Small Signal Stability of Fractional Frequency Transmission System With Offshore Wind Farms

Fractional frequency transmission system (FFTS) is a relatively new transmission technology that can be used to deliver the energy from remote offshore wind farms, and it can increase the transmission capacity through the reduced transmission frequency. However, the dynamic performance of a FFTS with the wind farm may be different from that of a traditional ac transmission with the wind farm. In this paper, a doubly fed induction generator (DFIG)-based wind farm is connected to the main grid via the FFTS. The detailed dynamic model of the FFTS with wind farm is established first, and then the eigenvalue analysis is carried out to evaluate the system dynamic damping performance. To verify the results of eigenvalue analysis, dynamic simulations are carried out on a single machine infinite bus and a multimachine system. Both eigenvalue analysis and simulations demonstrate that the FFTS will have a negative influence on the damping of the DFIG-based wind farm in comparison to that of the wind farm with the traditional ac transmission system. However, with the control of cycloconverter, this problem can be easily overcome, and the damping performance of wind farm with FFTS can be even better.