Nine-switch-converter-based DFIG wind power system and its dynamic DC voltage assigned approach for low voltage riding through (LVRT)

This paper proposes a Doubly-Fed Induction Generator (DFIG) Wind Power System, which is driven by the novel Nine-Switch Converter (NSC) instead of the traditional twelve-switch back-to-back (B2B) converter. Moreover, a new adapted control method is proposed to fully use the high dc voltage of the NSC, with which the dc voltage could be dynamically assigned to the rotor-side branch and grid-side branch of the NSC, depending on the different grid conditions. With the proposed control, the gird-side branch gets higher voltage to maintain dc bus steady under normal condition, and the rotor-side branch gets higher voltage to withstand the rotor-overvoltage during Low Voltage Ride Through (LVRT). The proposed system and adapted control method ensure the rating capacity of the power devices can be fully used at any time, and no extra capacity margin on the power devices is required. Considering a traditional twelve-switch B2B converter requires extra capacity margins on its power devices, or additional auxiliary circuit to realize LVRT, the proposed NSC based system is more cost-efficient because it not only saves three power devices, but also achieves LVRT without additional auxiliary circuit.