Research On Frequency Control Strategy Of Directly-driven Wind Turbine With Permanent Magnet Synchronous Generator

In the maximum power point tracking(MPPT)control mode,the rotor speed of directly-driven wind turbine with permanent magnet synchronous generator(D-PMSG)and system frequency are decoupled,its rotational kinetic energy has little contribution to the system inertia,and can not provide additional active power in frequency control.The increase of wind power capacity will threaten the grid security for the decrease of system inertia.In this paper,the following work has been done to improve the frequency response characteristics of D-PMSG:Firstly,the detailed model of D-PMSG is established,and a simplified model is proposed for the complexity and low calculation speed,mainly on the shafting model and generator converter model.Secondly,the principle of additional inertia control is studied,and an improved additional inertia control method is proposed in view of the mutual influence between additional inertia control and MPPT control.The proposed method utilizes the auxiliary power of additional inertia control to support system frequency,and can compensate the active power reference of MPPT control by adjusting the rotor speed according to the system frequency variation,so as to avoid the mutual influence of the two control methods in the dynamic regulation process.Thirdly,the principle of overspeed and pitch controls is analyzed,and a deloading and frequency control strategy to coordinate overspeed and pitch controls is proposed.The judgment method of the low,medium and high wind speed modes is presented first,then the deloading operation of D-PMSG can be realized by setting the initial power tracking curve coefficient and adjusting the pitch angle.In the frequency control,overspeed control is used first,and the pitch control is put into service when the speed reaches the rated speed,and the pitch control can be divided into three control modes,which are low,middle and high wind speed.Finally,the merits and drawbacks of the improved additional inertia control and deloading and frequency control are compared,and an integrated frequency control method of the improved additional inertia control and deloading and frequency control is proposed,that is,D-PMSG is operated in deloading mode,and the inertial support and primary frequency modulation can be realized by improved additional inertia control,overspeed control and pitch control when the system frequency fluctuates.