• Electrical India
  • Dec 5, 2014

SF6 Gas
Handling Techniques in Electrical Utilities
Upcoming trends in SF6 gas maintenance and management

SF6 gas has been classified as a potent Green House Gas by environmental and climate change organizations. Releasing of SF6 in atmosphere will be aggravating the current scenario of global warming and climate change. However, SF6 is also a popular dielectric used in High Voltage switchgears, and contrary to the popular notion, SF6 does decompose and lose its dielectric strength. This requires quality checks and regeneration of SF6 in a timely manner.

- Hrushaabh P Mishra


 SF6 gas is an essential gas for electrical equipments such as circuit breakers, transformers, CTs etc. Sulphur Hexa-Fluoride or SF6 is a non-toxic, inert, insulating and a cooling gas of high dielectric strength and thermal stability. It is particularly suited for the following applications.

  • In both High Voltage and Medium Voltage Power Circuit Breakers: SF6’s excellent arc quenching capacity is put to use here. 
  • In High Voltage Cable: SF6 gas insulated transmission cables can carry higher capacity of power. 
  • Power Transformers: On account of their high operational safety, SF6 gas filled transformers are employed in hazardous operational areas such as mining. Their light weight, compact design & low noise levels are decisive advantages.
  • Particle accelerators, X-ray equipments, and UHF transmission.

SF6: Chemical Properties

  SF6 is a very stable and inert gas, colourless and odourless, non-toxic and non-flammable and insoluble in water. It is one of the least reactive
of all known gases and in normal conditions, it attacks no known substance with which it will come in contact.

Physical Properties

  SF6 is 5 times heavier than air and is one of the heaviest known man-made gases. Due to leaking of SF6 in air, it will mix insufficiently with air, and will retain itself near-to-ground levels. SF6 gas cannot be separated easily from by air. SF6 exhibits 2-5 times better heat transfer properties than air.

Electrical Properties

  Electronegativity is an attribute of a molecule to attract and bind electrons towards itself and therb not allowing the build- up of electron avalanche. SF6 is highly electronegative. It has a pronounced tendency to bind free electrons forming heavy ions with low mobility making the development of electro avalanches very difficult. Corona, resulting due to high potential electric field present in SF6 gas, does not lead to increase in conductivity within SF6 gas. Additionally, SF6 gas is an excellent recombination gas. It has been observed that SF6 breaks down into free sulphur and fluorine atoms from 150OC onwards. However, as soon as the ambient temperature is attained, the free atoms of sulphur and fluorine once again recombine. During arcing of contacts, the released energy is utilized for dissociating the SF6 gas molecules, without letting to further build temperature inside the compartment. 

SF6 as Greenhouse Gas

  Man made pollutants, due to extensive human activity, have been harming the atmosphere since the industrial revolution. Two prominent outcomes of these pollutants are:

  • Stratospheric Ozone Depletion
  • Average Global Temperature Increase.

  Due to the reflectivity of CO2, H2O, O3, by default present in the atmosphere, and the increasing presence of man-made gases such as CO2, N2O (intensive agriculture), and CFCs (spray propellants & refrigerants), the IR energy being irradiated from the earth’s surface, fails to escape to the atmosphere and it eventually leads to increase in earth’s temperature. With a GWP of 23,900 times that of CO2, SF6 has been classified as a potent GHG by Inter-governmental Panel on Climate Change. The proportion of SF6 has been increasing at an annual rate of 7%, mostly because of its use in electric utilities. Once released to atmosphere, it has an extremely large lifetime of 800-3200 years. In Europe, SF6 gas falls under F-Gas directive which bans its usage for all applications except switchgears. The consequences of Global Warming are far reaching and some of them have been listed as below:

  • Unprecedented combinations of climate change along with associated disturbances such as floodings, drought, wildfires, insects, ocean acidification.
  • Approximately 20-30% of plant and animal species assessed are likely to be at increased risk of extinction if increase in global average temperature exceeds 1.5 to 2.5OC.
  • In dry and tropical regions, like India, crop productivity is projected to decrease for eben small local temperature increase (1-2OC) which would increase the risk of hunger.
  • Coastal territories are to be exposed to increasing risks due to sea level rise, leading to increased flooding.

ISO14064

  In line with IPCC directive & Kyoto Protocol, The ISO 14064 standard, published in 2006, revolves around environmental management and quantifying, monitoring, reporting and verifying greenhouse gas emissions. The standard is published in three parts.

