Why underground cables are a better long-term choice for utilities

Credit: Nexans

By Frédéric Lesur, Nexans

In a world rapidly impacted by climate change, extreme weather will only become more common. Across the US, heat waves are creating some of the largest wildfires on record and the Atlantic hurricane season drenched the southern part of the country – once again. 

These conditions create significant risk for grid reliability and result in power outages. These could look like planned blackouts to reduce wildfires or rolling blackouts during storms to keep demand under capacity. As the majority of the risk lies in overhead cables, taking power cables underground is an important consideration.

Underground power cables can strengthen utility systems and provide greater resilience and lower maintenance needs. Let’s take a closer look at why they are an excellent long-term choice by diving into what underground cables are, the benefits and the current state of cables in the US.

What are underground cables and how do they stand up against extreme weather?

Underground cable lines first appeared back in 1890, when the first successful installation was pioneered by Vincent de Ferranti using his famous 10,000-volt concentric cable in the UK. Since then, underground power transmission has evolved dramatically. 20 years ago in Europe, an agreement was signed with the French government to limit the length of overhead lines (OHL) after an unexpected storm damaged the French grid (3.7 million households without electricity) and caused blackouts for up to three weeks. In the years since, research and development resulted in a significant reduction in the cost of undergrounding, through longer lines and mechanized installation. Now 20 years later, we are having exactly the same discussion in the US.

Traditionally, underground cables are used in cities where overhead lines are not feasible; in areas where an overhead line may risk lives; or in scenic areas where aesthetic is an important consideration.  Cables are usually buried at a depth of three feet for distribution grids, and four or five feet for transmission grids. In terms of safety, there is no disturbance at the ground surface when a short circuit occurs or a very high amount energy is released, because the soil contains the fault. The soil also protects against third-party damage as long as proper authorization procedures are followed before digging, especially in urban areas. Additionally, the current generation of underground systems requires less maintenance, and refurbishment is only required every 40 or 50 years, which is the specified lifespan of a transmission line.

Underground cables are better protected against extreme weather and other catastrophic events than overhead lines. In addition to proper depth, installation matters too. It is generally recommended that cables are installed using ducts, where installers obtain one permit so they only have to dig once and then pull the cable inside.

The depth is sufficient for fire protection and means that heat will not damage the cables during a fire. On the other hand, sparks from OHL (overhead lines) are routinely identified as the cause of fires in dry areas. This has had dramatic consequences and has led to fatalities in California.

Superconducting system has developed its resilient electric grid to be sure to have two different parallel technologies to supply the same area. Superconductor cable systems are relatively new technology and play a key role in improving the resiliency of power grids by protecting against extreme weather and other catastrophic events. If one system fails, the second is there to provide reliability so that the whole network is guaranteed.

What is the current state of cables in the US?

Overall, the utility cable market is expected to grow at a +4.2% p.a. pace by 2030, according to a Utilities Cable Market study by Roland Berger. Currently in the US, the average grid is 35-40 years old and many are ageing beyond their lifespan. America’s energy transition will take a while and require large scale investments to ensure power supply security.

President Biden’s $2 trillion infrastructure plan could boost the country’s energy transition with a $100 billion investment in the power grid, which includes tax credits to incentivize the deployment of at least 20 GW of high-voltage capacity power lines, as well as funding new climate resilience and energy initiatives. The US is behind compared to its counterparts in Europe. France already makes about €1.75 billion investment per year for the transmission grid and more than €5 billion investment per year for the distribution networks. Considering the scale between the two countries, Biden’s plan seems to be ambitious, but also reasonable, taking into account the current stakes.

With the new administration’s commitment to energy transition and plan to develop an energy highway with miles of HVDC cable across the country, we at Nexans are involved in the development of offshore wind farms on the East Coast to accelerate the energy transition. We will be providing up to 1,000 kilometers of subsea high voltage export cables for offshore wind farms in North America. We’ve also recently announced a new facility in Charleston, South Carolina, which has been upgraded this year to manufacture submarine cables (in addition to land systems) for the American market.

What are the cost benefits of underground cables?

Underground cable lines have many advantages and benefits over overhead lines and are gaining momentum for safety, reliability and cost effectiveness. It is interesting to look at the total cost of ownership: CAPEX (cost of purchasing supplies, civil works, installations) + OPEX (maintenance, dissipated losses). Underground cable systems are often more expensive in CAPEX (overhead lines are bare conductors, while insulated conductors need more raw materials and manufacturing processes). Undergrounding needs more civil works (trenching) while tower or pylons need foundations and tracks for access.

However, the right of way – which depends on the local regulation – is often cheaper for underground systems and the required corridor is narrower by a factor of about ten. Underground systems need very little maintenance as the insulation is extruded plastic (a passive component). There is no fluid under pressure or potential leak. Overhead lines on the other hand need monitoring and maintenance to prevent from corrosion (frequent painting of the metal supports) and require maintenance against weather hazards (lightning impact, storms, icing, sticky snow). Studies also shows that dissipated losses due to heating by Joule effect are lower for underground cables due to use of pure copper and aluminum, while alloys of aluminum and steel are required for mechanical strength of the overhead bare conductors.

Engineering is different for both technologies, which have their own advantages and drawbacks. Buildings and trees must be avoided above underground circuits, for example very few people now build their house under overhead lines, but it is possible to grow plants with reasonable roots. The system operator must schedule a frequent pruning under the overhead lines to avoid contact and arcs with the conductors. Most damages during storms are caused by fallen trees.

Challenges for underground cables include the lifespan of a transmission line and high cost as well as the lack of public acceptance. However, underground cables not only protect against blackouts during peak load hours and severe weather events, but also from an environmental point of view provide massive electrification, which is the best way to decarbonize our energy system.

About the Author

Frédéric Lesur is a senior electrical engineer with more than 30 years’ experience specializing in cabling technology from underground land (HVAC and HVDC) to submarine and superconductor cables and is an active participant in the international standards community.

He serves as Senior Engineer in Nexans’ High Voltage Products and Cable Systems business helping to develop specifications for complex client projects as well as provide customer support. Lesur is also the host of ‘WHAT’S WATT,’ Nexans’ new YouTube channel devoted to teaching youth, tomorrow’s engineers and industry professionals about electricity, its applications and potential for social progress in the fight against climate change through sustainable electrification. He joined Nexans in 2017 serving as R&D manager in the company’s Calais, France research center for land high voltage cable systems.

Author

  • The Clarion Energy Content Team is made up of editors from various publications, including POWERGRID International, Power Engineering, Renewable Energy World, Hydro Review, Smart Energy International, and Power Engineering International. Contact the content lead for this publication at Jennifer.Runyon@ClarionEvents.com.

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