Miracle gas or not?
There are many published discussions and reports on various aspects of SF6 gas(sulphur hexafluoride) usage in electrical equipment. Most of them are based on facts and researches, but some are not.
34 Questions And Answers To Break the Myth About SF6 Gas In Electrical Equipment
Let’s try to answer the 34 questions and break the myth about this ‘miracle’ gas.
Note that most of answers are based on CAPIEL (Coordinating Committee for the Associations of Manufacturers of Industrial Electrical Switchgear and Controlgear in the European Union) researches and related IEC standards as well.
Your comment is highly appreciated!
By Edvard | November, 10th 2014
21. How much SF6 (quantified in kg) can escape due to “normal” leakage?
This depends on the filling quantity, which depends on the rating and design of the equipment (volume and pressure). For HV switchgear the emission factor ranges from about 0.1% per year to 0,5% (0,5% per year is the maximum acceptable leakage rate according to IEC 62271-203)
For sealed for life MV equipment a range below 0,1 % per year is common. For example, a 3 kg filling quantity (RMU) results in a calculated loss of 3 g per year.
22. How high is the MAC (Maximum allowable working environment concentration) for pure SF6 in the substation and how hazardous is pure SF6?
It is generally recommended that the maximum concentration of SF6 in the working environment should be kept lower than 1000 μl/l (*). This is the value accepted for a full time (8 h/day, 5 day/week) work schedule. This value is not related to toxicity, but an established limit for all non-toxic gases which are not normally present in the atmosphere.
Therefore, this limit does not mean that higher SF6 concentrations pose any toxic hazard. According to Clause 7.1 of IEC 60480: “In principle, a mixture of 20% of oxygen and 80% SF6 can be inhaled without adverse effect. Concentrations above 20% would cause suffocation due to lack of oxygen.
(*) TRGS 900, Technische Regeln für Gefahrstoffe
23. What decomposition products are created in the case of internal arc faults, and in what quantities?
Gaseous and dusty by-products will be generated. See IEC 60480, Table 1 and/or CIGRE Report Electra 1991 (“Handling of SF6 and its decomposition products in GIS”, Table 2 “Rough characterisation of the major decomposition products resulting from different sources”).
The decomposition products depend on the type of equipment and its service history; the quantities depend on energy (voltage, current, time) and the type of the equipment.
24. How hazardous are the decomposition products?
See IEC 61634, Annex C: “Release of SF6 from switchgear and control gear – potential effects on health”.
In this Annex a calculation method is given to evaluate the amount of by-products with toxic characteristics generated under different conditions. It is, then, possible to evaluate potential toxic hazard taking into account the volume of the switchgear room.
Calculations show that, in practice, only in case of an internal arc with a massive emission of heavily arced gas a real hazard is created. Evacuation and ventilation is therefore compulsory in such an event.
25. What has to be done after an arc fault in the switchgear?
In such accidental cases caution must be taken. If the encapsulation has been damaged some compounds with toxic characteristics may be present, generated not only from decomposition of SF6 but also from other sources (e.g. burning paintings, vapours of copper, etc) can be present.
Therefore, in all cases, evacuation of the switchgear room is the first measure to be taken irrespective the switchgear contains SF6 or not. See IEC 61634 sub-clause 5.3: “Abnormal release due to internal fault”.
26. Does a (passive or active) ventilation system have to be installed in the switchgear room or cable basement?
Buildings containing SF6-filled indoor equipment should be provided with ventilation; natural ventilation would normally be adequate to prevent the accumulation of SF6 released due to leakage (see IEC 61634, sub-clause 3.4: “Safety of personnel” and IEC 61936-1). Type and extent of required measures depend upon location of the room, the accessibility, and the ratio of gas to room volume.
27. What do I have to do when a GIS installation is damaged?
For example hole drilled in encapsulation or transport damage such as a panel dropped and cast resin broken) and SF6 escapes or when my GIS develops an abnormal leak?
Appropriate corrective action should be to deal with the leakage. If the equipment is in service and the leakage is high, it must be de-energised, in accordance with the organisation’s operational procedures. Loss of gas must be minimised by following the organisation’s procedures and using the services/recommendations of the manufacturer or qualified service organisation as appropriate.
The technical integrity of the equipment will need to be verified after such an occurrence and appropriate corrective actions taken by authorised personnel before refilling of equipment or placing in service.
28. Under which conditions needs SF6 gas in gas-insulated switchgear to be replaced?
Which are the parameters to be checked e.g. (concentration, dew point, decomposition products) and what are the related acceptable limits?
Normally the gas remains until disassembly. During a maintenance operation requiring the evacuation of the gas, it should be analysed. Guidance on how to proceed then is given in IEC 60480.
29. How do I evacuate and fill the system?
See IEC 61634, CIGRE Report 2004 (“Practical SF6 handling instructions”). Please, refer also to the instruction manual of your equipment.
