Connecting Renewable Energy to the Electric Transmission Grid
Trending Toward Innovative HVDC Technology
By Jennifer Rouda & Silvia Yanez
Developers of grids to serve large-scale wind and solar installations are now considering modern High Voltage Direct Current (HVDC) as an appealing business model due to the potential cost savings and environmental advantages for long-haul, point-to-point transmission systems. HVDC systems have unique characteristics that make them smart options for certain electric transmission problems and, from an environmental permitting viewpoint, may have significant advantages given their reduced footprint over long distances, the possibility to efficiently deliver power through existing energy corridors, and the reduced risk of agency and community controversy.
Renewables developers recognize HVDC’s economic value proposition
Although several of the first successful HVDC projects in the US have avoided significant land impacts by using subsea routes, several long-haul HVDC lines are currently proposed to move green power from the rich wind resources in Wyoming and the Plains states to hungry load centers in Las Vegas, Los Angeles, and the East Coast.
Although AC systems work well for interconnecting power to the grid locally, when transporting higher voltages over greater distances, developers can realize economic savings with HVDC. For example, compared to HVAC, construction costs for installing HVDC lines are reduced because fewer sets of conductors are required, overhead clearance requirements are reduced, and intermediate stops to deliver power locally (with their added permitting and materials costs) are not required. Additionally, HVDC is more efficient because less energy is lost in transport and can be more controllable in accommodating the load variability typical of renewable resources.
Environmental Advantages of HVDC: Reduced footprint saves time & money
HVDC systems can also have environmental advantages that translate into cost and permitting schedule savings. When compared to AC systems of similar capacity, a major advantage of HVDC systems is the reduced footprint resulting from narrower permanent rights-of-way (ROWs) and the smaller amount of land required to build and operate the converter stations at each end of the line, as opposed to multiple intermediate facilities. Additionally, while converter stations may have associated visual, cultural, noise, or land use impacts, a reduced footprint can equate to diminished biological and cultural survey requirements—both time consuming, limiting factors for permitting schedules—and fewer impacts on land use and wildlife habitat. This is particularly important where ROW grants are expensive to procure or threatened or endangered (T&E) species habitat is present, as T&E species mitigation can be a significant expense in and of itself.
The reduced ROW requirements for HVDC are also beneficial for existing congested corridors because replacing existing AC lines with DC can increase the power transmission capacity in the same available space. AC and DC lines can also coexist within the same already approved, dedicated corridors, assuming reliability requirements are met. According to Areva, a HVDC solutions supplier, the same tower body, foundation design, and ROW width can support a 220kV AC line or a 380kV DC line that could deliver 300% more power. This significant reduction of materials and land use requirements translates into valuable potential environmental benefits—not only in terms of the permitting process, but also in terms of the use of or effect on resources.
Additional HVDC environmental advantages include reduced visual impacts from shorter towers and fewer structures, reduced electric and magnetic fields (EMF), and lower corona effects and radio interference, which may be key community and agency concerns.
State renewable performance standards are ratcheting up across the nation with 10% to 40% renewable energy required by most states by between 2015 and 2025. To meet these mandates, the industry must implement creative solutions for delivering solar and wind power from remote locations to city users. According to the Western Wind Solar Integration Study released by the National Renewable Energy Laboratory last May, the Western US energy grid could accommodate up to 35% renewable energy by 2017—assuming significant grid upgrades are completed.
HVDC Success on Two Coasts: The San Francisco Trans Bay Cable & the Neptune RTS Projects
A unique, offshore HVDC success story is the San Francisco Trans Bay Cable (TBC) project. The 53-mile-long 400 MW HVDC submarine electric transmission line was installed to transfer energy from an existing Pacific Gas and Electric transmission substation in Pittsburg, California to the San Francisco peninsula. HVDC was selected since undersea AC cables incur more line losses, DC power is easier to control, and a DC system could reliably deliver 400 MWs of power to San Francisco. Due to proactive issue identification and active stakeholder engagement, the public/private partnership project completed environmental permitting on schedule and is expected to be operational by late 2010.
Another successful HVDC project is the Neptune Regional Transmission System (Neptune RTS). The 65-mile-long, 500-kV subsea HVDC transmission network connects Hempstead, Long Island to Sayreville, New Jersey with 660 MW of transmission capacity. More than 50 miles of the route is undersea in the outer New York Harbor and the Atlantic Ocean, with a small portion of onshore underground line built in an existing ROW. The environmental permitting process included state and federal applications for the converter station sites and onshore and offshore transmission line corridors. This process involved early participation of environmental, technical, and legal teams to develop a permitting strategy, meetings, and negotiation with regulatory agencies, and an early focused public involvement process. With this successful strategy, the project was completed ahead of schedule and within budget and constructed in an environmentally sound manner, avoiding major fishery and other environmental and cultural sensitive locations. The project was fully permitted in 2006 and became operational in June 2007.
For unique projects like the Trans Bay Cable and Neptune RTS, the cost of installing converters to connect DC segments to the existing AC grid is often warranted, even for relatively short distances. Considering the reduced environmental impacts and faster permitting schedules, HVDC is a cost-effective strategy, particularly for developers of long-haul transmission projects.
Jennifer Rouda is a principal transmission planner, and Silvia Yanez is an environmental engineer with Ecology and Environment, Inc. (E & E). E & E is a global consulting firm currently completing the environmental permitting for several long-haul HVDC projects.
Ecology and Environment, Inc.
www.ene.com