  • ISO 14064-1: 2006 specifies principles and requirements at the organisation level for quantification and reporting of greenhouse gas emissions and removals. It includes requirements for the design, development, management, reporting and verification of an organisation’s greenhouse gas inventory. 
  • ISO 14064-2:2006 specifies principles and requirements and provides guidance at the project level for quantification, monitoring and reporting of activities intended to cause GHGs emission reductions or removal enhancements. It includes requirements for planning a GHG project, identifying and selecting GHG sources, sinks and reservoirs relevant to the project and baseline scenarios, monitoring, quantifying, documenting and reporting GHG project performance and managing data quality. 
  • ISO 14064-3:2006 specifies principles and requirements and provides guidance for those conducting and managing the validation or verification of GHG assertions. It can be applied to organisational or GHG project quantification including GHG quantification, monitoring and reporting carried out in accordance with ISO 14064-1 and ISO 14064-2.

Origin of Contamination

Contamination during handling

  Mishandling during filling and recovering of SF6 are the major causes of contamination of SF6 with moisture and air. The residual air present in pipework and valves, air leaking in through sealings, and by the residual air left inside the enclosure after evacuation and before filling with SF6 go unnoticed and eventually dilute the purity of SF6. The ingress of air and gas-entrained dust can be minimised by:

  • Appropriate Design work of pipes & valves
  • Appropriate handling procedures
  • Careful evacuation of the air from the enclosure before filling with SF6 (a residual air pressure of 1 millibar is recommended).

Contamination by leakage

  The diffusion of air and humidity might happen from the outside to within the compartment because the partial pressure of air and water outside the enclosure is higher than that inside. The main leakage paths are enclosure porosity, sealings of mechanically moving transmission elements, and O-ring sealings. SF6 half-empty cylinders over a period of time may become corroded and might start leaking. The pressure within them may become destabilized, which might cause a potential burst too.

Contamination by desorption

  During the assembly of the equipment itself, accumulation of Humidity and gases within the inner surfaces of the equipment or in the bulk materials are released within SF6 during its normal operation and at elevated internal temperature. Polymeric materials such as epoxy are highly hygroscopic and have poor moisture retention capability at increased temperatures. Adsorbers which have not been properly handled may contain both humidity and adsorbed SF6 decomposition products which can be released during evacuation or at elevated temperature. The quantities of the desorbed substances are difficult to estimate because they depend on the specific materials employed, production methods, quality control and the assembling & maintenance procedures.

Decomposition by Electrical Discharges

  SF6 is partially decomposed by electrical discharges which can be grouped into four major types namely:

  • Partial Discharges of the corona type
  • Spark Discharges
  • Switching arc
  • Failure Arcs.

Partial Discharge

  PD is defined as a localised breakdown. PD activity is initiated within SF6 gas due to the presence of moisture and air, protrusions or abrasions on the insulator surface, freely floating particles within the gas. Corona is defined as a type of Partial Discharge that occurs in gaseous medium around the conductor. It is the ionisation of the surrounding gas and usually develops around the freely floating electrodes. Corona is an electron dominant process. The electron temperature is much higher than the gas temperature, since the gas exists under non-equilibrium conditions. The mean energy of the electrons in the corona is limited to 5 to 10 eV. This already exceeds the SF6 bond energy between SF5-F of 3.5-4 eV and the electron impact will dominate the decomposition process. The electron impact dissociation process can lead to the following reactions:

e + SF6 -------------> SFX + (6-x)F, x <5
e + SFX -------------> SFX + F

  Multi-step dissociation leads to the formation of lower fluorides of Sulphur such as SF4 & SF3. In the absence of contaminants in the gas or on the surface of insulators, the products of decomposition of SF6 quickly recombine though relatively rapid process to explain the thermal conversion of SF4 + F2 into SF6. However, in the process of oxygen and moisture, the recombination process can be interfered with by the reaction between the lower fluorides of sulphur and the contaminants to form sulphur oxyfluoride, HF and metallic fluoride. Some of the reactions can be expressed as follows:

SF5 + OH ---------> SOF4 + HF
SF5 + O ------------>SOF4 + F
SF4 + O ------------> SOF2 + 2F
SF2 + O ------------> SOF2
SF4 + OH ---------> SOF2 + HF + F
SF3 + O2 ----------> SO2F2 + HF
SF4 + H2O ----------> SOF2 + 2HF
SOF2 + H2O --------> SO2 + 2HF
SOF4 + H2O ---------> SO2F2 + 2HF

  Outside the corona discharge region, slower reactions between the long-lived lower sulphur fluorides such as SF4 and stable oxyfluoride with the contaminants will further lead to the formation of other types of compounds such as SO2, SO2F2 and SOF2.

Spark Discharges

  Spark Discharges may occur at large scale insulation defects such as floating conductors and during disconnector switching operation. The decomposition products generated are of the same kind as in corona discharges but their quantitative generation rates and compositions are different.