The work should be done in conjunction with the manufacturer or qualified service company in cases where an organisation’s own personnel is not trained appropriately.
30. How much SF6 gas is in my switchgear? Where do I find this information?
On the nameplate or in the operating manual. For older equipment please ask your manufacturer.
31. Does SF6 have to be disposed of when moist?
No, it is possible to dry the gas; moisture can be reduced to acceptable levels by adsorption; material such as alumina, soda lime, molecular sieves or mixturesthereof are suitable for this purpose (see also IEC 61634, Annex B.3: “Measures for the removal of SF6 decomposition products”). Maximum tolerable moisture levels for re-use can be taken from IEC 60480, Annex A.
32. What do I have to do when I came in contact with decomposed SF6?
See IEC 61634, Annex E: “General safety recommendations, equipment for personal protection and first aid”. Normally only trained and qualified personnel should deal with this and hence be aware of the necessary precautions and actions.
IEC 61634, Annex E: General safety recommendations, equipment for personal protection and first aid
For medium-voltage switchgear and controlgear using sealed pressure systems, the contents of this annex are applicable only during end-of-life treatment or in the very unlikely event of an abnormal release. For other types of equipment, information in this annex is provided for use in situations where workers have to make contact with SF6 decomposition products.
Such situations include:
Maintenance or any other activity involving opening the SF6-filled enclosures of equipment which has been in service;
Restorative activity after an internal fault or external fire provoking opening of the enclosure.
Experience over more than 25 years in working environments where contaminated gas is handled regularly has shown that personnel are unlikely to suffer adverse effects to their health, as long as they are suitably trained and equipped as indicated in this report and as recommended in the manufacturers’ instructions.
33. What environmental and safety at work aspects have to be taken into account?
See IEC 61634 [8], clause 4: “Handling of used SF6”.
The need to handle used SF6 arises where:
Topping up of the SF6 in closed pressure systems is carried out;
The gas has to be removed from an enclosure to allow maintenance, repair or exten-sion to be carried out;
The gas has been wholly or partially expelled due to an abnormal release;
The gas has to be removed at the end of the life of an item of equipment;
Samples of the gas must be obtained or the gas pressure measured through tempo-rary connection of measuring apparatus.
Situations 1 and 1 arise mainly with respect to high-voltage equipment and may arise with medium-voltage GIS equipment in particular if it is required to add further equipment to an existing switchboard. They do not arise with equipment using sealed pressure systems.
34. What has to be observed for cleaning of the switchgear room after an internal fault with emission of decomposed gas?
See IEC 61634, sub-clause 5.3/5.3.3:
Abnormal release due to internal fault (Indoor installations”, and national requirements.)
An internal fault occurs when abnormal arcing is initiated inside a switchgear and controlgear enclosure.
In certain types of equipment, particularly metal-enclosed medium-voltage switchboards, air insulation is used for the busbars between cubicles and around cable connections and SF6 is present only within switching chambers. In this case an internal fault could occur within the switchboard but outside the switching chamber, so that no SF6 is released.
An internal fault is a very rare occurrence but cannot be completely disregarded.
It can occur as a result of:
A defect in the insulation system;
A mechanical defect leading to a disturbance of the electric field distribution inside the equipment;
The mal-operation of part of a switching device due to faulty assembly, components or malfunction or misuse of interlocks.
An internal fault will cause an increase of pressure inside the enclosure, the effects of which will depend upon circumstances. The pressure rise is caused by the transfer of the electrical energy from the arc into the gas. The increase in pressure will depend upon the value of the arc current, the arc voltage, the arc duration and the volume of the enclosure in which the arc has developed.
Following an internal fault leading to pressure relief or enclosure burn-through, the SF6 and much of any solid decomposition products (powders) will have been expelled from the SF6 enclosure.
References:
CAPIEL (Coordinating Committee for the Associations of Manufacturers of Industrial Electrical Switchgear and Controlgear in the European Union) – Frequently asked Questions (FAQ) and Answers on SF6
IEC 61634 High-voltage switchgear and controlgear – Use and handling of sulphur hexafluoride (SF6) in high-voltage switchgear and controlgear
IEC 60480 Guidelines for the checking and treatment of sulphur hexafluoride (SF6) taken from electrical equipment and specification for its re-use
-The End-
For SF6 GIS equipment,it is necessary to use an Ultra High Sensitivity SF6 Trace Gas Leak Detector to find the leak and fault point. D-Industrial Q200, as the quantitative leak detector, as the main detection instrument of SF6 gas leakage of Circuit Breaker, GIS, Transformer and other SF6 equipment, it can detect and alarm SF6 gas leakage and track and locate the leakage quickly. It Is of high sensitivity (10-8 mL/S (0.01 PPM).