In this paper, a hybrid transmission system is presented for variable-speed constant-frequency wind turbines (WTs). This approach employs an electrically controlled speed regulating differential mechanism (SRDM) so that the synchronous generator is able to rotate at a stable speed and, thus, generate electricity in constant frequency regardless of the changing wind rotor speed. In order to find out the operating performance of the proposed SRDM-based WT, a detailed 1.5 MW simulation model is established by which the speed-regulating accuracy, the electrical harmonic pollutions, and the system low-voltage ride-through (LVRT) capability are all studied in depth and then compared with those of the existing direct-drive or double-fed wind power systems. Simulation results indicate that, under various wind conditions, SRDM can efficiently regulate the output power frequency with satisfactory deviations by using a low-power (less than 262 kW) permanent magnet synchronous motor. Compared to the existing WTs, the 5th, 7th, 11th, and 13th current harmonics of the proposed WT are all ideally reduced to 0.4% of the fundamental current. Moreover, around 0.45 p.u., reactive power can be provided for the rapid recovery of the grid during severe voltage sags. The output power quality, system transient stability, and LVRT capability are all improved. Theoretical analysis and case studies give good verifications of the feasibility and superiority of the proposed SRDM-based WT. Published by AIP Publishing.
