SF6 Gas Purification

Sulpher Hexafluoride (SF-6) gas is very widely used in High Voltage Circuit Breakers of both puffer & GIS types due to its excellent insulation, dielectric, electronegativity & arc quenching properties.  At pressures between 4 to 7 Kg/cm2 the dielectric strength of SF-6 gas is 2 times that of oil & 4 times that of air.  This permits smaller clearances with the result design & construction of equipments can be of reduced size and weight for the same KV ratings.
The presence of moisture content, in traces exceeding the permissible limits, can however drastically reduce the effectiveness of these properties and there is an increased possibility of arcing & internal flashovers due to condensation and tracking which in turn severely affects performance reliability of circuit breakers.  Additionally, the excessive moisture in the SF-6 gas & enclosed equipment also accelerates the formation of compounds such as SF5, S5F10 and Hydrofluoric acid (HF) as a result of reactions between SF-6 arc by-products such as SF2, SF4, S2 F2, SOF4, SO2, SO2F2, SOF2 & water, and these compounds being highly corrosive, toxic, and aggressive, degradation of insulation, insulating surfaces as well as erosion & rusting of metal surfaces result due to aging mechanism and all this in the end can lead to either SF-6 gas leakages and/or mechanical & electrical failures.Periodic Moisture measurement and control therefore constitute a major part of any effective maintenance programme for SF-6 gas circuit breaker & gas insulated equipments.  The moisture particularly in liquid phase is very detrimental to the dielectric withstand strength of gas insulated equipments & therefore moisture has to be maintained at a level such that it does not condense in to liquid for the entire range of expected operating temperatures.


Each manufacturer will have their own specifications for acceptable Dew Point limits.  A typical Dew Point value for Tropical country like India, of – 370C (180 PPMV) measured at atmospheric pressure is most satisfactory & favoured for SF-6 breakers up to 400Kv range, since this value is equivalent to – 15.90C at breaker operating pressure of 7 Kg/cm2, which is well above lowest operating Temperatures in India.  From this point of view, lowest acceptable Dew Point of  
– 300C (375 PPMV) can be fixed as a Danger Monitoring parameter, since it corresponds to – 80C Dew Point at 7 Kg./cm2 breaker operating pressure.  Dew Point readings below – 300C should never be allowed/accepted.  Similarly a typical `Dew Point’ of – 400C be made applicable for new SF-6 Gas Cylinder.  In superior quality SF-6 gas cylinder moisture content is kept less than 5 – 15 PPMV, thereby assuring Dew Point of above – 400C up to – 620C.


The necessary spot checks on SF-6 gas system at site can be easily & quickly made on sample SF-6 gas from breaker or gas cylinder by using good quality, reputed Hygrometers preferably from either Shaw Meters U. K. or Alpha Moisture Systems U.K.  All care must be taken to follow instructions given by the manufacturer of Hygrometer properly towards measurement of Readings.  Generally Dew Point Meters designed for site application use Aluminum Oxide sensors and these are suitable for Dew Point Measurement at (slightly higher) atmospheric pressure.  For accurate readings & repeatability one must take Dew Point readings at the same gas flow rate (3 to 5 L/min) and pressure (0.5Kg/cm2) & to facilitate this a suitable pressure gauge, regulator and flow rate tube assembly has to be deviced & used while using the Hygrometers.  In some top end hygrometers all such arrangements are integral in the instrument & these hygrometers can be connected directly to breaker system pressure.
There are few important points one should know, while taking Dew Point measurements on  SF-6 gas system at sites and same are given as under. 
1.Conversion from PPMV to PPMW:

The molecular weight of SF-6 is 146.  The molecular weight of water vapour is 18.Therefore a given volume of SF-6 weights 146/18 i.e. 8.1 times the same volume of water vapour.  Therefore conversion factor for PPMV to PPMW is 8.1.  For example

200 PPMW is equal to 1620 PPMV OR 300 PPMW = 2430 PPMV.

2.The PPMV level is independent of pressure.  That is SF-6 gas having 300 PPMV
at 3 atm pressure would still have 300 PPMV at 1 atm pressure.

3.When converting/comparing PPMV/PPMW and dew point readings, the pressure
at which the readings are recorded is extremely important as Dew Point Temp
increases with increase in pressure for same PPMV moisture readings.  For example : SF-6 system having 300 PPMV at 22 psig gauge pressure will have dew point of – 230C but for same moisture reading of 300 PPMV, Dew Point would be – 120C at
87 psig gauge pressure.


4. Dew Point measurements vary from season to season.  In summer when Temperatures are high, Dew Point readings will be showing comparatively higher moisture content than in winter when Temperature are low.   This is because at higher temperatures the internal surfaces of various items enclosed in SF-6 breaker will tend to de sorb any moisture absorbed in them previously.
We, at Ovac Switchgear Services Pvt. Ltd, are now properly geared up & equipped to guide & record readings of moisture content or Dew Points on SF-6 GCBs of any make & type, at sites for our customers.  For this purpose, we have procured one modern & CGL recommended compact model type SADP mini, Hand held, Dew Point hygrometer of Alpha Systems U.K. make, recently, and same is complete with leather carrying case, pressure regulator, flow indicator meter & a pressure gauge, to enable us measure readings very accurately with consistency.
In the event Dew Point readings taken show higher moisture content than permissible we can
help customers, in undertaking to purify & dry the SF-6 gas at site with the help of our imported filter/purifier units, to the acceptable limits.  Our filter/purifier unit has recently proved its effectiveness, at Chandrapur sub station of MSEB when in case of three separate 400Kv GCBs, Dew Point readings of – 120C were improved to – 390C even with only ONE pass, to MSEB’s full satisfaction.
The recommended frequency of monitoring Dew Point in SF-6 breakers as per M/s.CGL, is three-year after Installation & once in a year thereafter, every year.  We request our customers to take full advantage of our facility, for the purpose of auditing, monitoring, as well as preventing avoidable failures in future. 
For the guidance, and ready reference, various useful data interpretation, control charts, specifications, definitions, etc., on subject matter, are enclosed along with this write up, and we hope same would prove useful to our valued customers.     

ANNEXURES
1.      Typical SF-6 Gas specifications IEC 376.

2.      Graph of PPMV against dew point & some basic definitions.

3.      Moisture Equivalents.

4.      Photo of Model 8 ADP mini. & features/customer list.

5.      Note on DEW POINT & Conversion (Page 4 ) (Clause 5.4)

6.      Only Figure 2 Flow chart for moisture determination & comparison.

7.      Fig. 3 - Seasonal variation of moisture content.

8.      Recommended Moisture Limits – Table – 2 (Page 13) + CGL, Limits.

9.      Construction of Filter/Purifier & gas analysis.