Introduction: 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.

Objectives: The overall aim of the project is to identify, characterize and verify the long term performance of new insulating liquids which could be used in Cable Sealing Ends.

Expected 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.