| Renewable clean energy as a key breakthrough of energy technology in the Fourth Industrial Revolution has been received more and more attention and support from countries.When power electronic equipment such as inverters is used as an important interface for generating clean energy and is connected to the grid in large numbers,the damping and inertia of the power system will be reduced.By simulating the performance of synchronous generators,Virtual Synchronous Generator(VSG)technology has attracted much attention to have similar virtual moment of inertia and damping characteristics.During actual operation of the VSG,not only the normal operating conditions need to be considered,but also the operating status under the unbalanced grid connection point caused by factors such as parameter asymmetry,unbalanced fault and load unbalance.This paper takes the three-phase three-wire gridconnected inverter as the main body of research,and conducts in-depth research on the failure of traditional VSG control strategy under unbalanced grid voltage.The specific content includes the following aspects:(1)In order to solve the problem of grid-connected current distortion and the problem of output power containing two times frequency fluctuation under unbalanced grid,an improved VSG control strategy based on current control is proposed.First,the output power characteristics under unbalanced conditions is analyzed,the negative sequence current is selected as the control variable and then the negative sequence current command expression is calcuiated under the unbalanced grid.Secondly,according to the derived current command expressions,a unified current command expression for balanced current,constant active power and constant reactive power control is established to achieve coordinated control of multiple goals.Meanwhile,to solve the overcurrent problem caused by grid-connected voltage changes,a dynamic voltage compensation control strategy is proposed to suppress excessive peak current.Finally,simulation and experiment verify the effectiveness of the proposed control strategy.(2)To deal with the fluctuation of DC side voltage and output active power in unbalanced grid,an improved VSG control strategy based on power control is proposed.First,in order to improve the control accuracy of grid-connected current,a voltage and current double closed-loop structure is constructed by simulating the stator equation of a synchronous generator.Secondly,using the feature that the inverter power output contains two times frequency ripples under unbalanced conditions,the 2 times frequency active fluctuating current command is introduced in the dq coordinate system and superimposed with the current command output by the VSG voltage outer loop to realize the suppressing ripples in active power.Thirdly,transform the superimposed current command to the abc coordinate system,eliminate the third harmonic term contained in the original three-phase current command,and the improved 2 times frequency current command is obtained in the dq coordinate system,which improves the current quality injected into the grid.Finally,simulation and experiment verify the effectiveness of the proposed control strategy.(3)In response to the problem that the output current command of the voltage outer loop changes drastically due to the grid voltage fluctuation and the current inner loop cannot track accurately,an improved VSG control strategy based on the disturbance observer is proposed based on the balance current control target.First,the state space equation of the grid-connected model is established and feedback linearization control is used to design the current controller.On the basis of the above,the disturbance observer is introduced and designed to achieve the goal of quickly tracking the changing current command while eliminating errors caused by parameter changes,measurement accuracy,etc.Finally,simulation and experiment verify the effectiveness of the proposed control strategy.This thesis includes 50 figures,5 tables and 84 references. |
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