SF6 in Gas-Insulated Switchgear

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).

Flowchart of SF6 gas decomposition

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

Maximum concentration of contaminants in SF6 gas for reuse (per IEC 60480)

air and/or CF4 (carbon tetrafluoride)

3%

gaseous decomposition products

50 ppmv

dew point temperature (humidity)

−9.4°F/−23°C (filling pressure < 14.3 psig/200 kPa abs.) or

−32.8°F/−36°C (filling pressure > 14.3 psig/200 kPa abs.)

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.

Gas Detector

GIR-10 gas detector

Leak locators: SF6-IR-Leak

With a measuring range from 0 up to 2,000 ppmv, 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.

SF6 gas emissions monitor

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.

 

SF6 Tracer

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 ppmv, 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.



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