Research On Converter Control And Power Quality Control In Microgrid

Because the droop control cannot avoid the large impact generated when large-scale power electronic devices are connected to the power grid,the virtual synchronous generator emerges.The virtual inertia that simulates the traditional synchronous generator is added,and the impact of the micro-grid is greatly reduced.At the same time,there will be large power quality problems under the condition of multiple virtual synchronous generators.To complete the access problem of multiple virtual synchronous generators in a microgrid,it is necessary to analyze and improve each power quality problem.The main tasks include:(1)The hierarchical control strategy architecture based on sagging control is introduced,and an improved control strategy in grid-connected state is implemented for this control architecture.(2)Analyze the modeling principle of the active and reactive loops of the virtual synchronous generator,and illustrate the influence of the inertial coefficient,excitation coefficient,active droop coefficient,and reactive droop coefficient of the virtual synchronous generator on the overall system.A parallel equivalent model is proposed to obtain the corresponding relationship between P-f and Q-E,and the influence of line impedance and virtual impedance on the power distribution of the system is analyzed.(3)Explained the basic mechanism and improvement method of power quality of multi-parallel synchronous generators,introduced the basic concept of power quality control strategy,and proposed the use of virtual impedance,harmonic imbalance suppression ring.The secondary control adds voltage offset compensation to the common synchronous coupling point for the virtual synchronous generator power loop;the virtual impedance is used to compensate for different line impedance ratios,resulting in unbalanced power distribution caused by multiple VSG parallels;harmonic imbalance suppression loop For the off-grid environment of the microgrid,the voltage harmonics and unbalance caused by the non-linear,unbalanced loads at the PCC point.(4)Perform small-signal stability analysis on the power quality improvement control strategy of the multi-parallel virtual synchronous generator,including single virtual synchronous generator model,improved control model,and aggregated micro-grid overall model.Finally,the eigen-root analysis method is used for virtual analysis.Synchronous generator related key parameters analysis and stability determination.(5)Transiently improve the traditional virtual synchronous generator control strategy,adding model predictive control to replace the traditional voltage and current double closed loop,can effectively improve the voltage response speed,reduce the voltage distortion due to the addition and subtraction,by modifying the virtual Synchronous generator parameters can still guarantee the inertial characteristics of the current.