Underground transmission assets are critical to any electricity network. Maintenance and life extension are the core work of cable asset managers as replacements of these assets would usually incur substantial capital investments. Cable assets are complex physically and chemically and thus significant trailing and testing in different environment are crucial to understand their behaviour, performance and failure modes. Statistically diverse data sets are also important for improving their management. In this project, NGET will collaborate with EPRI to carry out fundamental research and various experiments regarding cable assets to understand their behaviour, performance and failure modes under different conditions and environment. NGET will use the learning gained to improve its own cable asset management practices to deliver benefits for consumers.
Benefits
The assumptions for the benefit estimation include the following: the learnings and knowledge learned will allow deferment of CSE replacements in the next 15 years and reduction in outages. Should this be the case, a conservative estimate would provide a cost saving over the period of next 15 years of around £1.78m (NPV).
Learnings
Outcomes
Year 2021/2022:
P36.001 Design, Construction, Ratings, and Operation and Maintenance of UT Systems
- A series of accelerated ageing tests have been performed on transmission cable terminations with results analysed and documented. The inspections on surface materials have generally indicated lowered levels of hydrophobicity after ageing. The test results regarding electric field stresses will be compared with Finite Element Analysis (FEA) simulations and further updates on the analysis will be reported next year.
- A new version of the Underground Transmission Workstation (software) has been issued.
- A robotic inspection system for underground vaults using 360° cameras have been developed and demonstrated at EPRI’s laboratory in Lenox, USA.
P36.002 Extruded Dielectric Cable Systems
- Full-scale thermo-mechanical experiments have been performed on two cable samples with results analysed and documented. The current results indicate that the forces experienced by the individual phase conductors within the same cable sample could be very different. In one case, the force experienced by the conductor for Phase 1 was almost twice of the force experienced by Phase 2. The results also suggest that the thermo-mechanical forces may not return to pre-test levels even after a cooling period of 7 days. Further experiments are planned to investigate whether such changes are permanent, or a longer cool-off period is required.
Year 2022/2023
P36.001 Design, Construction, Ratings, and Operation and Maintenance of UT Systems
- A series of accelerated ageing tests have been performed on transmission cable terminations with results analysed and documented. The investigative results on termination samples removed from service were also reported. All results can be used to improve operation and maintenance practices.
- A new version of the Underground Transmission Workstation (software) has been issued. A total of 9 modules are included in the software, including the Induction module and others.
Year 2023/2024
P36.001 Design, Construction, Ratings, and Operation and Maintenance of UT Systems
- Accelerated ageing tests continued on transmission cable sealing ends with results analysed and documented. Forensic analyses of cable sealing ends were completed, and results shared. All results can be used to improve operation and maintenance practices.
- A new version of the Underground Transmission Workstation (software) has been issued. A total of 9 modules are included in the software. The NSPAN module that calculates thermo-mechanical forces of a cable route was now fully tested. Preparation to create another module for fault current evaluation is complete and the module will be implemented by the end of 2024.
P36.002 Extruded Dielectric Cable Systems
- Experiments on thermo-mechanical behaviours of cables have been performed on a laminate sheath cable, in addition to the two cable samples completed before with results analysed and documented. The results showed different thermo-mechanical behaviours of different cable types. The results can be used for design and operation of cable systems.
Year 2024/2025
P36.001 Design, Construction, Ratings, and Operation and Maintenance of UT Systems
- Accelerated ageing tests of transmission cable sealing ends, with installation of the dry type sealing end that NGET installed recently, are in progress. Forensic analyses of cable sealing ends from EPRI members were completed, and results shared. All results can be used to improve operation and maintenance practices of NGET cable sealing ends.
- A new version of the Underground Transmission Workstation (software) has been issued. In addition to the existing modules, another module for fault current capability evaluation is completed and available.
P36.002 Extruded Dielectric Cable Systems
- Experiments on thermo-mechanical behaviours of cables have been performed on a laminate sheath cable, and results reported. The results showed different thermo-mechanical behaviours of different cable types. The results can be used for more effective design and operation of cable systems.
- Meaningful information is provided in the document on extruded cable system condition monitoring and diagnostics. The document covers considerations for application of distributed temperature measurements and field partial discharge measurements. The results can be useful to understand background of the technologies and to assess and apply the available diagnostic techniques to NGET extruded dielectric underground transmission cable systems.
Year 2025/2026
- A new version of the Underground Transmission Workstation has been issued. Along with the software, a set of documents were also updated and published to provide engineering support in the design, operation, and maintenance phases of a project.
- Forensic analyses of cable sealing ends from EPRI members were completed, and results shared. All results can be used to improve operation and maintenance practices of NGET cable sealing ends.
