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 thermal-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 thermal-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.
Recommendations for further work
· EPRI has been assisting National Grid on condition assessment of aging self-contained fluid-filled transmission cable systems. National Grid could continue such practices to enhance our cable maintenance and replacement management programs.
· The software tools that EPRI developed (Underground Transmission Workstation or UTW) has also become an unique and useful tool for engineering design and verification.
· EPRI’s research results on cable sealing end performance, thermo-mechanical design, cable monitoring techniques, and on shared forensic analyses can be applied to National Grid systems to improve operation and maintenance practices.
Lessons Learnt
Year 2021/2022
Discussions with EPRI showed that the effects of thermal-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 an important NGET tunnel cable scheme may have contributed to the frequent 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.
