Reducing Cost of Undergrounding

In April 2023, I wrote about climate change, its impact on overhead lines and the switch to underground cabling. While the cost of MV and LV cables have come down, the benefits of undergrounding are currently overshadowed by its high cable-laying construction costs (largely driven by standards of 70 years ago). This article outlines a new technology that has been commercially implemented in EU, Australia and Canada over the past few years which substantially lowers the cost of undergrounding.

?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:

1.?????? 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.

2.?????? 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.

3.?????? 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. ?

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):

1.?????? 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

2.?????? 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

3.?????? Execution Time: MV Underground at 25 days/mile (15 days/km) versus 15 days/mile (9 days/km) for overhead MV lines

4.?????? 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 is the use of impact resistant (5mm or 10mm thick Schedule 40/80) PP-EPDM “split pipes” that wraps around the cable like a duct (Cable Protection System | Biosirus Inc.). These 1m (3.25 ft) long, light weight, split-pipe halves are interlocked by tabs and connected to each other by flex collars enabling a continuous pipe protection system that also accommodates cable bends and terrain hugging. These split pipes can be installed from the cable-reel-end (along with the cable being pulled) or in the trench after the cable is laid. Since it offers continuous cable protection, they can be laid at ground-level (GLDS) or shallow at 0.6m (2 ft) deep with no sand bedding or brick or concrete covers. Its short section enables much easier access to cable repairs.

This technology has now been implemented over the past few years in EU, Canada and Australia across many verticals (utilities, solar/wind farms, waterbodies, rail transit). Construction cost reductions are substantial at 50-60% with 100% reduction in “fill” material (no sand beds and bricks/flagstones or concrete). It is 60-70% faster due to simpler process, just use of hand-tools and one inventory system that traverses all terrains/waterbodies. This split-pipe cable protection system can be applied to HV, MV, LV and communication cables.

In general, global jurisdictions that have rugged terrain (high trenching costs), high labour costs, remote project sites or heavy vegetation management will see the best cost reductions. Emerging nations with lower labour costs, high vegetation management and high fuel costs will also see cost reductions and improved productivity.

This technology is now gaining traction globally. Pilot trials for a wide variety of applications using split-pipes methods are being mooted in NA, Middle East and India for both cold weather and hot climate applications.

Cabling costs can be drastically lowered using new technology and methods.