Courtesy of Nexans
Nexans supplies underground cables to RTE.

Hardening The Grid Is Manageable

Sept. 28, 2021
The threat of extreme weather events call for innovative thinking.

Lately, the trending topic around the virtual watercooler is reinforcing or hardening the power grid. As topics go, it’s ideal for discussions. Everyone has an opinion, and no one is shy about expressing them. They all have solutions, which run from simple and straightforward to highly complex methods requiring the application of advanced digital technologies. One of the most lively debates is undergrounding transmission and distribution lines.

All of this dialog is driven by the phenomenon of extreme weather events and the prolonged outages they cause. Mention extended outages in any group of regulators, customers, or utility personnel and be prepared for high-spirited and lengthy discussions. That should not be surprising since everyone is under such pressure in our homes, businesses, and workplaces. Electricity has been tightly woven into the fabric of our society and there is no tolerance for power outages of any duration.

Even an outage of only a few hours can cost customers tens of thousands of dollars, impact work, and in some cases be life threatening. As global climate change produces more extreme weather events, prolonged outages are becoming more commonplace, and it’s only getting worse. Many of the experts have gone so far as to say these abnormal events are becoming the new reality and we better adjust to them.

Following that statement, there is usually someone quickly adding, “the electric grid was built for weather conditions that no longer exist.” They also immediately point out the power grid needs to adapt to those changes, and why isn’t undergrounding being done. It’s an argument that is getting more nods of support from industry stakeholders.

Growing Threats

Last winter’s Winter Storm Uri — that’s correct, the Weather Channel started naming winter storms several years ago because of their increasing numbers and intensifying severity. Uri nearly brought ERCOT’s (Electric Reliability Council of Texas) power grid to its knees with an almost complete grid failure. It was a massive storm that impacted North America from Canada, across the U.S., and into Northern Mexico.

In all fairness, the problems brought about by Uri are complicated and not quickly solved. Keep in mind, weather events aren’t limited to winter storms either. Hurricanes are also a major factor for prolonged outages and utilities like Florida Power & Light Co. (FPL) are spending a lot of resources to harden their systems with underground circuits.

According to a news release from FPL, they have been installing underground service for decades and they are committed to increasing their underground facilities. About 48% of FPL’s distribution system is already underground, compared to about 30% for other utilities nationwide. In addition, about 90% of the new distribution lines FPL is installing are underground. 

Beginning in 2018, FPL started a pilot program known as the Storm Secure Underground Program (SSUP). The focus of the program was undergrounding of overhead power lines in targeted communities. As of Dec. 31, 2020, more than 240 SSUP projects have been completed throughout the FPL service area and the results have been very positive as far as customer satisfaction.


Let’s not forget the severe droughts and brutal heatwaves that are responsible for producing wildfires across the globe. These intense wildfires have grown so massive in size that a new term was coined to describe them. They’re being called megafires. A wildfire becomes a megafire when it surpasses 100,000 acres (40,468 hectares) in size. According to Reuters there were eleven megafires in 2020 in the U.S alone, and so far, the 2021 fire season has been much worse.

On a global scale, these massive fires consumed many millions of acres last year, which caused billions of dollars in damage and lives were lost.. A few months ago, Pacific Gas and Electric (PG&E) announced a major initiative to expand the use of underground cabling in their electric distribution power lines. PG&E reported they have more than 25,000 miles (40,234 km) of their overhead distribution power line in high-risk fire areas.

PG&E said they will bury about 10,000 miles (16,093 km) of these high-risk overhead power lines in those high-risk fire zones, which will significantly reduce wildfire risk. It will also benefit customers by lessening the need for Public Safety Power Shutoffs (PSPS). PSPS events have been responsible for prolonged outages that really impacted customer satisfaction.

Critical Infrastructure

Undergrounding has been an increasingly important tool in everyday network reinforcement. WEC Energy Group has implemented what they refer to as a system modernization and reliability project (SMRP). The SMRP has improved reliability in remote areas of Northern Wisconsin by replacing aging overhead lines with underground circuits. WEC Energy Group reported that the US$425 million project resulted in a 97% reduction in electric outage minutes in the targeted service areas. See the August 2021 issue of T&D World ( for details.

