High Voltage Direct Current (HVDC) Lines Seen as Crucial to Bring Renewable Energy to the Grid
The growth of renewable energy is not only reshaping the way society consumes power, it is disrupting conventional thinking in the energy industry on many levels, including storage, grid management, and distribution.
A sea change in the method of long-distance power transmissions is one example. Traditionally, transmission of electric power has been performed by high voltage lines carrying alternating current or HVAC (High Voltage Alternating Current). Increasingly, that role is being usurped by lines carrying high voltage direct current, or HVDC (High Voltage Direct Current), because of their superior ability to transmit power generated by renewables.
This shift has been so significant that many in the energy industry believe a large-scale conversion to an HVDC transmission grid is essential. Fully assessing the strengths and weaknesses of HVDC and taking the necessary steps to properly expand its use is likely to become a strategic imperative in the fight against climate change.
What is High Voltage Direct Current (HVDC)?
Electricity, invisible to the human eye, is a fascinating and mysterious phenomenon. On a very basic level, it is typically perceived as tiny electrons, like little balls of energy, travelling along wires to form a current that turns on homes’ lights and powers household appliances.
But the concept gets more complex. There are two very different forms of electrical current. A current can flow in one direction from the source (power plant) to the receptor (device or appliance). It flows through and out of that receptor, returning to the source again—always in the same direction in a continuous circular movement. This is referred to as direct current or DC.
Somewhat counterintuitively, electricity may also flow in a back-and-forth motion, or alternating current, also known as AC. This flow travels from the source to the receptor, and back, also on a loop. But instead of always travelling the loop in one direction, it switches its direction once it completes one round and then travels in the opposite direction in the loop repeatedly and rapidly, back and forth, constantly alternating its direction. A device or appliance harnesses the energy from that current by connecting to and drawing power from the back-and-forth motion on the receiving end.
What does all this have to do with renewable energy and high voltage transmission lines?
When the nation's electric transmission infrastructure was first being built, DC power systems were more expensive and more complex to install. Consequently, AC became the predominant and standardized form of high voltage transmission, and most power from utilities is currently transmitted over HVAC lines.
With advanced technology, high voltage direct current (HVDC) is increasingly recognized as superior to transmit renewable energy over long distances.
However, because of the rapidly expanding capacity of renewable energy, the role and the value of HVDC is also rapidly expanding. With advanced technology, HVDC is increasingly recognized as superior to transmit renewable energy over long distances, and many see it as an indispensable tool for meeting the larger goal of bringing sufficient supply of renewable power online to meet carbon reduction goals.
According to the U.S. Department of Energy (DOE), HVDC is more efficient and less expensive at long distances and can connect asynchronous systems or grids that operate on different frequencies. The reasons for this can be found in the distinct and unique characteristics of AC and DC power.
Disadvantages of High Voltage Alternate Current (HVAC)
The alternating nature of AC creates phenomena that do not exist in DC power. For example, the back-and-forth motion of alternating current generates a type of energy called reactive power. It is an essential element of AC current, although it is not part of the power that is consumed by appliances and devices on the receiving end.
This reactive power leads to losses of power in an AC system and causes it to not travel well over long distances. Reactive power in HVAC lines also creates a magnetic field between the lines and the ground below. This magnetic field, or capacitance, also contributes to a loss of power, which is referred to as capacitance line loss.
In addition, AC current creates a so-called corona discharge, which involves the casting off of electrons into the air surrounding a conducting wire (audible sometimes along overhead long-distance HVAC lines as a humming sound). This also leads to power loss. And finally, AC is affected by the so-called “skin effect,” in which power travels only at or near the surface of a conducting wire.
HVAC lines require larger, bundled lines to compensate for the loss of power. These larger, thicker wires also add to the cost.
Consequently, HVAC lines require larger, bundled lines to compensate for the loss of power. These larger, thicker wires also add to the cost. Moreover, HVAC lines are typically installed high above ground to minimize capacitance line loss, although this leaves the lines vulnerable to outages caused by high winds.
Advantages of HVDC
In contrast, reactive power and the skin effect do not occur in DC, and the corona discharge is much less significant. HVDC lines also do not need to be installed high above ground because they do not experience capacitance.
All these factors make HVDC transmission lines more efficient, less expensive, and able to carry more energy over longer distances than their AC counterpart.
Because DC power does not operate on a frequency, it can connect multiple asynchronous power systems without disrupting the frequency of either one.
The DOE notes that the US power grid is divided into three such AC systems: the Eastern interconnection, the Western interconnection, and the Texas interconnection. Transferring power between these grid regions without disrupting the frequency of either system is only possible with direct current (DC) links.
