Study On SOCVF Feedforward And Phase Compensation Control Of Inverter In Weak Grid

The use of renewable energy has developed rapidly in recent years.Direct-drive wind power generation system is one of the important forms to make full use of wind energy.The grid-connected inverter is an important part of the direct-drive wind power generation system.However,the distribution of wind energy is not balanced with the demand area.Because of the influence of long-distance transmission and other factors,the traditional power grid is weak.so the traditional grid is weak.Wide range of grid impedance and severe background harmonics are two characteristics of weak grid.This brings a threat to the stable operation of grid-connected inverter in direct-drive wind power system.Therefore,it is of great significance to study the control strategy of the grid-connected inverter in weak grid and make the grid-connected inverter better serve the direct-drive wind power generation system.The stability of grid-connected inverter in strong grid is the premise of grid-connected inverter in weak grid.In order to ensure the stability of grid-connected inverter in strong grid,this paper adopts a method of designing PI regulator parameters according to shear frequency,and introduces the design process in detail.In order to suppress the resonance peak of LCL,active damping control method of capacitive current feedback is adopted.The math model of grid-connected inverter in weak grid is established.From the perspective of loop gain,it can be concluded that when the grid impedance changes in a wide range,the phase angle margin and shear frequency of the system will decrease greatly.From the point of view of the output impedance of inverter,the distortion degree of grid-connected current will be larger when the grid impedance and low background harmonics exist at the same time.In the case of rich background harmonics in the grid voltage,In order to realize accurate phase-locking,a Dual Second Order Generalized Integrator(DSOGI)is added on the basis of SRF-PLL.Because the? axis and ? axis of DSOGI are coupled,a complex vector method is adopted in this paper to establish the model of DSOGI and realize the decoupling of ? axis and ? axis.DSOGI is essentially a Second-Order Complex Vector Filter(SOCVF).In order toimprove the stability of grid-connected inverter in weak grid,a control method based on SOCVF feedforward is proposed.In order to suppress the background harmonics of the grid,This paper introduces a special frequency resonance regulator(SFRR)method.When the maximum frequency of SFRR is greater than the shear frequency of the system,the system is unstable due to the affection of grid impedance.Therefore,the regulator is improved.and on the basis of the improved regulator,a phase compensation control method based on the phase compensation factor switching based on the judgment of the grid impedance mode value is proposed.This method not only improves the stability and rapidity of the system but also effectively restrains the influence of low subbackground harmonics on the incoming grid current.In this paper,a simulation model is built in Matlab/Simulink software,and a three-phase two-level LCL grid-connected inverter prototype is built in the laboratory.The rationality of the theoretical analysis and the effectiveness of the control strategy adopted are verified by simulation and experiment.