Fig. 1: Traces of flashover on a busbar

Fig. 1a: Surface carbon deposition due to failure arcs

Switching Arcs

  Switching arcs occur during the load breaking operation of power circuit breakers. At the centre of high current breaking arc, the temperature rises to be as high as 20000 K. As the arc cools, recombination of sulphur and fluorine to form SF6 occurs rapidly, often in the microseconds scale. In the presence of oxygen, H2O and metal vapour, resulting of electrode heating, the recombination process is altered which leads to the formation of various arc by-products. The high current flow in these arcs leads to substantial erosion of the contacts and insulation material by the hot arc. The main cause for SF6 decomposition is the reaction of these erosion products with the fragments of thermally dissociated SF6 and other trace gases such as moisture and oxygen. The most important of these reactions can be expressed by the below three formulae:

Cu + SF6 ---> CuF2 + SF4
W + 3 SF6 ---> WF6 + 3 SF4
CF2 + SF6 ---> CF4 + SF4

  The first two reactions are associated with eroded material from the arcing contacts for which copper tungsten (Cu-W) is normally used. The last reaction is due to eroded PTFE (a CF2 polymer), which is employed in most switchgear to contain the arc.

Failure Arcs

  Failure Arcs are a result of insulation breakdown or switchgear interruption failure and occur extremely rarely. In these events, the arc burns mainly between metallic minerals, which are not designed for arcing such as aluminium, copper and steel. These materials have relatively high arc erosion rates.

Mechanical generation of dust particles

  Metal dust particles may be generated by mechanical friction of metal surfaces. In properly designed equipment, these particles usually fall into areas where they have no effect on the insulation integrity of the installation. If however they fall into the area of high electric field stress such as an insulating barrier, they may cause tracking on the insulator surface and flashover.

SF6 Impurities

  The impurities resulting in the SF6 gas are Air, Moisture, HF, CF4, and SO2. Permissible limits have been laid for the aforementioned impurities according to IEC60376 & IEC60480. IEC60376 lays down the guidelines for checking brand new SF6 gas and IEC60480 lays down the guidelines for checking used SF6 gas.

Fig. 2: HF dust accumulated at the bottom of HV contact plate

Effects of contamination

  A functional deterioration of equipment by SF6 contaminants can, in a general sense, be viewed in the following six respects:

  • Health risk
  • Corrosion
  • Insulation Performance of gas gaps
  • Insulation Performance of insulator surfaces
  • Switching capability (for switchgear only) 
  • Heat transfer.

Health Risk

  SO2 and HF constitute a health risk, whereby they might cause irritation effect in eyes and nose and lead to difficulty in breathing. HF, one of the highly corrosive gases, causes severe skin burns if it comes in contact with the skin of human personnel.

Corrosion

  HF, corroding the contacts and the internal metallic parts of a gas compartment will lead to the formation of metallic fluorides in form of dust particles. Reaction of SO2 and HF with insulator surfaces leads to pitting, and decrease in surface resistivity. This results in surface tracking and increased conductivity.

Insulation Performance

  Air and CF4 are chemically inert gases. SF6 is usually diluted with these two gases in order to reduce the release of SF6 & the associated environmental impact. However, the dielectric strength of a mixture of SF6 with Air or with CF4 is always lesser leaser than that of pure SF6. SF6 mixture with air/ CF4 has greater arcing time and the interruption derating occurs. If the performance has to be maintained, then the pressure will have to be risen or the redesign of the arc extinguishing zone would need to be done.

Heat Transfer

Air and CF4

  Modern Designs however avoid the use of corrosion sensitive materials. Some of the contaminants are chemically inert such as Air, CF4 and moisture may affect the gas insulation capability and the circuit breaker switching performance, if present in too high concentrations. They may also have an influence on the convective heat transfer by the insulating gas. The main conducting liquid contaminant is water condensing from moisture in the form of water droplets or films. As water has an extremely high dielectric constant and high electric conductivity, it causes local field enhancements at droplets and conducting surface films along insulators both of which deteriorate insulation performance . Moisture is mainly introduced by desorption from surfaces and from the bulk of polymers. Its condensation is controlled by the absolute moisture which is expressed as partial water vapour pressure pH2O. Non-conducting solid decomposition products are generated from arc eroded metals by reaction with dissociated SF6. They mainly consist of copper fluoride CuF2, tungsten oxide WO3, the tungsten oxyfluorides WO2F2 and WOF4 originating from switchgear contact erosion and Aluminium Fluoride AlF3 in case of internal arcing. They are non-critical for insulation as long as they are not exposed to excessive moisture. Conducting solid contaminants such as carbon and metal dust may become critical when deposited on field exposed insulator surfaces as conducing layer. Carbon may be generated by carbonisation of polymeric materials. Metallic dust particles generated by mechanical friction may be transported by gas flow.

