Existing polymeric cable sealing ends (CSE) typically contain silicone oil as an insulating liquid between the cable core and the outside of the CSE. In the past year, a number of performance issues have been identified with silicone oil filled CSE. Although these devices are intended to be maintenance free, there is evidence that the insulating liquid is degrading. In some cases, this has led to the electrical degradation of CSEs. A deeper understanding of the chemical degradation of the silicone oil is needed in order to understand the possible reasons for this behaviour. It is also desirable to determine if different insulating liquids (for example, synthetic or natural esters, which have been used in transformers) could be deployed in CSE. The possibility of retrofitting existing CSE would need to be explored, along with any design changes which might be necessitated by the new insulation. Understanding of the long-term ageing of these liquids will be of paramount importance.
Benefits
The replacement of a new cable sealing end is estimated to cost tens of thousands of pounds. National Grid is currently planning on replacing over 100 cable sealing end sets due to concerns around the performance of silicone oil. The cost of managing these assets while they are considered to be at a higher risk has been conservatively estimated to be £1 million per year.
The ability to identify poor quality oil during installation of the CSE would prevent this type of degradation occurring again and will allow this type of degradation to be managed more effectively in the future.
Understanding how the oil degrades with time and how this changes for different types of oil may allow a clear definition of ‘good quality’ silicone oil to be made, which may offer the ability to reduce replacements by allowing the oil to be changed. Developing a list of alternative oils would provide an alternative opportunity to reduce replacement cost; as the oil could be changed, rather than the entire termination.
Learnings
Outcomes
Year 2018/2019
A key outcome of the project to date is the identification of typical in-service temperatures and electric field strength within two general CSE designs. This information is critical to understanding the likely ageing mechanisms that are present within CSEs.
Year 2019/2020
Replacing silicone oil within the XLPE terminations with an alternative oil is unlikely to be an improvement, due to the problems foreseen and the quantities of tests that would be required in order to alleviate the concerns. Many properties of silicone oil are currently found to be superior to those of the alternatives considered in this project.
XLPE has not been found to: accelerate the ageing of the silicone oils; increase the moisture level in the oils, release remnants from XLPE manufacturing process into the oils.
Year 2020/2021
Tests to date have shown that the integrities of the silicone oils tested have not been significantly impacted by either ageing or contaminants or combinations of both.
Recommendations for further work
The impacts from sulphur and chloride contamination could be investigated further and a deeper root cause analysis could also be performed. This would help to identify the source of these contaminants and understand at what level of concentration these may or may not become an issue.
Further tests could be carried out combining combined heat stress with electrical stress. This could help to identify whether there is any ageing mechanism would require such a combination, and also to investigate whether such mechanisms would cause concerns.
Long term ageing tests should also be performed on silicone oils with a range of different stress cones and tapes. These would help to identify ageing markers within the oils and potentially higher order effects from breakdown of the materials as they age, which could potentially release debris into the oil and might cause deterioration.
Lessons Learnt
- It is crucial for different work packages/streams to establish and maintain effective communication as they could inform each other with valuable discoveries which may impact significantly on the directions of the work planned
- Using Karl Fischer titration to measure moisture content could be difficult for high viscosity oils since water tends to accumulate in these oils rather than dissolve
- Levels of moisture content in silicone oils seem not have major impacts on their electrical integrity
- It is worth noting that based on the wide range of oils and contaminants used, with the extensive tests carried out, the project has not identified any significant issues for using silicone oils within CSEs. In fact, this project has shown that silicone oil is likely to be the best choice and that silicone oil has remarkable abilities to withstand ageing and contaminants. The project has however identified that there is a significant lack of public knowledge of the behaviour and properties of silicone oil. This project, LiCaSE, has made substantial progress to address this.
- Disparities have been identified in IEC and IEEE standards for silicone oil. Though they are not inconsistent enough to cause major concerns, these disparities do cause confusions.
Dissemination
Year 2018/2019
As this project is ongoing, dissemination outside of the organisation has been limited to date. Further dissemination is planned as part of this project.
The following paper has been submitted for consideration:
“Review of Properties and Aging Characteristics of Ester Based Dielectric Insulation Liquids in Comparison to Mineral and Silicone Oils” S. Virtanen, X. Lyu, G. Callender, H. Walker, T. Andritsch, J. Pilgrim and O. Cwikowski published in IEEE Transactions on Dielectrics and Electrical Insulation
Year 2019/2020
The above paper was accepted for publication, in addition the following papers have been submitted for published:
"Techniques to Monitor the Aging of Silicone Oils” S. Thakur, G. Callender, A. V. Shaw, R. C. Brown, O.Cwikowski
Published in: International Conference on Dielectrics (ICD), IEEE conference 2020
DOI: 10.1109/ICD46958.2020.9342001
“Electrical Ageing of Silicone oils” Ian L. Hosier, Alun. S. Vaughan, O.Cwikowski
Published in: Electrical Insulation Conference (EIC), IEEE conference 2020
DOI: 10.1109/EIC47619.2020.9158574
“Degradation of silicone insulating fluids in cable sealing ends” Suvi Virtanen, George Callender, David Wheatley, James Pilgrim, Thomas Andritsch, Richard Brown, John Langley, Oliver Cwikowski.
Published in: Jicable: 10th International Conference on Insulated Power Cables, Versailles, France
URI: http://eprints.soton.ac.uk/id/eprint/430613
Year 2020/2021
IEEE Electrical Insulation Magazine “A review on the relevance of standards for silicone oils used in Cable Sealing Ends” S. Virtanen, G. Callender, T. Andritsch, J. Pilgrim, D. Wheatley, R. Brown, G. Langley, O. Cwikowski