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SF6 Management and Alternative Gases
- Status:
- Complete
- Project Reference Number:
- NIA_NGET0163
- START DATE:
- END DATE:
Project summary
- Funding mechanism:
-
- Network Innovation Allowance
- Expenditure:
- £1,200,000
Leak Detection and Refilling
The aim of this work package is to develop automatic and more accurate leak detection techniques, for both outdoor and indoor use, to more efficiently manage GIS and SF6 filled equipment. Success in this area will allow utilities to move from reactive to proactive planning of resource by applying modern monitoring techniques to order equipment where bigger benefits would be delivered. By also developing improved and automated gas handling techniques, the level of SF6 used can be significantly reduced.
Leak Sealing and Repair
The aim of this work package is to identify ways of reducing leak rates in order to achieve a significant leak reduction through development of techniques for SF6 switchgear that are sustainable and long-lasting. This work package will investigate and develop new technologies and installation methods and test them on-site to analyse their performance.
SF6 Capture and Reuse
The aim of this work package is to identify a methodology for efficient gas capture which allows for gas cleansing and quality re-use of SF6. Initial work will involve the development and laboratory testing of the technology. If successful, the technology will be trialled on-site.
SF6 Alternatives
This work package is looking at two new gases both at different stage of development, G3, a novel gas composed of 4% NOVEC and 96% CO2, and CF3I mixtures. This programme is stage-gated and divided into three phases detailed below. This project will look at the first one.
Phase 1: Use of G3 on two-life gas insulated busbar sections terminating in cable-sealing ends. By trailing this initially in passive sections, implementation will be de-risked and practical experience gained.
Phase 2: Installation of G3 in a full substation on all assets except circuit breakers.
Phase 3: Trialling of G3 on active sections.
Phase 1
G3 is a new gas matches the dielectric strength of SF6 when operated at a higher pressure and reduces the global warming potential from 23,500 to 345.
This new gas has never been installed on life equipment and to do so, modification to the GIS wall sections and flanges need to be made. From an operational perspective, new gas-handling systems and parts need to be developed and tested. For this reason, in order to trial this new gas, the following steps will be taken:
1) Modification to the GIS-to-air bushings
2) Modification to the flanges
3) Installation of the two modified GIS busbar sections
4) Development and testing of gas-handling equipment and maintenance practices
5) Monitoring of the gas performance for a period of 3 years
The scope is limited to two busbar sections on a substation.
The second gas requires further development in order to understand whether it may be of use to the industry as a 400kV insulator based on CF3I gas. In order to develop this gas further, we are supporting an iCASE studentship to look at the properties of this gas and mixtures in gas insulated busbars on a laboratory demonstrator.
Benefits
Leak Detection and Refilling
A successful outcome of this work package would see would see the three deliverables, described in the objectives above, successfully developed and tested.
Leak Sealing and Repair
If successful, this work package will develop a technology and/or compound capable of being used for large gases and for both porcelain and metallic interfaces.
SF6 Capture and Reuse
If successful, this work package will deliver a technology capable of capturing any leaking gas, filtering and if required, refilling the tank.
SF6 Alternatives – Phase 1
A successful project will see the modifications to the GIS equipment being performed without impacting the reliability of the whole substation as well as see the introduction of G3 into the system for a period of three years without faults. It will also see the development of a CF3I mixture capable of being used in transmission applications.
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Learnings
Outcomes
Most of the learning from the SF6 alternatives element of this project will be identified with the ongoing monitoring of the busbars which are now commissioned and connected to the HV system. National Grid will use the results of the non-invasive monitoring (techniques like PD and gas tests) of the equipment to inform its decision on the future use of G3.
The main advantage of CF3I gas mixtures compared with SF6 is its significantly lower global warming potential. The dielectric properties of CF3I gas mixtures were shown to be adequate in comparison with SF6. Both AC discharge thresholds and impulse breakdown were found very suitable to insulation applications. However, published literature points towards issues related to toxicity and health impact. These properties are not very well studied and require further investigation.
Also, the research findings to date from other investigators indicate that the arc-quenching capability of CF3I is lower than that of SF6.
