Wolfgang Degen, Dr. Hartmut Knobloch and Klaus Schuler , Siemens AG, Erlangen, Germany
Although the use of SF6 in high voltage engineering contributes only marginally to the greenhouse effect, responsible management and handling are nonetheless essential.
Sulphur hexafluoride (SF6) has been used in the power transmission and distribution sector for more than 40 years. An excellent insulating and arc quenching medium, SF6 permits powerful and compact system solutions like no other technology, particularly in the high-voltage sector. SF6 switchgear copes with the highest voltages and switching capacities in the smallest of spaces and has in many cases replaced oil and air as an insulating and arc extinguishing medium.
To ensure reliable operation throughout their service life, switching devices rely on top-quality components. With regard to possible SF6 emissions, the quality of enclosures including their materials, the machining process used, the mechanical design of seals and the seal material itself are all crucially important. The leak-tightness of gas-insulated switching devices throughout their service life is not only a prerequisite for the reliable operation of these devices but also essential to avoid SF6 losses.
Properties and alternatives
Modern high-voltage switchgear contains SF6 as an insulating and arc quenching medium. The switching capacity and dielectric properties of switching devices are crucially dependent on the SF6 gas density which is continuously controlled by a density monitor. Leaks through seals or the enclosure will automatically shut the devices down. Normal operating pressure (filling pressure at 20°C) for these switching devices is 0.45 to 0.7 MPa, corresponding to use in a minimum temperature range from -40 to -25 °C. In terms of arc quenching performance, SF6 is better than air by several powers of ten under the same conditions. The SF6 is stored in sealed enclosures for the service life of the switching device and filter material is used to ensure a constant gas quality in the switching device throughout this period until a first inspection is performed after 25 years.
SF6 is non-toxic, does not endanger soil or water, is non-flammable and has no ozone-destructive effects. However, SF6 is currently assumed to persist in the atmosphere for up to 3200 years and it has a greenhouse potential that is 22 200 times greater than that of CO2. Recent findings based on measurements suggest a lifetime of 650 years and a global warming potential 20 900 times higher than CO2. Although the impact of SF6 on the greenhouse effect is comparatively minimal at 0.2 per cent, with only a fraction of this due to its use in the electrotechnical industry, the 1997 world climate conference in Kyoto included it in the list of greenhouse gases to be monitored.
Figure 1. Shipment of a complete GIS bay with SF6 priming filling
Developments over the last 30 years have resulted in smaller switching devices with the same performance data leading to a substantial reduction in the amount of SF6 used.
Operating pressures mean that all gas compartments have to be considered as pressure vessels in mechanical design terms. Vessel design and material selection comply with the rules of national and international pressure vessel codes.
The sealing points between the individual enclosures and fittings have to be fabricated in a closed system and require particular care. For this reason, there should be as few sealing points as possible with the shortest possible seal length. Shipment of the largest possible preassembled and prefilled units results in a better quality installation than if these tasks are performed locally at the substation, and this in turn critically affects SF6 losses during commissioning and operation.
The structure encapsulating the gas insulated high-voltage switchgear also encloses the live high-voltage components and in many cases provides the basis for the mechanical structure as well. Enclosure design is influenced more by dielectric than by mechanical requirements, while state-of-the-art engineering criteria apply in the case of material selection and dimensioning. Most importantly, enclosures reliably retain the full complement of SF6 insulating and arc quenching medium, safeguard components from external influences and provide protection against contact with live parts.
They also minimize possible injury to operating personnel as a result of internal arcing faults, prevent emissions from electrical fields through their shielding effect and reduce magnetic fields as a function of the reverse current flowing through the enclosure.
Suitable enclosure materials are essentially welded steel and aluminium constructions as well as cast iron and cast aluminium. It was initially only possible to manufacture small enclosures out of ‘gastight cast aluminium’. ‘Gastight cast aluminium’ means that a fine-grained microstructure with a sufficiently small leakage rate is achieved at the time of casting without the need for subsequent ‘impregnation’ of the enclosure to ensure leakage integrity.
