Electric power delivery faces the brunt of severe weather (flash floods, ice/snowstorms, hurricanes, wildfires, extreme heat). Hundred-year-old records are being broken resulting in economic losses (NA, Caribbean islands, S/SE Asia, EU, others). Each event costs hundreds of millions of dollars in restoration, lasting several months. Public patience is wearing thin and political pressure is mounting to fix this. Attention is turning to underground power distribution (and even ground-level distribution systems or GLDS). The suburbs and rural areas will see most such conversions.
Underground cables have their own characteristic relative to overhead lines, (a) immune from lightning/ tree/ animal contacts; (b) better public safety from downed conductors (albeit hazards exist from excavation); (c) no ongoing vegetation management; and (d) no visual clutter. The downside is (a) less overload capability; (b) longer fault location/restoration; and (c) high capital cost (4x to 7x of overhead lines).
Most cabling methods today are driven by 70-year-old standards and their construction costs (machinery, fuel, fill materials and labour) have increased substantially over the years. Added to this are recent environmental rules related to the management of boring-fluids, excavated material, backfill and fresh-fill materials. Today, the construction costs alone make up 70% of a cable project.
The following are the most adopted cabling methods for HV, MV and LV cables:
- Duct Method: Cables are pulled through underground steel/concrete/PVC/fiberglass conduits and pipes, laid in a trench, backfilled with soil or concrete. The potential cable-pull “burn” damage to outer PVC jacket is an unanalyzed and unresolved issue (the best friction factor of 0.39 for the smoothest fiberglass pipe is still high). Large 3-core cables (despite heavy greasing) are still vulnerable.
- Direct Buried Method: Cables are laid in an excavated trench, duly bedded below and above the cable by 150 mm (6”) sand, then covered by bricks or flagstones (cable protection) and backfilled. Sand/clay soils are “shored-up” during installation or made with concrete walls. Procuring and delivering sand and bricks and the labour associated with its implementation is substantial.
- Marine Applications: Generally, these are bare cables laid in water weighted by cement bags. The cables experience small movements due to wave/current/marine traffic. In cold-weather lakes/shorelines, cable movements occur due to frost-heaving/icefloes. All these small movements lacerate (or tear) the outer PVC jacket over time (particularly in rocky areas), causing water ingression in cables.
The above methods were the best in class for their yesteryears (deeper depths and other cable protections due to vulnerability of damage). No recent reviews have been undertaken of newer materials and technologies that can better address cabling methods while reducing construction costs. Even the gas distribution industry seems to have made changes keeping up with new technologies. With climate change and the need for cabling, we need to re-examine cabling construction methods. Â
To reduce construction) costs, we need to (a) better protect the cable; (b) enable shallow depths or even ground-level cabling; (c) have a simplified less-engineered process (methods, inventory) across all terrains; and (d) eliminate expensive and time-consuming excavation and backfill. Given the millions of overhead circuit conversions globally, any significant cost reduction would be a huge savings to the utility and comfort to their regulators.
Cabling has four major cost components, (a) the cable itself; (b) cable laying/construction process; (c) execution speed; and (d) project delays. Typical North American costs (USD) highlight this challenge (note: the wide cost spread):
- Total Cost (2023): MV Underground at 2.3 - 3.8M$/mile (1.4 - 2.3M$/km) versus 500 - 800k$/mile (300 - 500k$/km) for overhead MV lines
- Construction Cost: MV Underground at 70% of total cost at 1.6M$ - 2.7M$/mile (1M$ - 1.6M$/km) verses 50% total cost at 250k$ - 400k$/mile (150 k$ - 250 k$/km) for overhead MV lines
- Execution Time: MV Underground at 25 days/mile (15 days/km) versus 15 days/mile (9 days/km) for overhead MV lines
- Project Risk: MV Underground delays due to equipment rental, trade scheduling, excavation, backfill and cable pulling versus overhead MV line delays due to pole installation every 150 feet (45m) and line stringing. Due to multiple elements involved, cable projects invariably run into project delays.
An alternative innovative solution would be to use (say) shorter 3-feet lengths of interconnected, impact resistant EPDM, “split pipes” that wraps the cable (like a duct) and can follow the cable terrain in its x-y-z degrees of flexibility. These light weight, interlocked pipes enable a continuous pipe protection system that accommodates cable bends and terrain hugging. This eliminates the need for trench bottom grading required with 10/20 feet conduit pipe lengths or sand bedding/cable protection flagstones associated with direct buried cables. These split pipes can be installed either "wrap and lay" (pulled from the reel-end with the cable) or "lay and wrap" (after the cable is laid in the trench). Due to  its flexibility they can laid as a single inventory application across its entire cable route including (a) at ground-level (GLDS) in rocky terrain; (b) shallow depth with no sand bedding or brick or concrete covers; and/or (c) in water crossing. Its short interconnected openable sections enable much easier cable repairs.
This technology has now been implemented over the past few years in EU, across many verticals (utilities, solar/wind farms, waterbodies, rail transit) and recently in Canada (shoreline cables) and Australia (rail transit) for MV , LV and communication cables alike. With large overhead conversions slated for NA, its time utilities rethink cabling systems.
Comments and feedback welcome.
Â