Research On Control Strategy Of Microgrid Inverter Under Unbalanced Load

With the large-scale development and application of renewable energy,distributed generation technology has developed rapidly.As the main organization form of distributed generation,microgrid can give full play to the economic benefits and environmental protection advantages of distributed generation.As the basic interface unit of the microgrid,the microgrid inverter needs appropriate control strategies to ensure the safe and stable operation of the microgrid.Virtual Synchronous Generator(VSG)control not only has the flexibility of electronic equipment,but also draws on the advantages of the synchronous generator(SG)to the grid.It can make the microgrid inverter have similar external characteristics and electromagnetic transient characteristics as SG,which can effectively alleviate the problem of underdamping and low inertia caused by the high penetration rate of distributed power,and effectively promote the microgrid system to participate in the operation and regulation of the grid.However,due to the asymmetric impedance of the microgrid line and the random single-phase load,the phenomenon of unbalanced output voltage of the microgrid inverter is particularly common.In this dissertation,the research on the control of microgrid inverter under unbalanced load is carried out.The main content of the dissertation is as follows:First of all,according to the topology of the microgrid inverter,mathematical models in different coordinate systems are established.Drawing on the control principle of SG,the active power-frequency controller and excitation controller unit of VSG are designed and analyzed.At the same time,based on the mathematical model of the microgrid inverter,the voltage and current double-loop control in two coordinate systems are analyzed and parameterized in detail.Secondly,the performance limitations of the traditional phase-locked loop in the face of complex voltage conditions of the microgrid are discussed in depth.The cascaded generalized integrator composed of second order general-integrator and third order general-integrator is analyzed in detail,and the effect of parameters on the filtering and dynamic characteristics of the cascaded generalized integrator is discussed.On this basis,a phase-locked loop structure,DCGI-PLL,is proposed.It is simulated and compared with the traditional phase-locked loop under different working conditions to verify the effectiveness of the proposed phase-locked loop structure.Then,starting from the generation mechanism of the unbalanced voltage,the cause of the unbalanced output voltage of the microgrid inverter is analyzed.An unbalanced voltage compensation control is proposed based on the voltage and current double-loop control structure.It can enable the microgrid inverter to output three-phase symmetrical voltage under unbalanced load condition,and improve the ability of the microgrid inverter with unbalanced load.Finally,the above-mentioned control strategies are comprehensively simulated in MATLAB / simulink to verify their rationality and effectiveness.At the same time,a microgrid inverter experimental platform was built and a low-power experiment was conducted to initially verify the effectiveness of the VSG control strategy.