Figure 2. Equipment for handling SF6
Design of switchgear
Gas insulated switching devices account for most of the SF6 used in the power supply sector. According to the present state-of-the-art, systems feature both single and three-phase encapsulation. Three-phase encapsulated systems are typically de-ployed at the distribution level, while system types featuring single-phase encapsulation are used for the higher transmission voltage level.
Recent developments in the field of gas-insulated switching devices have been characterized by optimized material use and reduced resource exploitation, with further improvements in reliability from a previously already high level.
Crucial steps in this development were:
- Advances in switchgear technology allowing the number of interrupter units to be reduced despite increasing switching capacities.
- Technical advances in the casting and machining of aluminium leading to the development of smaller, design-optimized enclosures.
- Use of computer-aided production and test techniques with high quality standards.
- Use of integrated components with combined functions in a single gas compartment, e.g. disconnector and earth electrode.
- Use of intelligent monitoring and diagnostic tools to prolong service intervals and to avoid unnecessary maintenance work.
Small, compact devices are now commercially available as a result of these developments. These differ from the earlier systems as follows:
- Up to 85 per cent less SF6
- Delivery of fully sealed and tested switchbay units up to 245 kV
- Leakage rates reduced to
As a result of these improvements, the mean time between failures – MTBF – according to international fault statistics has now increased to between 400 and 1000 years, depending on the type of switching device.
The switchgear itself is divided into various gas compartments with gas monitoring of every compartment centralized or decentralized by means of a density monitor.
The design of installation compartments for static seals must be such that the sealing surfaces conform with the specified surface qualities. In the case of cast parts, the sealing surfaces must be absolutely pore-free after machining. Installation compartment dimensions are a function of the direction of pressure (generally from the inside out), the required degree of compression and the extent to which the seal is packed.
Reducing SF6 emissions
Leakage rates and handling losses are considered here. The term leakage rate is used to define the amount of SF6 lost from a closed, sealed and pressurized system. The leakage rate is given in percentage per year per gas compartment. With older types of switchgear, leakage rates of three per cent and more were usual. Based on developments over the last 30 years, leakage rates of
In order to minimize SF6 losses, the following rules must be observed in development labs and test bays, during routine testing and during switchgear operation and maintenance:
- SF6 may not be released to the atmosphere.
- Flexible tubes, which are disconnected from the gas compartment following operations to fill and remove SF6, must have self-sealing couplings at both ends.
- Prior to venting gas compartments, their SF6 contents must be extracted until a specified partial vacuum is reached. It is important to ensure that the gas compartment is leaktight.
- Suitable maintenance devices are required for removal of SF6.
- Contaminated gas must be reconditioned or returned to the manufacturer for recycling.
- Laboratory, installation and service personnel must be appropriately trained.
Current figures for SF6 mass balances published by ZVEI/VDN in Germany indicate even lower relative SF6 emissions. For example, manufacturer-related emissions from development, production and installation in Germany in the year 2000 amounted to only 2.5 per cent of total stocked SF6 in the high-voltage sector and 1.7 per cent in the medium-voltage sector, while SF6 losses due to leakages and user handling in 2000 accounted for only 0.9 per cent of total installed SF6 in the high- and 0.1 per cent in medium-voltage sectors respectively. In the latter case as a direct result of “sealed-for-life” technology.
This positive trend is not only attributed to a growing awareness and care in the management of SF6 among both manufacturers and users, but also to permanent improvements in switchgear construction and improved handling techniques designed to minimize SF6 emissions. These and other measures summarised by organisations like Capiel and Eurelectric underpin self-commitment declarations.
There is currently no alternative to SF6 for high-voltage switchgear without significant performance losses. Continued use of SF6 as an insulating and arc quenching gas is therefore essential to ensure the reliable operation of power supply networks. Even though the use of SF6 in high-voltage engineering contributes only marginally to the greenhouse effect, responsible management and handling are nonetheless essential. It will be possible to ensure the safe use of SF6 in switchgear while drastically cutting emissions despite further likely increases in SF6 consumption.