Indirect Frequency Control Method for Rotor Power Reduction of Large DFIG Individually Linked to Medium Voltage DC Grid

On the condition that doubly-fed wind power generator is individually linked to the medium voltage DC power grid through diode rectifier, the indirect frequency control method is proposed to adjust stator frequency with rotor speed in wider range, which can reduce rotor power and the cost of the rotor side converter for large-capacity DFIG wind turbine. In the vector control system of DFIG, the stator flux linkage is substituted for reactive power to indirectly control frequency following the change of rotor speed dominated by MPPT. Furthermore, a frequency control strategy is suggested and used for limiting rotor power to a given value in the whole range of wind speed change. Simulation results of large capacity DFIG-DC system proved the validity that rotor power can be reduced below a given limited value. Moreover, experimental results of 5kW DFIG-DC system verify the feasibility of the wide-range frequency control method.