Infra-Red Technique based SF6 gas Analysis

  Infra-red based analyzers require the smallest amount of gas for analysis. It gives the readings at the fastest rate with no cross interference or contamination. Infrared based SF6 gas analyzers are preferred today since they differ from the conventional sensors on following grounds:

Removal of SF6 impurities

  While SF6 itself is not consumed in significant quantities, its performance can degrade due to contamination by air, moisture, decomposition products. It is desirable both from an ecological & economic point of view, to keep SF6 at a low contamination level by careful handling so that it can be reclaimed on site many times. SF6 continually should be reused during equipment development, product testing, commissioning, maintenance and repair and decommissioning where the criteria of IEC60480 can be achieved. It thus goes through a continuous cycle of reuse. Such a systematic re-use of SF6 requires that the gas be kept at the stated quality level at which it can perform its functions. With properly maintained reclaiming equipment, humidity and reactive decomposition products can almost always be removed on-site so that non-reusable gas can be transported. In the rare case that the gas cannot be purified sufficiently on-site to achieve the reclaimed criteria it still has reuse potential when treated by a specialised processing company. This allows it to be rendered reusable in the majority of cases & some SF6 producers are already offering such a purification service. Only a very small fraction of the residue will have to be processed for the final disposal in an environmentally compatible way.

Reclaiming Equipment

  Gas reclaimers have been used successfully since the introduction of SF6 technology. They are commercially available in a variety of sizes, gas processing capacities & storage capacities & range from units that can be hand carried to larger trailer mounted systems. The appropriate type & size of the reclaimer should be chosen according to the gas capacity to be handled. 

Block diagram of a reclaimer

  The basic functional schemes of a reclaimer are as follows:

Filters

  The various types of filters used in reclaiming carts are as follows:

Vacuum Pump

  The vacuum pump module is used to extract air from SF6 insulated equipment and associated piping prior to refilling with SF6 and for dehydration (removal of residual moisture). It is also used to remove air from various sections of the gas processing system itself, eg, after maintenance work.

SF6 Vacuum Compressor

  The SF6 vacuum compressor module is used to recover SF6 from gas insulated equipment and to assist the series connected SF6 piston compressor.

Gas/Humidity Filter

  Filter elements should be adequately sized to remove moisture, gas by-products and particles larger than 1 micron in size (as a second means of trapping and larger particles which may have been transmitted via a non efficient particle filter).

Storage Module

  The SF6 storage module is used to store SF6 processed by the reclaimer. It must have enough capacity to store the amount of SF6 to be recovered. It can be an integral part of reclaimer or an external item. If used for liquid storage of SF6, it should be rated for a pressure of atleast 50 bar.

Refilling

  The reclaimer must have the provision to allow refilling of gas, from the storage vessel into an electrical equipment, the refilling procedure requirements will vary according to storage method employed.

Hose Connection

  Hose connections should be self sealing to prevent air and moisture from entering the gas reclaiming equipment. As the equipment will often be left in a state of vacuum these SF6 valves need to be pressure and vacuum tight.

Pipe Work

  Gas piping & pipe unions used must be of high quality & preferably use a metal to metal re-usable sealing system, proven in its performance with SF6 & its decomposition by products. The following characteristics are essential: Pressure and vacuum tight; Vibration proof; Re-usable (indefinite refitting possible) & Temperature change resistant.

  All pipe-work should be of copper tubing silver soldered to tube unions and having inline covering of PTFE (Polytetrafluoroethylene). All components (gauges,valves, filters, etc.) should be securely mounted to the frame of the gas cart, such that pipe-work does not have to support them. This prevents stress cracks causing either gas losses or inadvertent gas mixtures. Heavy components (i.e. compressors and vacuum pumps) should be "shock-proof" mounted and be connected to the fixed pipe-work via flexible connections. Equipment used to process SF6 (i.e. compressors) should be oil less, of gas tight construction and should not contain any internal components that can be corroded by decomposition products (e.g. galvanised metal).

Conclusion

  SF6 gas can be reused for electricity utility field purposes by periodic checking of the quality of the gas with a NDIR based instrument. If impurities are found to be beyond permissible limts, one has to do purification of gas for reusing it.


Author is Technology Resource Head with Syselec Technologies Pvt. Ltd. He is a Gold Medallist in BTech Electrical Engineering from Government College of Engineering, Amravati, M.S. He has been specializing in SF6 Gas Testing services for reputed utilities the past 4 years. His other areas of interests include Partial Discharge testing for GIS, Power Transformer Residual Life Assessment and Switchgear protection testing.