Hybrid Undergrounding
Risk Reduction Category
Technology Description
Overhead lines are exposed to many natural hazards such as storms, ice, tree cover, and animals. Attempts to harden overhead distribution include covered conductors, spacer cable, and construction techniques among others. When these measures are insufficient to meet the requirements – whether reliability indices, public safety, or other governance – utilities consider taking the circuit underground. Traditional undergrounding is effective but very expensive, and therefore, used as a last resort. It can be prohibitively expensive when the terrain is rocky.
Hybrid undergrounding is defined in a 2020 EPRI technology publication as any approach that gets power lines out of the air where they are no longer exposed to wind, ice, trees, and animal damage threats without having to be buried with traditional undergrounding methods. [3] While the safety related challenges of hybrid undergrounding are numerous, the cost benefit and ignition reduction efficacy are significant.
Ground Level Distribution System (GLDS) is an implementation on the hybrid undergrounding concept. GLDS is a “no excavate” approach that negates the costs of excavation whilst providing similar benefits of traditional undergrounding
GLDS conceptual diagram from US patent [1]
Technical Readiness (Commercial Availability)
This is an emergent technology in the ideation and small-scale pilot stage. A solution is not commercially available. A patent was issued in 2022 for a Ground Level Primary Electric Distribution System [1] and a proof-of-concept is being deployed by PG&E. [2] In the proof-of-concept, PG&E is packaging the electric cable in conduit in a specially molded tray, tied in with a basalt rebar, then sealed with a special geopolymer cement, placed at ground level, and capped in thermoplastic. It is a reinforced box-like package that wraps the electric wire in materials to ensure safe operation and is secured to the ground to prevent movement [4]. Recently, the GLDS pilot project at PG&E just reached a significant milestone as the first half mile of GLDS circuit was energized in early November on the Woodside 1101 circuit in San Mateo County, California [4].
Outstanding challenges include:
- How can the GLDS be monitored to ensure continued operation? – as the system exists at ground level it may be impacted by uncontrolled surface excavation groundworks (landscaping which changes the thermal environment)
- What level of public education will be required to roll out this new system? – GLDS will have a different appearance to traditional systems
- What new training and tools will the Line Crews require to work on this system? – the cables use conduits and long lengths thus different tooling and work methods are likely required to undertake the work
- How to make a GLDS protection system robust to protect the cables, yet accessible to enable maintenance?
- What are the available repair / replace strategies and components? – although well protected by the geo polymer; large impacts (snowplough etc) or the need for expansion / relocation will require that the cable system will need to maintenance
- Quantifying the ability of GLDS to survive the passage of a “Wildfire Event” and continue to provide power thereby supporting other restoration efforts?
- Establishing the impact of the novel installation environment upon the power carrying capacity? – do the cables and joints run hotter or cooler, is there a summer derating requirement
Implementations/Deployments
Through a field trial, PG&E is exploring the idea of moving overhead powerlines to ground level, using GLDS, to eliminate ignition risk and enhance grid resilience. PG&E anticipates this innovative approach could provide comparable risk reduction and to traditional undergrounding. [2] EPRI is concurrently working with the patent holders on some custom testing geared toward safety and robust lifecycle operation of this type of system.
Innovations as of Mid 2023
Potential Enrichment Work Opportunity
References
[1] Movafagh, Roozbeh. Ground Level Primary Electric Distribution System. US 2022/0216682 A1, United States Patent and Trademark Office, 7 July 2022.
[3] EPRI, “Wildfire Risk Reduction Methods”, 2020.