Learnings

Outcomes

A gas research facility has been developed with the capability to accurately determine the gas mixture concentration, conduct high-voltage testing and to recycle and reuse all test gases, which facilitates the comprehensive gas characterisation required for investigation of SF6 alternatives.

Involved in the development of this was the construction of a test object and the development of the dielectric withstand tests as detailed in the progress section of this report.

A mixture ratio of 20/80% NOVEC™ 4710/CO2 was retro-filled in the full-scale demonstrator and successfully passed the type tests for 420 kV and 550 kV BIL as per IEC standards. Results have shown that a 20/80% NOVEC™ 4710/CO2 mixture can have comparable electrical performance with pure SF6 in this design of GIB – specifically GE TB155.  

The work will inform a more in-depth investigation of full-scale re-testing of selected in-service GIS busbar designs as it has shown this to be technically feasible.

Recommendations for further work

The next step for this work could be to conduct a long-term energisation test campaign at the Deeside Centre for Innovation where electrical and optical monitoring may be set up to monitor performance.  This needs to be evaluated against the wider program of areas for investigation as part of the strategic aim of removing SF6 from the network.

Lessons Learnt

2017/18 Lessons Learnt

The following conclusions were made during the first year of the project:

• In the tested geometry, the 20% NOVEC™ 4710 & 80% CO2 mixture has the equivalent dielectric performance to SF6
• In the tested geometry, the 16% NOVEC™ 4710 & 84% CO2 mixture has equivalent AC and SI withstand to SF6 but slightly reduced LI withstand
• In the tested geometry, the 10% NOVEC™ 4710 & 90% CO2 mixture has equivalent AC withstand, slightly reduced SI withstand and significantly reduced LI withstand compared to SF6
• Partial discharge signatures of NOVEC™ mixtures are substantially comparable to those of SF6

2018/19 Lessons Learnt

• A 20/80% mix of Novec/CO2 passed the 420 kV and 550 kV withstand tests in accordance to IEC standards (60060-1, 62271-1, 62271-203 and 62271-204) in the GIS test object designed for SF6
• A small-scale coaxial cylindrical prototype was tested for both SF6 and 20% NOVEC™ 4710 & 80% CO2 mixture:
• Initial positive lightning impulse tests shown that Novec/CO2 mix has slightly lower breakdown results than SF6.
• Initial negative lightning impulse tests shown that Novec/CO2 mix has comparable breakdown results than SF6.

2019/20 Lessons Learnt

• For LI breakdowns, 20/80% mix of Novec/CO2 has a comparable breakdown performance to SF6 in quasi-uniform field configuration. However, SF6 outperforms the Novec/CO2 mixture under non-uniform field configurations.
• Non-standard type tests were carried out using impulse waveforms for a 20/80% mix of Novec/CO2 where the number of impulse applications was increased from 15, as per IEC standards, to 30. The gas mixture passed the LI and SI tests of both polarities with increased impulse applications at the specified 420 kV BIL.
• EPDM gaskets were analysed with a NOVEC™ 4710 mixture. The gas purity is not affected by the elastomer material even when exposed to elevated temperatures at 105°C, which only resulted in less than 0.2% reduction in gas purity after 112 days.

2020/21 Lessons Learnt

• The pressure for the 20/80% mixture may be reduced to 4 bar while still being able to achieve LI and SI requirements.  This lowers the temperature at which the mixture will remain a gas.
• Laser measurement of roughness provides an accurate assessment of fabricators materials for testing in conditions where this needs to be known
• Reducing the concentration of NOVEC™ 4710 in CO2 from 20% to 16% reduces the lightning impulse strength at 4.5 bar, the difference increased particularly in the positive phase sequence compared with lower pressures.

Dissemination

The project was presented at the Low Carbon Networks and Innovation Conference in December 2017, the International Conference on High Voltage Engineering and Application (ICHVE) in September 2018, the Conference on Electrical Insulation and Dielectric Phenomena (CEIDP) in October 2019 and the International Conference on Dielectrics (ICD) in July 2020.  

In November 2020, a summary of the work once completed was shared in an open webinar advertised primarily externally through LinkedIn.  More than 200 registrants were recorded with more than 120 people attending the live event.  A similar presentation was also given at the virtual EuroDoble 2020 conference in October 2020.

A separate recording made of the webinar presentation was made available to ENIC participants in December 2020.  It may be watched here: https://player.vimeo.com/video/484123343 

The following papers were published or submitted:  

L. Loizou, L. Chen, Q. Liu, I. Cotton, M. Waldron and J. Owens, “Technical Viability of Retro-filling C3F7CN/CO2 Gas Mixtures in SF6-designed Gas Insulated Lines and Busbars at Transmission Voltages,” IEEE Transactions on Power Delivery Volume: 35, Issue: 5, Oct. 2020.  https://doi.org/10.1109/TPWRD.2020.2967582.  An extract from this paper was shared under the title “Retro-Filling Existing SF6 Insulated High Voltage Equipment with Alternative Gases: National Grid UK & Manchester University Perspective” as a LinkedIn article  

L. Loizou, L. Chen, Q. Liu and M. Waldron, “Lightning Impulse Breakdown Characteristics of SF6 and 20% C3F7CN/80% CO2 Mixture under Weakly Non-Uniform Electric Fields,” IEEE Transactions on Dielectrics and Electrical Insulation, Volume: 27, Issue: 3, June 2020. https://doi.org/10.1109/TDEI.2020.008762

L. Loizou, L. Chen, Q. Liu, M. Waldron, G. Wilson, R. Fernandez Bautista and M. Seltzer-Grant, “Partial Discharge Characteristics of SF6 Gas and 20% C3F7CN / 80% CO2 Gas Mixture Using UHF Method,” J. Phys. D: Applied Physics (submitted)

L. Loizou, L. Chen and Q. Liu, “A comparative study on the breakdown characteristics of SF6 and 20% C3F7CN/80% CO2 gas mixture in a coaxial configuration,” IEEE Conference on Electrical Insulation and Dielectric Phenomena, Richland, Washington, USA, 2019. https://doi.org/10.1109/CEIDP47102.2019.9009877

L. Loizou, L. Chen and Q. Liu, “Breakdown Characteristics of C3F7CN/CO2 Gas Mixtures in Rod-Plane Gaps”, IEEE Conference on High Voltage Engineering and Application, Athens, Greece, 2018. https://doi.org/10.1109/ICHVE.2018.8641943

L. Loizou, R. Fernandez Bautista, L Chen, M. Seltzer-Grant and Q. Liu, “Evaluation of UHF Partial Discharge Measurements for SF6 and 20% C3F7CN / 80% CO2 Gas Mixture,” IEEE International Conference on Dielectrics, Valencia, Spain, July 2020.