Research on MW-Level Half/Full Direct-Drive Doubly-Fed Induction Generators for Fractional Frequency Transmission System

Doubly-fed induction generators (DFIG) are widely used in wind power generation systems due to their low cost and high power quality. However, traditional DFIGs are designed with high-speed rotors and require the use of multi-stage gearboxes to increase the speed, which decreases the reliability of the system as well as increases the cost. This paper studies the feasibility of half/full direct-drive DFIG to improve the performance of generators for the Fractional Frequency Transmission System (FFTS). By comparing with traditional DFIGs, the results show that half-direct-drive and direct-drive DFIGs are larger and heavier while having higher copper losses and lower efficiencies. In addition, direct-drive DFIGs have a higher apparent rotor power, which means that a larger capacity converter is required. As can be seen, although the multi-stage gearbox can be reduced or omitted when the speed of the DFIG is lowered, it also poses some new challenges.