Control Strategy On Wide Frequency Operation Of Doubly Fed Induction Generator At Offshore DC Wind Power Field

Wind power generation has been sustained by its clean and renewable development.But the onshore wind has some problems such as consumptive difficult and long-distance transmission.Therefore,offshore wind power is more promising than onshore wind power.The offshore wind farm with a distance of more than 100km and a capacity of more than 200MW is more suitable for HVDC transmission.The all-DC offshore wind farms,the present study focus,using DC converter technology can simplify the AC transformation link and save the cost.All-DC offshore wind farms can be divided into DC series wind farms and DC parallel wind farms according to the form of network.In this thesis,the topology of the wind power generation system is used as the stator side converter with uncontrolled rectifier devices and the rotor side converter with a fully controlled device.This topology is more economical because it uses only one fully controlled device.There are obvious differences in the control targets of the DFIG wind power generation system in these two types of wind farms.The control target of the wind power generation system in a series wind farm is to realize frequency regulation and DC voltage constant.The control target of the wind power generation system in parallel is to realize frequency regulation and power control.In this thesis,two different control strategies are proposed based on the above control targets.The two control strategies need to adjust the frequency.Based on the relationship between frequency and magnetic chain,an indirect control method using magnetic chain control frequency is proposed.Because the DC system does not need to limit the frequency to 50Hz,the operation of the machine at wide frequency can be used to optimize the performance of the motor and reduce the capacity of the fully controlled converter.For wind turbines in series,the influence of wide frequency on the control effect of DC voltage is studied.The frequency has no effect on the DC voltage when the motor is running steadily.But in the case of large disturbance,the frequency will affect the fluctuation of the DC voltage of the power generation system.In this thesis,the influence of the wide frequency operation on the transition process of the power generation system is studied.For wind turbines in parallel,the maximum wind energy tracking of a DC wind power generation system under the broadband frequency operation.And the proposed control strategy can ensure that the frequency change has no effect on the wind energy tracking effect.Therefore,the wide frequency range of the machine can be operated to achieve some optimization targets of the power generation system without affecting the power control.The objective of this thesis is to improve the efficiency of the generator and reduce the loss of machine.And the corresponding control mode of motor optimization is proposed.First,the components of the machine loss are analyzed.Screening out the adjustable loss components and combined with the mathematical method,the optimal magnetic chain solution method is obtained with the minimum loss function as the objective function.Use MATLAB/Simulink to simulate the above control strategy.The control strategy proposed in this thesis can achieve the control target of the power generation system.The simulation results show that the operation and efficiency of the machine can be optimized by using wide frequency.