In the past six months, transmission lines have been destroyed by hurricanes in Puerto Rico, singed by wildfires in California, and bitterly opposed by residents in Utah and Pennsylvania who want to stop utilities from building more.
Such problems have grid operators literally thinking deeper. Increasingly, utilities in the United States and elsewhere are routing power underground. Puerto Rico’s grid rebuild is a prime example: A proposal, crafted by an industry-government consortium late last year, calls for “undergrounding” transmission to harden a power system still recovering from Hurricanes Irma and Maria.
Much of the plan’s outlay for transmission—US $4.3 billion—would create hardy overhead circuits interspersed with underground cables in areas where gusts could snap even the strongest lines and towers. A $601 million line item also provides for a buried high-voltage direct current (HVDC) cable around the territory’s southeast corner, where most big storms strike first. This underground bypass would create a secure path from the island’s most efficient power plants to the heavily populated area around San Juan.
By heading below ground, transmission grids are following a path laid by their lower-voltage cousins—distribution grids. In some cities, power distribution occurs entirely out of sight. This is possible thanks to specialized cables, whose metal conductors are wrapped in cross-linked polyethylene, a heat—stable insulator, as well as metal and polymer layers providing electrical shielding, impermeability to water, and puncture resistance.
Utilities have been slower to bury transmission because of the expense, according to power consultant Ken Hall, a former transmission and distribution director at the Edison Electric Institute, a Washington, D.C.–based utility trade group. Transmission lines operate at higher power levels than distribution lines and generate more heat, which is harder to dissipate underground.
Doing it properly can mean burying up to a dozen cables at a time to carry the current, and specifying thick copper conductors that have lower resistance and generate less heat than the cheaper aluminum employed in overhead lines. Each cable must be shipped in roughly 1-kilometer-long links and stitched together on-site, adding further to the tab.
Tally it all up, says Hall, and underground transmission costs roughly 5 to 10 times as much per kilometer as overhead circuits. “Every utility in the United States has underground distribution. But not every utility has underground transmission,” he notes.
Despite the expense, utilities are now investing more in underground transmission, prodded by regulators and public outcry. Denmark was among the first to mandate it in 2008, requiring most new AC and HVDC transmission to be routed underground, with the exception of the highest-voltage, 400-kilovolt AC lines. (Burying high-voltage AC is harder than burying HVDC, largely because AC flows with greater resistance and thus generates more heat.)
In 2015, Germany mandated underground transmission for HVDC systems unless they could be strung alongside existing power lines. Most of the HVDC projects that Germany is counting on to supply North Sea wind power to southern cities, originally designed as overhead lines, are now being replanned as underground links.
Germany’s reforms also encourage burying short segments of high-voltage AC to reduce public conflicts for transmission routes through towns and scenic areas. Operators in Germany are planning or have completed 11 AC pilot projects, according to Heinrich Laun, a regional manager for Bürgerdialog Stromnetz, a publicly funded initiative that helps communities understand and negotiate grid planning.
In the United States, utilities have promised to bury lines along nearly one-third of the long-disputed 309-kilometer Northern Pass project—a set of AC and HVDC links meant to deliver more Canadian hydropower to the northeastern United States. The intent was to eliminate the “visual impacts” of tall towers and suspended lines on New Hampshire’s White Mountains, which are popular with tourists. State regulators will vote on the project in February.
For all its benefits, Laun says undergrounding has also caused new headaches for transmission operators. Farmers can plant and plow over buried cables, but Germany’s farm lobbies are concerned with potential soil impacts from cable heat and altered drainage, and have demanded compensation.
Replanning underground cables have already delayed most new HVDC projects, Laun says, which were supposed to provide an alternate supply of electricity for the south before Germany’s last nuclear power plants are shut down in 2022. Now, most HVDC projects are not expected until 2025, and schedules could slip further behind if faced with more opposition.
To Laun, these short-term challenges will beget long-term acceptance of new transmission lines that are buried out of sight. He says this acceptance is critical if Germany is to build the many more lines necessary to integrate more renewable power into its grid. “In the long run, it’s worth it,” he says.