Globally, transmission underground cabling projects are also becoming more mainstream. Early this year Réseau de Transport d’Electricité (RTE), France’s TSO (Transmission System Operator reported they had signed a three year contract (with an option for 2 additional years) with Nexans. Nexans will deliver underground cables, accessories, and provide installation services for underground grid connections at 90 kV (kilovolt), 220 kV, and 400 kV. RTE’s underground projects will ensure the reliability of critical network infrastructure and accommodate new renewable generation capacity.

Swedish distribution network operator Ellevio signed a four year agreement with NKT for low and medium voltage power cables to replace overhead lines with underground power cables. The undergrounding will reinforce the power grid in urban areas around Stockholm. According to Ellevio, the overhead cable replacement will weatherproof the power grid and minimize the risk of weather related outages.

HVDC Undergrounding

For the purest, high voltage direct current (HVDC) undergrounding circuits may seem like it’s pushing the envelope a bit, but think of it as reinforcement on steroids! Seriously, HVDC should be considered in all grid reinforcing discussions because the technology offers so many benefits that HVAC transmission can’t provide.

The distance HVAC underground cables can transmit power is limited due to capacitive charging current increases. HVDC underground cables, however, have no capacitive charging currents. Distance isn’t an issue so HVDC underground cables can deliver higher capacities of power over longer distances.

Also HVDC is more compact. A 2 gigawatt system uses 2 cables vs. 3 for HVAC, which allows for narrower rights-of-way. Plus HVDC power flows are controllable from source to load. Because of these and other benefits, there are several projects in various stages of development worldwide that are proving the HVDC undergrounding to be a useful technology.

Europe, Asia, and other parts of the world have long considered HVDC technologies as a building block for making their power grids stronger and more resilient. Hitachi ABB points out that modern HVDC underground cables have been available since 1990 and used in many successful projects. The German TSO TenneT, has been using HVDC underground transmission cable systems for many years.

November of 2020 saw the German TSO Amprion and Belgium TSO Elia Group commission the 56 mile (90 km) Aachen Liège Electricity Grid Overlay (ALEGrO) interconnection. The ALEGrO link uses HVDC underground technology supplied by Siemens Energy. It has a capacity of approximately 1,000 megawatts (MW).

Another interconnection link is getting some attention because of its uniqueness. The 2,100 MW  SOO Green HVDC Link project is a co-locate HVDC underground transmission line installed on Canadian Pacific Railway’s right-of-way. According to SOO Green, the innovative partnership provides the link with nearly all of the necessary right-of-way for its 350-mile (563.3 km) route.

The multi-state project will connect MISO (Midwest Independent System Operator) in Iowa with the PJM system in Illinois. Siemens Energy is a financial investor. They will also provide the fully dispatchable, bi-directional HVDC voltage source converter technology including black start capability for the link.

Recently, SOO Green announced it has selected Prysmian Group as its preferred supplier for the ±525 kV HVDC cable systems to be installed on the project. The project will require 700 miles (1126.6 km) of paired ±525 kV cross-linked polyethylene class underground cables.

The in-service date of the US$2.5 billion SOO Green link is estimated to be around the fourth quarter of 2024. It should be an interesting project to follow with all of its innovative approaches for building an interregional HVDC project in the US.

Probably the biggest argument against undergrounding transmission lines and distribution circuits is the expenses involved. But most of the figures usually only include the upfront costs. There have been very few comparisons that include reduced maintenance expenses or the decreased trimming expenditures to name a few.

There is no one size fits all solution when it comes to reinforcing or hardening the power grid and experts continue to disagree on remedies. Undergrounding, however, offers more advantages than drawbacks. It’s an off-the-shelf technology that has been proven on the grid for many years. Maybe outside-of-the-box thinking might be improved by under-the

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