HVDC and Green Power
All HVDC’s advantages make it ideally suited to transmit power generated by renewable energy sources. Most renewables, such as utility-scale solar, wind, hydro, and geothermal, generate power in remote locations that are far removed from populated areas and thus have the greatest need for power. These generating sites require sufficient transmission infrastructure to carry power over long distances—and over large bodies of water and land masses—to meet the targeted consumer demand.
The nation's existing transmission infrastructure is insufficient to meet this challenge. In some cases, adequate infrastructure does not exist near renewable generating sites and needs to be built from scratch. In other situations, existing HVAC lines are not up to the task, so they need to be replaced or upgraded.
In some cases, adequate infrastructure does not exist near renewable generating sites and needs to be built from scratch.
HVDC transmission technology has improved over the years, and the costs have been greatly reduced. It is now widely recognized as the superior technology to get more renewable energy capacity to the grid where it can meet consumer demand and help cut carbon emissions.
Because HVDC does not experience capacitance line loss, transmission lines can be installed underground and even under the water, dramatically reducing the cost of their installation and making the renewable generating sites feasible and cost-effective.
HVDC Installations Around the Globe
Developers recognize these advantages, and projects are underway across the globe. Some have been completed and are operational. One such project was an international collaboration: NordLink was developed jointly by Norwegian power company Statnett, Norwegian and German grid company TenneT, and German investment bank KfW.
The NordLink transmission line travels over 623 kilometers (about 387 miles) between a hydropower facility in Tonstad, Norway, to wind power facilities in Wilster/Nortorf, Germany. Most of the transmission line, about 516 kilometers (320 miles), travels under the North Sea.
The project, which became operational in 2021, creates a unique and vital link between the two countries. It has the capacity to transmit 1,400 MW (megawatts) of renewable energy, enough to power about 3.6 million households.
Perhaps more importantly, it can transmit power both ways. The transmission line consists of a double cable made of positively and negatively poled cables. With this setup, NordLink can transmit power from the Norwegian hydropower station to Germany or from the German wind farms to Norway, according to supply and demand. This allows the Norwegian hydroplant to act as a sort of battery that stores power for the wind farm that can be discharged when the wind is not blowing. It also allows both countries to dramatically increase their access to renewable power sources.
Another project, the Champlain Hudson Power Express® (CHPE), will deliver 1,250 MW of clean power to more than a million homes in New York City. Renewable energy generated by existing hydropower facilities in Canada will travel over nearly 340 miles of HVDC transmission cables that will be installed along the route.
When completed, the lines will run along waterways, roads, and railroad rights-of-way to achieve the most direct route and minimize visibility. They will also run underground and underwater, through Lake Champlain, and along and under the Hudson River.
This $6 billion project will help New York meet its clean energy goals. The city has been striving to increase its access to renewable energy ever since the nearby Indian Point nuclear power plant was retired in 2021.
The CHPE project owner, Transmission Developers, says the project will have many benefits. Environmentally, the project will help New York City access cleaner renewable power. In doing so, it will reduce carbon emissions and displace fossil fuels. Economically, the project will help lower electricity costs for consumers, as well as increase jobs, economic activity, and tax revenues for the region it serves.
The project is not without critics. Opponents argue that the route may harm local fish populations and Native American communities.
The Future of HVDC
Many more HVDC transmission projects have been completed or are underway, and future expansion is expected worldwide. However, HVDC does have its disadvantages. The most significant of these is high capital costs caused by the need to invest in expensive converter stations that transform AC power to DC before it can be transmitted.
For this reason, HVDC is only cost-effective for distances that exceed certain break-even points, such as more than 60 km (or 37 miles) for lines underwater and 200 km (or 124 miles) for overhead lines. It’s at these points that the advantages of HVDC outweigh its costs.
As the DOE notes, transitioning from AC systems will require “adjustments to grid planning standards and modeling techniques to adequately plan for the technical differences of HVDC systems.”
Nevertheless, the benefits of HVDC and its potential role in helping the world transition to more green power generation cannot be overlooked. Developers recognize this and appear to have embraced the technology. The market research firm DNV projects at least 46 new HVDC projects to be installed around the world over the next decade, equating to a 94.3 GW addition of HVDC transmission capacity and at least 18,000 km (close to 11,200 miles) of HVDC cable.
The American Council on Renewable Energy (ACRE) asserts that the US is “lagging behind” in the deployment of enough HVDC lines to meet increased demand. It argues that a combination of misconceptions, lack of standards, supply chain challenges, and regulatory hurdles combine to impede progress. It recommends collaboration among grid planning authorities, transmission owners, equipment manufacturers, industry groups, the DOE, and others to address these challenges and remove barriers so that the industry can properly expand.
*Rick Laezman is a freelance writer in Los Angeles, California, US. He has a passion for energy efficiency and innovation. He has covered renewable power and other related subjects for over ten years.