Dewpoint measurement 6 gas-insulated switchgear

1 Box 26, FIN-00421 Helsinki, FINLANDTel: +358 9 894 91 Fax: +358 9 8949 2485 Email: (2) Dewpoint measurementin SF6 gas-insulatedswitchgearSulfur Hexafluoride (SF6) is an inertinsulating gas of high dielectric strength andthermal stability. The electric power industryuses SF6 to insulate high voltage lines, circuitbreakers and other equipment used inelectricity transmission and measurement is an important partof SF6 equipment is a greenhouse gas, with a globalwarming potential 23900 times that of this reason, the gas is used sparingly in SF6 Power switching or load current interruption in aswitchgear results in arcing between two contact insulation properties of SF6 control effectively the arcformation.

2 Under the influence of an arc, a small portion ofSF6 gas dissociates into its atomic constituents sulfur andfluorine, then again recombining naturally into , in the presence of moisture, the SF6 reformationis disturbed and secondary decomposition products such ashighly corrosive hydrogen fluoride (HF) start to weaken the insulation properties of SF6 and maycause rapid deterioration of a high-voltage order to avoid the formation of unwanted secondarydecomposition products, the amount of water vapor in thesystem should be kept at environmentA typical Dewpoint range for servicing the SF6 equipmentis C.

3 Pure SF6 may be even drier. The mosttypical operating pressure within the switchgears is , but Dewpoint measurement can also be performed atatmospheric industry also uses ppm units, especially ppmw (partsper million by weight) to indicate humidity. For SF6, theconversion between ppmv and ppmw isppmw = ppmv / the measurementTypically, Dewpoint measurement in the switchgear isdone at 5-8 years intervals by specialized SF6 measurement can either be performed at system oratmospheric pressure. The conventional dewpointmeasurement technique (chilled mirror) has typically beenrestricted to measurement at atmospheric pressure due torather high liquefaction temperatures of pressurized Box 26, FIN-00421 Helsinki, FINLANDTel: +358 9 894 91 Fax: +358 9 8949 2485 Email: (2)StandardsIEEE standard guides for moisture measurement andcontrol in SF6 gas-insulated Equipment (IEEE std 1125-1993).

4 International standards specify the recommendedmaximum moisture content in circuit 376 specifies the limit for new gases to be introducedinto the GIE compartment as 120 ppmv/ Td = -40 US standard ASTM D2472-92 states 71 ppmv / Td =-45 SF6 contains very little moisture. A typical moisturelimit set by SF6 gas manufacturers is -60 C limit recommendations of the gas-insulatedequipment (GIE) vary between 70 .. 810 ppmv,corresponding to -45 .. -22 C Td in the atmosphericpressure, or -29 .. -3 C Td in 5 barg for measuring Dewpoint inSF6 The Vaisala DRYCAP Hand-held Dewpoint MeterDM70 with the DMP74C probe is optimized formeasuring the Dewpoint in SF6 gas.

5 The sensor purgefeature of DM70 allows fast response in low dewpoints,and only a small amount of SF6 is needed for response time is especially important because of theenvironmental restrictions on SF6 release to instrument outputs several measurement quantities,including Dewpoint temperature, ppmw, ppmv, andeven Dewpoint temperature calculated from gas pressureto atmospheric 10m extension cable between the MI70 indicator and theDMP74C probe is useful for measurement points thatlocate high up from ground DSC74B and DSC74C sampling cells are optimizedfor measurement in the SF6 gas insulated equipment, and itenables measurement at both gas system and atmosphericpressure.

6 The sampling cells also enable collecting andrecycling SF6 when measuring in atmospheric of Dewpoint measurement technology, it is tobe noted, that SF6 combustion gases such as HF should notbe present when making the measurement . The industrystandard is typically to ensure these gases are not presentin notable quantities before performing dewpointmeasurement. In a well serviced switchgear , the presenceof HF is very rare. Combustion gases may appear in placeswhere the gear is heavily burdened and short circuitscommonly take place.