- Accelerated ageing tests of the newly installed dry type sealing end that NGET installed in their systems recently are in progress. The outcome can help to better understand their long-term performance.
- Experiments on thermo-mechanical and thermo-electrical ageing behaviours of extruded dielectric cables were performed. The results can be used for more effective design and operation of extruded cable systems.
- Meaningful information is provided in the document on extruded cable system condition monitoring and diagnostics. The document covers considerations for application of distributed temperature measurements and field partial discharge measurements. The results can be useful to understand background of the technologies and to assess and apply the available diagnostic techniques to NGET extruded dielectric underground transmission cable systems.
Lessons Learnt
Year 2021/2022
Discussions with EPRI showed that the effects of thermo-mechanical forces on cables might be more significant than previously thought and might be able to explain some of the faults observed. This matter will be further investigated in the next stages of the project.
Year 2022/2023
The high mineral content in the cooling water used for a cable scheme may have contributed to increased sheath faults observed. Results on oversheath cracking investigation have been documented and provided in the tunnel cable investigative report.
Year 2023/2024
It is important to understand the long-term performance of the dry-type cable sealing ends newly installed in NGET systems. Long-term ageing tests are being performed using the EPRI outdoor long-term ageing test facility in Lenox, Massachusetts.
Year 2024/2025
The ongoing long-term performance investigation of the dry-type cable sealing ends newly installed in NGET systems is very important to develop effective operation and maintenance strategies. The lessons learned from cable accessory failures shared by EPRI are also helpful in understanding failure mechanisms and reducing the number of failures in NGET systems. In addition to the discussed findings, EPRI encouraged all members to share more experience on the same topic within the user group so that the whole group can work together to advance this dry-type cable sealing end technology.
Year 2025/2026
Movement of extruded dielectric transmission cable aboveground sections below GIS or transformer cable sealing ends (CSEs) can result in failures of the sealing ends. The failures can be caused by fluid leaks from fluid-filled sealing ends or the interface condition disturbance because of the loss of interface pressure for dry-type sealing ends. These movements may be driven by thermo-mechanical forces introduced by the feeding transmission cables, sealing end supporting structural movement and vibration, or other environmental conditions. One possible solution to such challenge is to apply distributed optical fibre monitoring technologies to detect such movement. A plan has been developed to address this challenge in future EPRI research. The development aims to monitor the condition of the cable sections connecting to the sealing ends and detect any movements that may potentially lead to sealing end failures with the overall objectives of improving overall performance of the cable systems and grid reliability.
Dissemination
Year 2021/2022:
- Three Transmission and Substations Task Force meetings have been held:
- Mid-year summary webcast in June 2021
- Webcast in August 2021
- In-person Task Force meeting in Charlotte, USA, March 2022
- Two EPRI advisory meetings were held in 2021 and 2022.
- A two-day in-person Underground Transmission Task Force meeting was held in Warwick, UK, May 2022
Year 2022/2023
- Three Transmission and Substations Task Force meetings have been held:
- Mid-year summary webcast in June 2022
- Task Force meeting in Charlotte, USA, August 2022
- Task Force meeting in Charlotte, USA, March 2023
- Two EPRI advisory meetings were held in 2022 and one in 2023.
- A two-day Underground Transmission Task Force meeting was held in Warwick, UK, January 2023
Year 2023/2024
- Three Transmission and Substations Task Force meetings have been held:
- Mid-year summary webcast in June 2023
- Task Force meeting in Lenox, USA, August 2023
- Task Force meeting in Charlotte, USA, February 2024
- Two EPRI advisory meetings were held, one in 2023 (Indianapolis, Indiana) and one in 2024 (Atlanta, Georgia).
- A two-day Underground Transmission Task Force meeting was held in Warwick, UK, January 2024
Year 2024/2025
- Three Transmission and Substations Task Force meetings have been held:
- Mid-year summary webcast in June 2024
- Task Force meeting in Charlotte, USA, August 2024
- Task Force meeting in Charlotte, USA, March 2025
- Two EPRI advisory meetings were held, one in 2024 (Los Angeles, California) and one in 2025 (Charlotte, North Carolina).
- A two-day Underground Transmission Task Force meeting was held in Warwick, UK, January 2025
Year 2025/2026
- Three Transmission and Substations Task Force meetings have been held:
- Mid-year summary webcast in June 2025
- Task Force meeting in Charlotte, USA, August 2025
- Task Force meeting in Charlotte, USA, February 2026
- Two EPRI advisory meetings were held, one in 2025 (Houston, Texas) and one in 2026 (Charlotte, North Carolina).
- A two-day Underground Transmission Task Force meeting was held in Warwick, UK, January 2026
