A Priority for SF6 Gas Users: Leak Detection and Gas Analysis

The transmission and distribution industry relies on sulfur hexafluoride to insulate medium- and high-voltage equipment. Leak detectors and gas analysis instruments ensure that the SF₆ is pure enough – and in sufficient quantity – to quench electrical arcs. 

The Problem with Contaminants in SF₆ Gas

Pure sulfur hexafluoride is a very stable and nontoxic gas with excellent dielectric strength (able to withstand high voltage without breaking down). During an electrical discharge, this gas quenches arcs by splitting apart. Its six fluorine atoms capture errant electrons, and then the molecule quickly recombines. It is much more effective than air or nitrogen as an insulator.

That’s under ideal conditions, though. No matter how careful service technicians are, tiny amounts of reactants found in ambient air – namely water (H₂O) and silica (SiO₂) – invariably find their way into a gas compartment. Water compromises the gas’s insulating strength by preventing SF₆ from fully recombining after an electrical discharge. When this happens, sulfur and fluorine combine with hydrogen and oxygen to form other compounds (Figure 1).

Fig. 1: Decomposition byproducts of SF6 gas

Over time, higher humidity levels lead to hazardous conditions:

• Toxic decomposition products that harm human and environmental health.
• Highly reactive decomposition products that corrode SF₆ gas compartments.
• Less SF₆ gas to properly insulate medium- and high-voltage equipment.

compound	stability (in air)	byproducts	usual limit values (ppmv)*	odor
SF6 (sulfur hexafluoride)	stable	see below	1,000	odorless
S2F10 (disulfur decafluoride)	stable	SF4, SF6	0.01	acrid
SF4 (sulfur tetrafluoride)	fast decay	HF, SO2	0.3	acrid, sour
SO2F2 (sulfuryl fluoride)	stable		0.3	odorless
SOF4 (thionyl tetrafluoride)	stable	SO2F2	0.5	sour
SiF4 (silicon tetrafluoride)	fast decay	SiO2, HF	0.5	pungent
SO2 (sulfur dioxide)	stable		1.0	acrid
SOF2 (thionyl fluoride)	slow decay	HF, SO2	1.5	acrid, pungent
HF (hydrogen fluoride)	stable		2.0	sour

* The lower the limit value, the more toxic the substance

The IEC (International Electrotechnical Commission) and CIGRE (International Council on Large Electrical Systems) have developed criteria and limit values for SF₆ gas. Their quality directives specify the limits at which a contaminant exists, and how the SF₆ gas used in switchgear should be handled. The permissible standard values are stated in IEC 60480:2019, Specifications for the re-use of sulphur hexafluoride (SF₆) and its mixtures in electrical equipment

With analytic instruments, operators can quickly determine the gas’s current condition: level of humidity, percentage of purity, and concentration of decomposition products. Operation is easy. Simply connect the device to the gas compartment, and then manually start the analysis sequence. Automatic flow control provides precise and reproducible results. After 5–10 minutes, the analysis is complete and the system compares the result to applicable benchmarks in accordance with IEC or CIGRE.

WEgrid Products for SF₆ Leak Detection

Leaks in switchgear are expensive. What’s more, depending on its size, a leak can quickly become a safety and environmental hazard. To prevent fugitive gas emissions, the EPA strongly recommends a robust program of leak detection and repair (LDAR) using techniques ranging from simple soap and water to thermal imaging. For reliable leak detection and emissions monitoring, WEgrid Products uses dual-wavelength non-dispersive infrared (IR) technology.

GIR-10 gas detector

Leak locators: SF₆-IR-Leak

With a measuring range from 0 up to 2,000 ppm_v, this portable measuring instrument is ideal for locating leaks on site and making quantitative measurements. Simply move the sensor across the tank’s exterior. As SF₆ level increases, the console beeps faster. Because the SF₆-IR-Leak (model GIR-10) uses infrared technology, environmental factors – humidity, wind, common VOCs (volatile organic compounds), etc. – will not distort results.

GA35 emission monitor for SF6 gas

Emission monitoring: SF₆-IR-Monitor

This stationary instrument effectively and continuously monitors the concentration of SF₆ gas in ambient air, thereby guaranteeing the occupational safety of enclosed spaces. SF₆-IR-Monitor (model GA35) checks the room’s air quality with a non-dispersive infrared sensor, and immediately sounds a loud alarm if the concentration of any hazardous compounds reaches a certain threshold. In order to contain escaping gas and repair leaking equipment as early as possible, this instrument is usually placed near gas tanks and GIS.

GA65 SF6 Tracer

Leak testing: SF₆-Tracer + SF₆-IR-Leak

Using photo-acoustic IR spectroscopy, SF₆-Tracer (model GA65) is able to detect the smallest concentrations of SF₆ gas, in the parts per billion (ppb) range. It is commonly used for leak testing during the final inspection of GIE, but can also monitor SF₆ levels in enclosed spaces. Use the tracer in conjunction with SF₆-IR-Leak, which has a detection rate of 0.6 ppm_v, to pinpoint the exact location of the gas emission.

WEgrid Products for Gas Analysis

WEgrid Products offer a comprehensive portfolio of innovative equipment and accessories for measuring not only the quality of SF₆ gas, but also alternative gases for insulating medium- and high-voltage equipment.

The GA11 gas analyzer comes in three versions: for SF₆ gas, for g³ (Green Gas for Grid from 3M), and for N₂ gas. Model GFTIR-10 is a measuring system for laboratory analysis of decomposition products in SF₆ gas. WEgrid also offers accessories for gas analyzers, such as the GA05 inlet pressure control unit and the GA45 gas recovery bag.

WEgrid Solutions, a wholly owned WIKA subsidiary, provides the products, expertise, and peace of mind that the T&D industry needs. Contact us for more information on SF₆ leak detectors, SF₆ gas analyzers, and a range of other products for protecting people, the environment, and your power plant.