Connecting the Country with HVDC

Today, the U.S. Department of Energy’s (DOE) Office of Electricity (OE) and Wind Energy Technologies Office (WETO) released a $10 million funding opportunity announcement to fund research to drive innovation and reduce costs of high-voltage direct current (HVDC) voltage source converter (VSC) transmission systems. This investment is intended to enable future cost-effective grid upgrades required to integrate increasing renewable energy generation on to the grid, both onshore and offshore. 

HVDC transmission has significant advantages compared to conventional alternating current (AC) lines, including greater efficiency over long distances, lower costs at these distances, and the ability to connect asynchronous systems. The latter is important because the U.S. power grid is divided into three asynchronous systems: the Eastern interconnection, the Western interconnection, and the Texas interconnection. Transferring power between these regions without disrupting the frequency of either system is only possible with direct current (DC) links.

In the early years of our country’s electric grid, AC became the standardized system for transmitting electricity because DC power systems were expensive and technically complex. Since then, advancements in power electronics have made HVDC transmission more economically feasible. On a typical HVDC link, power is sent to a converter station, where the current is converted from AC to DC. Power is then transmitted over HVDC cables to a second converter station, which converts the power back to AC to be sent to end users.

In 1970, the country’s first HVDC system—Pacific DC Intertie—was completed. This system enabled the delivery of low-cost hydropower from the Pacific Northwest to load centers in Southern California. Since then, several additional HVDC lines have been added, as shown in figure 1. In recent years, as the country works to decarbonize and reimagine a grid with high shares of renewables, there is potential that HVDC transmission could play a critical role in developing a least-cost, reliable, carbon-free grid.

Many of our country’s natural resources are far from load centers and the hourly availability of resources does not always correlate with energy demand. Long distance transmission lines could play a critical role in delivering low-cost renewable energy from areas with excess supply to areas with high demand. This could facilitate decarbonization in high-load regions and reduce the amount of unused electricity, helping enable efficient use of the country’s resources.

HVDC transmission systems do face challenges. By design, AC current periodically goes to zero. This characteristic is exploited for conventional protection systems; in the case of a disturbance on the grid, electric current can easily be stopped. DC current does not have the same property and stopping the current can cause significant wear to the system. Innovators are working to design technology to detect, isolate, and mitigate faults on DC systems.

Transitioning from AC systems requires adjustments to grid planning standards and modeling techniques to adequately plan for the technical differences of HVDC systems. While AC systems are standardized so components from various manufacturers can easily be integrated, DC systems do not have the same level of standardization and interoperability. 

Ultimately, HVDC technology has many benefits for the American grid as it is cost-effective over long distances. Converter stations required for HVDC deployment are expensive, however, with a breakeven distance of approximately 37 miles for submarine lines and 124 miles for overhead lines, HVDC systems offer cost savings that outweigh these high converter costs. These savings include relatively low loss rates and cheaper construction costs. Over long distances, HVDC losses can be up to 30-50% lower than comparable HVAC systems, reducing long-term costs.

HVDC's connection to asynchronous grids could provide flexibility and reliability advantages. The ability to transfer energy between the Eastern Interconnection, Western Interconnection, Texas, and Quebec could enable reserve sharing, especially during contingency conditions, which would improve grid reliability and resilience. Additionally, the ability to connect asynchronous systems could enable greater integration of microgrids, offshore wind resources, and fractal grids through HVDC connections.
 
HVDC technology — including voltage source converters — also allows for control of energy flows. System operators can inject power onto the grid to minimize the impact of disturbances and recover from blackouts. This ability could be particularly useful to accommodate high shares of variable resources, which otherwise could compromise system frequency and stability.

OE supports research and development of HVDC technology due to its significant advantages over conventional AC lines. Our office was excited to announce the recent winners of the American-Made HVDC Prize. The knowledge gained from this prize will help reduce technology gaps that hinder HVDC deployment in our country and enable new innovative solutions to technical challenges through the development of new hardware, controls, and advanced concepts. Furthermore, OE also recently released the HVDC Cost Reduction (CORE) Initiative that supports research and development to reduce HVDC technology and long-distance transmission costs 35% by 2035 (35 by 35). The CORE initiative requires coordinated action, guided by an aggressive goal: to develop and domestically manufacture HVDC transmission technologies to meet all U.S. market demands by 2035 in a cost competitive manner. Congress directed FY 2023 appropriations for OE to fund this initiative.

As a partner in this space, WETO also supports HVDC technology advancement to enable deployment of offshore and land-based wind. They recently announced the selection of four HVDC-based projects to initiate standards creation, develop a HVDC benchmark system, develop requirements for multi-terminal HVDC systems, and create a HVDC curriculum for workforce development.

By incentivizing innovation in HVDC technology we not only accelerate progress, but empower a new generation of thinkers to shape the world we want to live in. Together, we can unlock the full potential of HVDC and usher in an era of cleaner, more accessible energy for all.