High voltage testing of a leak sealant material in accordance with IEC 60587 showed that the material passed with Class 1A 4.5. This means the material performed well under fault conditions, and the tests showed how it would perform in worst case scenarios. As a result, the material is a good fit for joints with metal-to-metal configurations.
Recommendations for further work
More large-scale testing is needed for the electrical insulation properties of the CF3I gas mixtures, in particular 30%/70% CF3I/CO2 gas mixture.
There are still questions as to the suitability of the leak sealing material in use for composite joints as a result of the voids identified in the material resulting from the mixing and curing process. This would need further investigation for composite joints such as a GIS to a bushing connection as the enhanced electric field at the location could potentially cause partial discharge in the voids and reduce the effectiveness of the material.
Lessons Learnt
2015/16 Lessons Learnt
A number of learning points have already been identified:
- although the equipment on site hasn’t yet been HV tested or energised, National Grid is confident that G3 will meet all the necessary performance criteria for its unrestricted application to the UK transmission system (for indoor and outdoor use). The equipment has been fully tested in accordance with the existing framework of standards for high voltage switchgear with no problems identified.
- handling a gas mixture is more complex than handling a single gas fluid, but the solutions and hardware developed and demonstrated within this project indicate that this complexity won’t prevent it from being widely adopted.
- safety impacts and precautions in the event of (i) a high-volume leakage of the “clean” gas mixture and (ii) an internal fault that causes arcing by-products to be ejected have been assessed. In both cases the risks and safety measures are identical in scope to those for SF6-filled equipment.
- the HSE exempted National Grid from the PSSR in line with the existing exemption for SF6-filled equipment, but a broader exemption or legislative change may be more appropriate in the long term.
2016/17 Lessons Learnt
During 2016/17 the focus of the Sellindge project has been primarily upon site delivery, construction, testing and commissioning aspects of the G3 solution. We have demonstrated that, with only minor changes and modification, SF6 free solutions can be delivered within the existing delivery framework for gas insulated equipment. The equipment will continue to be monitored over the next few years.
2017/18 Lessons Learnt
From the Cardiff investigations, the potential of CF3I gas and its mixtures with CO2, as an insulating gas in GIS, have been demonstrated through dielectric/breakdown testing. Furthermore, the by-products from repeated electrical discharges have been analysed using the gas chromatography mass spectrometer (GCMS) gas analysis system and encouraging results have been seen. Measurements of the CF3I gas properties under PD, corona, and effect of particles are on-going. These measured results will be compared with the current published data for SF6 properties.
2018/19 Lessons Learnt
The by-products of CF3I gas mixtures with CO2 have been extensively studied and quantified under high voltage impulse breakdowns and AC corona/partial discharges. Also, the effect of discharges in the gas mixture on the test electrodes has been determined using microscopic analysis. The presence of Iodine following breakdown events has been detected. If not absorbed properly, this Iodine deposit could weaken the dielectric strength of the gas. Absorbent or adsorbent materials need to be explored to remedy this shortfall.
2019/20 Lessons Learnt
No new lessons
2020/2021 Lessons Learnt
The use of simulations in the project showed that there is little effect of the electric field on the external parts of a GIS joint and therefore further modelling of the GIS with the electric field was not needed. The modelling, however, showed that the thermal outputs should be considered.
Optical microscopy of the leak sealing material showed voids within the material resulting from the mixing and curing process. These have little to no effect in a metal-to-metal joint due to the coaxial arrangement and reduced electric field external to the joint.
Dissemination
The Sellindge project was presented at the Low Carbon Networks and Innovation Conference in October 2016 and December 2017.
A paper was also published at the CIGRE 2016 conference and subsequently re-published in a peer-reviewed journal “CIGRE Science & Engineering” Issue No. 7, February 2017, pp 102-108. This is available online.
A site visit to Sellindge was arranged for SSE to discuss G3 and if it could be adopted at their sites.
In October 2017, we attended the CIGRE working group B3.45 meeting (Application of non SF6 gases or mixtures in medium voltage and high voltage gas insulated switchgear) where the main discussion was around how the solution could be adapted to suit other utilities.
The work on CF3I was presented to the Power Networks Research Academy (PNRA) on 18th January 2018. During 2018/19 the CF3I work was further disseminated through CIGRE working group D1.67.