4 Gas Detection
GAS INSULATED SWITCHGEAR (GIS) MONITORING OF SF6 USING PRECISE NDIR SENSOR TECHNOLOGY
The integrity and safety of electrical switchgear are paramount in any power distribution system. Sulphur Hexafl uoride (SF6
) is widely used as an
insulating and quenching medium in switchgear because of its outstanding electrical properties and chemical stability. However, monitoring SF6
gas
is critical, as it is a potent greenhouse gas. Cambridge Sensotec stands at the forefront of SF6
gas analysis, ensuring
the safe and effi cient operation of electrical switchgear across various industries. This was achieved by using the latest NDIR sensor technology developed and produced by Wi.Tec- Sensorik GmbH.
Introduction
A gas-insulated switchgear (GIS) is a completely gas-tight encapsulated switchgear for high voltage and medium voltage, which surrounds the electrical conductor(s) with sulfur hexafl uoride (SF6
Gas-insulated switchgear is much more compact than air- insulated switchgear because SF6
strength of air at normal pressure. In addition, SF6 ) as a protective gas for insulation. In contrast
to air-insulated switchgear (AIS), this allows compact switchgear to be installed in confi ned spaces.
has four times the dielectric supports
the extinguishing of spark gaps more effectively than air. Gas- insulated switchgear is generally designed for indoor use but can easily be converted for outdoor use.
The insulating gas is kept at a pressure of 5 bar to 10 bar to ensure insulation capability. The reason for the higher gas pressure compared to normal pressure is that the average path length of the free electrons in the gas is reduced. Due to the electric fi eld strength, electrons cannot be accelerated as strongly on average at high gas pressure as at normal pressure and collide with the SF6
molecules. As SF6 is a greenhouse gas with a GWP1 of 23,500 (EPA), great
) and carbon dioxide (CO2 absolute proportion of SF6
efforts are being made to prevent leaks. However, there are now exemptions for gas-insulated switchgear, which are justifi ed on the grounds of small quantities of escaping gas and proper disposal. Furthermore, various gas mixtures such as nitrogen (N2
) can be used to reduce the in the insulating gas. For gas-insulated
content can be reduced to around 5 %. However, these mixed gases are more sensitive to impurities. Impurities lead to a reduction in the insulation strength and require a higher gas pressure, which increases the mechanical effort and system costs.
pipelines (GIL) without switching operations in the pipe sections, the SF6
Sulfur Hexafl uoride SF6
GIS Several applications for measuring SF6
the SF6 exist in a GIS. The fi rst
is the gas concentration in the close GIS chamber. Ideally, this concentration is always 100 % by volume. Due to unavoidable leakages, this concentration can reduce over several years. To record this process reliably, the measuring range between 90-100 vol.% must be particularly accurate. This application is described in this report. This measurement situation is shown in Figure1. This measurement is repeated at regular intervals to determine the course of the SF6
concentration and to identify leaks. For an online monitoring a gas density meter is also integrated into the system.
It is also possible to detect leaks in the system and the supply lines with a leak detector. In this case, the typical measuring range is 0-1000ppm SF6
. In the room air control (accumulation of SF6 in the room), gas
concentrations around the zero point are required. the typical measuring range in this case is 0-50ppm SF6
. itself. Most of the modern-day test equipment is highly
portable and battery powered so there are huge challenges in obtaining accurate SF6
readings in differing environmental
conditions of temperature and altitude around the World, as well as withstanding mechanical shocks from transportation and rough handling. Furthermore, the new requirements for zero- emissions gas testing mean that the equipment not only has to remove a sample of pressurized gas from the GIS to measure it, but also has to store the sampled gas and then return it back to the pressurized compartment afterwards without any leaks. This makes any zero-emissions SF6
gas analyzer hugely complicated
and internal space is therefore at a premium. The Rapidox SF6
gas analyzer range has for a number of years used a Wi.Tec-Sensorik NDIR gas sensor module supplied in an ultra-compact housing with bespoke circuitry supplied on a fl ying lead to allow insertion into a very tight space. The gas is depressurized before it enters the IR module and fl ow controllers regulate the fl ow of test gas to a precise 0.5L.min-1
allowing for
changes in gas density as composition changes. After the gas is tested it enters an internal defl ated storage bag which slowly fi lls until maximum capacity of fi ve liters is reached over ten minutes. In reality, a full test takes a maximum of eight minutes, but this is largely to accommodate the H2 has a much slower response. Typically, the SF6
O measurement which reading from the
IR sensor requires a maximum of two minutes to become stable and reliable.
With the storage bag full of gas, the operator can decide to either remove the gas to an external storage bottle or bag for recycling or, more commonly, pump back the gas to the pressurized vessel where it came from. This is done using a powerful internal compressor which can achieve pressures up to 10 bar on the return. Generally, SF6
where signifi cant toxic gases are detected, safety measures protect the NDIR gas sensor by rejecting contaminated gas before any harm can be done to the equipment.
is an extremely stable, non-fl ammable and highly
electronegative gas with excellent dielectric properties. It is commonly used in medium and high-voltage electrical equipment as an electrical insulator, arc-quenching and cooling medium. However, SF6
is classifi ed as a greenhouse gas and must be kept
within a closed circuit to avoid any deliberate release into the atmosphere. The international Kyoto agreement protocol has mandated reductions to harmful emissions amongst its member
states. For the power transmission and distribution network, SF6 technology remains essential. To protect personnel, equipment and the environment regular SF6
analysis should be adopted
within the maintenance schedule. The early identifi cation of any decomposition products and moisture within the SF6
avoid unnecessary shutdowns, outages and failures, in addition to reducing maintenance expenditures.
gas will help
Figure 1: Checking a GIS with a zero emissions portable gas analyzer from the Rapidox series.
Instruments
portable and bench testing equipment for more than fi fteen years. Typically, three important gases are measured: SF6
Cambridge Sensotec Ltd, has been manufacturing a range of SF6
purity (typically 97-100%), H2 , O2 , CO and H2 where required.
In accordance with IEC and CIGRE requirements, the preferred measurement technology for SF6
analysis is nondispersive
infra-red (NDIR), which, unlike other methods such as speed- of-sound or thermal conductivity, is a direct measurement of
IET ANNUAL BUYERS’ GUIDE 2024/25 O water content (target <-36°Cdp) and SO2
arc indicator toxic gas (target < 1ppm). Other gases such as HF, CF4
S are measured using electrochemical sensors For a successful and accurate measurement of SF6 , the NDIR
module must be fully temperature and pressure corrected over a wide range to accommodate all locations where humans live on planet Earth. For example, the Rapidox analyzer has to work correctly both in the 40°C heat of Dubai and the -10°C cold of Canada, as well as in high altitude places such as La Paz in South America at 3700 meters above sea level where atmospheric pressures fall below 700mbar.
Although the use of SF6 Worldwide is expected to increase year
on year for the rest of this decade, there are greener alternatives now available that are being slowly introduced. Gas mixtures of NovecTM
C4 and C5 combined with CO2 are in occasional use
albeit with limited life, as well as mixtures of Dry Air and SF6 N2
1 Global Warming Potential /
mixtures. All these gas types are relatively straightforward to measure using Wi.Tec-Sensorik NDIR gas sensors.
is extremely dry and clean but on occasions
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