EHV AC and HVDC transmission working together to integrate renewable power

Renewable resources in North America are extensive and are expected to represent a growing share of total resources. Large hydro and geothermal resources are concentrated in fixed locations and can be readily dispatched. Wind and solar resources are more dispersed, remote from major load centers, and variable in their output. New electric transmission infrastructure is needed to access newly developed renewable resources. For optimal transmission development, it is essential to take a long term approach that reflects the characteristics of the sources being connected. The transmission system must have capacity to aggregate these diverse renewable sources, transport usable blocks of power to remote load centers and disperse the power to load serving entities in an efficient and reliable manner. This paper outlines strengths and complementary features of extra-high-voltage alternating current (EHV AC) and high voltage direct current (HVDC) transmission systems and advocates a hybrid approach, tailored to integrating renewable resources. Variability of generation can be moderated across larger zones, losses can be reduced through higher voltage and improved grid efficiency, reliability can be strengthened with enhanced system control, and the use of resources and rights of way can be optimized with respect to the need, economics and environment. AC systems provide flexible access to sources and loads alike. An expansive overlay of the highest practical AC voltage forms the network backbone, serving as a high capacity platform for system integration. HVDC transmission offers express, efficient delivery of large blocks of power from point to point over long distances, continuous power flow control, fault current limitation and links that can tie asynchronous AC systems. HVDC is also well suited to submarine or underground links, where AC connections are difficult or impractical. Underground HVDC can provide easier access into densely populated urban areas as a creative "off ramp" or extension of the grid. Combining the strengths of EHV AC and HVDC technologies can focus the effectiveness of renewable energy and extend the practical range of its reliable delivery. These technologies' particular strengths can work well together in many settings. A long term view with a broader planning scope requires coordination and recognition that the ideal system is greater than a collection of disparate projects.