Distributed Energy Resources

Report eyes role of HVDC lines with high levels of renewables

High-voltage direct current (HVDC) lines could help mitigate some of the problems that higher levels of renewable energy penetration pose for the electric power grid, according to a new report commissioned by the Energy Information Administration.

The EIA commissioned the study from ICF in order to inform its long-term planning models and projections. The consulting firm was asked to review the extent to which HVDC lines might mitigate challenges resulting from additional renewable generation and to assess the advantages and disadvantages of using HVDC lines to transmit electricity generated from renewable sources, as well as the potential costs of building additional HVDC lines.

In the report, ICF reviewed several publicly available reports and prepared case studies for three major market regions in the United States to support EIA’s effort to assess the potential for HVDC transmission networks to mitigate the impacts of nondispatchable generation sources such as wind and solar power.

Among the negative effects of non-dispatchable renewable generation, the report identified generation curtailment, depressed or negative energy prices, system stability issues because of mismatch of generation and demand, an increased need for ancillary services, and inefficient unit commitment and dispatch.

The report found that HVDC lines can mitigate the impact of nondispatchable resources because DC power flows are controllable. They also have low losses over long distances, and they are they are decoupled from AC systems and are suitable for asynchronous interconnection, which when properly located can act to contain a cascading failure on a grid.

Currently there are relatively few HVDC lines in operation in the United States. Most of the 700,000 circuit miles of power lines in the U.S. operate using alternating current (AC). Typically, HVDC lines have been used to transfer large amounts of power over long distances because DC lines lose less power in transmission than do AC lines. On average, HVDC lines lose about 3.5% of their power per 1,000 kilometers, compared with 6.7% for AC lines. HVDC lines also can lose up to 1% of their power at the converter station, but DC line losses are still between 30% and 40% lower than AC line losses, the report said.

Relatively low line losses make HVDC a good technology for moving electricity from remote areas with high quality wind resources to population-dense load centers. According to the report, moving electricity from areas of high renewable penetration to areas with lower levels of penetration could also help reduce the need for increased ancillary services associated with renewable generation.

HVDC lines can also operate at overload capacity for a limited period of time – usually at 10% to 15% higher than rated capacity for less than 30 minutes – which AC lines are not capable of doing. That ability could provide grid operators valuable time to balance a grid with higher amounts of intermittent resources that experiences surges in output.

DC transmission lines also are more suitable for underwater applications, which could be an important consideration as more regions in the U.S. begin to explore the deployment of offshore wind farms.

To date, one of the biggest barriers to HVDC deployment has been the cost of converter stations to transform DC power into AC power. The per-mile cost of HVDC projects ranges between $1.17 million and $8.62 million, according to the report. The conversion station alone can account for as much as 60% of the total fixed cost of an HVDC project, ICF said.

The report also identified other factors inhibiting the deployment of HVDC lines. These include regulatory matters, such as siting issues; difficulty in deploying multi-terminal HVDC networks; preference for smaller, lower-cost solutions, and a lack of standardization across projects.

In the report, ICF noted that several issues remain to be addressed concerning the wider use of HVDC technology. Among those issues is a better understanding of how regional characteristics affect the benefits and deployment of HVDC lines. For instance, whether or not some system configurations and topologies of AC and DC interfaces are more effective at mitigating some or all impacts from nondispatchable generation.

ICF also said more study is needed on the penetration levels at which HVDC would be required for specific systems and on the quantitative measures of the extent to which regional and other factors can affect penetration levels.

Additional details about the report are available here.