National Grid Electricity Transmission (NGET) currently has a population of several thousand Sealing Ends (SEs) of all types (including fluid filled and XLPE) across Great Britain. With the expected increase in undergrounding technologies and the difficulties with new OHL routes, NG’s SE population is expected to increase.
Experience has shown that since rapid uptake of XLPE cables in the early 2000s, that the Cable Sealing Ends (CSEs) used to terminate the cables have suffered several bulk replacements due to performance issues; for different reasons. Which has required NG to manage the increased risk presented by these assets during their early replacement, and has ultimately led to a rapid replacement timeline. With the advent of Condition Monitoring Surveys (CMS), there is opportunity to identify the poor condition assets and manage their risk effectively, without the need for immediate replacement.
However, there is a lack of understanding on the degradation mechanisms for silicone oil filled CSEs, in particular the interaction between oil contamination/degradation and discharge/breakdown process for silicone oils used in these CSEs. This poses challenges to conducting effective assessment of the condition of the CSEs in order to inform the management of CSEs, their replacement strategy and manage their end-of-life.
Benefits
The replacement of 153 sets of cable sealing ends is presently being considered at an estimated cost of over £10m. This project will help inform the development of a new specification for silicone oils and improve our ability to manage ones that have shown signs of damage. This could prevent the replacement of CSEs in the future and reduce the risk associated with these assets.
Learnings
Outcomes
The project has established the differences in PD characteristics between new-dry silicone oil and ex-service silicone oil.
Year 2019/2020
The project has compared the differences between high and low viscosity silicone oils. This has included the impacts of temperature, moisture and bubble formation for these two types of oil.
Year 2020/2021
The project has identified that the PD behaviour at the solid/oil interface is intermittent and no consistent phase resolved pattern was observed. This highlights the challenges faced by using this technique to identify the growth of electrical trees on the surface of stress cones within CSEs. The work also identified that electrical tree “bridges” are only likely to form when there are surface contaminants already present on the stress cone.
Lessons Learnt
- PD characteristics of low viscosity silicone oils: Increased moisture level results in a higher PDIV and a lower PD repetition rate
- PD characteristics of high viscosity silicone oils
- When subjected to sustained AC stress, small bubbles can form
- These bubbles can easily accumulate if the temperature is elevated, e.g., 60 ̊C
- Prolonged heating can reduce the viscosity of the oil, for example, 24-hour heating at 80 ºC
- Creepage discharge characteristics of low-viscosity silicone oils
- With the rod-to-tape configuration, the contaminations on the surface of the silicone rubber and those inside the oil can lead to different patterns formed on the surface of the silicone rubber
- With the needle-to-plane configuration, black tracks originated from the needle tip can form towards the plane electrode. The breakdown can occur before the tracks reach the plane electrode.
- Creepage discharge characteristics of high viscosity silicone oils
- With the rod-to-tape configuration, it is difficult to induce PD and no obvious pattern can be observed on the rubber surface
- With the needle-to-plane configuration, black tracking patterns are again observed
- It takes less time for the breakdown to happen in high viscosity oils
- PD and creepage discharge characteristics of aged silicone oils:
- The used oil has a higher PDIV and lower PD repetition rate, which is caused by higher moisture content
- The effects of particles in oil on PD characteristics are not obvious.
- The PDIV of the laboratory-aged oil is similar to that of new low viscosity silicone oil
- The laboratory-aged oil has a higher PD magnitude but lower PD repetition rate than the new low viscosity oil at higher voltages.
Dissemination
Year 2018/2019
This project is still ongoing. For this reason, no dissemination has taken place to date but dissemination events are planned upon project conclusion.
Year 2019/2020
This project is still ongoing. For this reason, no dissemination has taken place to date but dissemination events are planned upon project conclusion.
Year 2020/2021
This project was disseminated on a virtual event called “Cable Research Virtual Conference” at 3 Sep. 2020.
A journal paper for IEEE Transaction is currently under drafting and will be submitted later this year.
