Research On Control Strategy Of Microgrid Inverter Under Unbalanced Three-Phase Load

In recent years,China has been actively promoting the concepts of "carbon peaking" and "carbon neutrality" in order to address the prominent issues of resource and environmental constraints.This is an important step for China to achieve sustainable development.As a result,renewable energy sources have been extensively applied in various sectors of production and daily life,especially in microgrid systems composed of new energy sources.These systems often adopt a distributed structure,which greatly enhances the flexibility of power generation.However,microgrids themselves present two problems.Firstly,traditional power grids are mostly composed of synchronous generators,while new energy devices are predominantly comprised of inverters using power electronic devices,lacking inherent damping inertia.Secondly,the inherent three-phase impedance asymmetry in microgrid lines and the randomness of single-phase loads on the user side make the unbalanced phenomena in microgrid systems particularly noticeable,especially in islanded conditions where this problem becomes more severe.Therefore,studying control strategies for microgrids under unbalanced conditions is especially important.The main contents of this article are as follows:Firstly,addressing the first problem,traditional inverters in microgrid islanded operation often adopt control strategies such as PQ and VF.However,these control strategies cannot provide inverters with damping inertia.Therefore,this article selects Virtual Synchronous Generator(VSG)technology as the main control strategy for inverters.By modeling and analyzing synchronous generators,the control structure of VSG can be divided into four parts: frequency-active power controller,mechanical motion controller,excitation controller,and voltage-current dual-loop controller.Traditionally,the voltagecurrent dual-loop control strategy can generally be classified as QPR and PI control.Each method has its own advantages and disadvantages.QPR control does not require decoupling and can directly control the AC signal without error,but it has the disadvantages of lowfrequency gain,poor dynamic response,and difficult adjustment of Kp and Kr parameters.On the other hand,PI control can only perform error-free control on the DC signal,so decoupling of the controller is required,but it has the advantages of good dynamic response and simple parameter tuning.Therefore,this article adopts PI control.To address the issue of PI parameter tuning,the article utilizes the idea of series calibration to calculate the PI parameters.At the same time,the system’s dynamic response and harmonic content can be adjusted by setting the cutoff frequency and mid-frequency bandwidth.Secondly,addressing the second problem,the article first analyzes the issue of output voltage imbalance caused by unbalanced loads in off-grid(islanded)scenarios.The main cause of microgrid imbalance is that the inverter output voltage contains not only positivesequence components but also negative-sequence components.Since this article discusses three-phase three-wire systems without zero-sequence components,the focus in the third chapter is on how to correctly extract the positive-sequence and negative-sequence components from the output voltage.In terms of extraction methods,the article adopts a double generalized second-order integration-based positive-negative sequence separation method and compares its advantages and disadvantages with the 90° positive-negative sequence separation method based on the stationary coordinate system.Furthermore,to address the issue of output voltage imbalance under unbalanced conditions,the article proposes a VSG control strategy based on unbalanced conditions.To tackle the issue of second harmonic components contained in the active and reactive power components caused by unbalanced conditions,the article designs an improved notch filterbased scheme that can suppress both second harmonic components and higher-order harmonics.To overcome the insufficient dynamic response of the loop,the article references the current source VSG voltage feedforward strategy and proposes a voltage source VSG voltage feedforward strategy.Finally,simulation modeling and verification are conducted using MATLAB/Simulink software.The simulation results demonstrate that the unbalanced control strategy...