AC/DC Hybrid is a Powerful Solution
Researchers at the University of Tennessee-Knoxville (UTK) have developed a hybrid solution that combines alternating current (AC) and direct current (DC) in the same AC line, forming a hybrid AC/DC line that maximizes the benefits of both. The project is an initiative of the Engineering Research Center (ERC) for Ultra-wide-area Resilient Electric Energy Transmission Network (CURENT), which is co-funded by the National Science Foundation and the Department of Energy and headquartered at UTK.
This research is part of CURENT’s efforts to develop technologies that advance a nationwide electric power transmission grid that is efficient, reliable, low-cost, and makes the best use of renewable resources. While high voltage DC (HVDC) lines are recommended in modern power transmission systems, approvals for new lines are difficult to obtain. By using existing AC lines for DC current, greater power can be transmitted without major modification of the original power grid, and purchasing new land for the line can be avoided. This research shows that the hybrid concept is feasible for practical application.
AC vs. DC has been an eternal debate topic since the days of Edison and Westinghouse. Historically, AC won the victory due to the easy control of voltage step up and down by AC transformers. However, DC can transfer more power for a given transmission line and has lower cost in long-line transmission. Therefore, high voltage DC (HVDC) has been recommended in modem power transmission systems because approvals for new lines are difficult to obtain. Still, it is expensive to rebuild and replace existing AC lines with new HVDC lines.
The solution: combining AC and DC current in the same AC line to form a hybrid AC/DC line. With a proper DC injection link, DC current can be carried along the existing AC lines. To implement this hybrid AC/DC concept, the UTK team developed different approaches for the DC injection link and a proposed methodology for system design. Analysis shows that this hybrid scheme can increase power transfer capability with lower cost than that of converting AC line to pure DC line.