s partial discharge status, temperature and humidity. For GIS, it is important to monitor the gas’s pressure, temperature, density and humidity. Switchgear has a typical service life of 20–50 years, with gas-related failures increasing sharply 15–25 years after installation. Therefore, monitoring is especially key during the second half of the equipment’s lifetime.
Options for monitoring gas-insulated switchgear
- Reactive approach: In this approach, a mechanical instrument, such as a switch contact, sets off an alarm when the gas density has dropped to a certain level. Mechanical instruments have an accuracy of 1–2.5 % of span. For humidity, technicians might manually analyse the gas periodically (every 1 to 6 years) on a predetermined schedule with a portable test instrument and read the gas density monitor.
- Proactive approach: Both analogue (4 … 20 mA) and digital instruments offer an early warning system with remote monitoring functions. Analogue instruments continuously monitor the gas density in GIS and transmit that data to a control room or another central location. Analogue instruments have an accuracy of 1.5–2.3 % of span for compensated pressure. Digital instruments continuously monitor not just the gas density but also the parameters of pressure, temperature, humidity, etc. This gives a more complete picture of the condition of the insulating gas. Digital instruments have an accuracy of 0.6–0.8 % of span.
Reactive vs. proactive maintenance of GIS
Reactive vs. proactive maintenance of GIS
- The more parameters that a sensor can detect and transmit in real time, the better operators can react to changes in the condition of the insulating gas and take steps to avoid safety issues. Besides remote monitoring functions, analogue and digital sensors also allow for gas condition forecasting and predictive maintenance.
Maintenance of gas-insulated switchgear
If necessary, the technicians will take the necessary corrective actions, such as:
- Taking the equipment offline to clean and/or replace the gas with a handling equipment
- Dehydrating the gas while the switchgear is operating
- Repairing a leakage and refilling the gas compartment
Gas condition forecasting: The better way to protect assets
One-time raw data measurements of gas density can give a false or misleading picture of an asset’s performance, leading to misjudgments such as unnecessary or insufficient maintenance. Changes in ambient temperature and humidity, which vary constantly, are also hard to detect when measurements are taken sporadically or periodically. With continuous monitoring, operators get not only a truer picture of the current gas condition, but also its development. They can use the data to gain a deeper understanding based on historical data and then use this knowledge for predictive maintenance and other next steps. If smart sensors are used, the instrument will also automatically compensate for ambient conditions to arrive at an accurate gas density.
Data quality in gas condition forecasting
Large vs. small ambient impact
Ambient conditions (temperature and humidity) and event anomalies have a notable impact on the precision of forecasts.
The figure shows that a large ambient impact, combined with an anomaly early on in the data collection, makes for a very uncertain and imprecise forecast. Under the same ambient conditions, but without the anomaly, the results are more precise. A small ambient impact, in turn, produces the most precise forecast.
Adding to the complexity, humidity has a complicated correlation with temperature. The humidity condition of an asset can be misjudged when only the raw measured data are taken into account. Furthermore, every gas compartment has its own unique correlation between humidity and temperature. This correlation can be extracted from historical data and compensated for. Thus, a temperature-compensated gas density can be forecasted with precision.
Since both data quality and temperature compensation have a significant impact on the precision of a forecast, it is necessary to use smart sensors with high accuracy and consistency and to work with a provider who understands how to preprocess, manage and interpret the data.
Smart sensors for the remote monitoring of insulating gases
Density of different insulating gases (click to enlarge)
The more information that operators have about their equipment’s insulating media, the better they can prevent failures and hazardous conditions. WIKA offers sensors based on temperature and pressure that, with the proper configuration, are easily adapted to calculate the current density of different gases. WEgrid, a wholly owned subsidiary of WIKA, offers a suite of smart sensors that forecast the condition of all types of insulating gases, including: SF6, N2, SF6/N2 mixtures, CO2, O2, CF4 (tetrafluoromethane), air, g3 (a syngas with 3M’s Novec 4710). No matter the equipment or insulating media, the WIKA gas density sensors can be configured for special applications.
WIKA sensors: Tailor-made for the monitoring of insulating gases
Gas density sensors for the model series GD-20 (click to enlarge)
WIKA offers several measuring instruments for reliable monitoring of SF6 and alternative insulating gases. One of them is the GD-20. The analogue version focuses on gas density, transmitting a temperature-compensated pressure reading via a 4 … 20 mA output signal. The digital version has an RS-485 interface that communicates using the Modbus® RTU protocol and provides data for pressure and temperature in addition to gas density. The GDHT-20 is a high-quality transmitter, capable of everything the digital version of the GD-20 can do with the addition of measuring the humidity content of the insulating gas. This additional parameter enables monitoring within the terms of the CIGRE directives for power systems, as well as in accordance with IEC standards.
GD-20-W: Wireless Monitoring of Insulating Gases
In addition to wired sensor solutions, WIKA offers the GD-20-W, a wireless alternative for continuous gas density monitoring in switchgear. This sensor measures not only gas density but also temperature and pressure, transmitting data via the LoRaWAN® protocol. With its integrated battery, the GD-20-W operates autonomously, eliminating the need for an external power supply. An intelligent alarm function detects critical changes early and can immediately trigger a warning before safety-critical conditions arise. This combination of high measurement accuracy and wireless transmission enables efficient predictive maintenance.
Outlook: SF6 and alternative gases
While SF6 will remain the primary insulating gas for the near future, less harmful alternatives and other climate-friendly technologies are on the horizon. Governments are part of the push, too. In 2022, the European Commission proposed banning fluorinated greenhouse gases in new medium-voltage switchgear by 2026 and in high-voltage switchgear by 2031. About a dozen US states have initiatives and regulations to phase out SF6-filled GIS.
Note
WIKA is a leading global supplier of instruments for analysing and monitoring both traditional and alternative insulating gases. We also offer comprehensive project planning for power transmission and distribution companies – from engineering and installation of the instruments to data management and analytics using our proprietary software. Contact us if you would like to learn more about how we can help make your business safer and more sustainable. On the WIKA website you will also find a wide range of information on the topic of power transmission and distribution (SF6) as well as on our SF6 gas solutions, in particular on our gas density sensors (including model GD-20 and model GDHT-20).
Also read our posts
High-accuracy moisture measurement in SF6 switchgear
Responsible handling of sulphur hexafluoride
You can also find out more about WIKA’s SF6 expertise in the following video: