| In recent years,the level of productivity has greatly improved,accompanied by the global energy crisis and the growing contradiction between people’s demand for electricity consumption.Traditional power systems that mainly burn fossil fuels are difficult to meet the requirements of high electricity consumption and low pollution.Microgrid based on clean energy such as solar energy and wind energy has gradually become a hot research topic in various countries.Microgrid systems can operate in grid connected and island mode,which is important for making full use of various distributed power sources and solving remote power supply needs.Currently,parallel connection of multiple inverters is commonly used to improve the capacity of micro sources and system redundancy.However,in the isolated mode island type,there are issues such as randomness and inconsistency among micro sources,especially the issue of inconsistent line impedance,which leads to difficulty in accurate allocation of reactive power.Common traditional virtual impedance methods cannot solve the problems of difficult measurement of line impedance and frequent changes in impedance angle in practical engineering;At the same time,due to the loss of support from the large power grid,the stability of the system voltage and frequency cannot be guaranteed after primary control.In response to the above issues,this article mainly does the following research:Firstly,a mathematical model of a single three-phase inverter is established,and the mathematical models of the inverter in three coordinate systems are derived in detail.Then,a power,voltage,and current three-loop control loop is designed to adjust the controller parameters and filter circuit parameters.The model can be extended to multiple inverters in parallel.Secondly,based on the parallel model of two three-phase inverters,the principle of power distribution for low-voltage micro grid parallel inverters based on droop control is introduced.Through simulation analysis of power distribution under various operating conditions,the limitations of traditional droop are recognized: Although active power can be accurately distributed according to the droop coefficient,due to the impact of line impedance,the accurate distribution of reactive power must meet the condition that the micro source impedance is inversely proportional to the droop coefficient.Therefore,in this paper,the self-adjusting virtual impedance method is used to improve the problem of inconsistent line impedance,while overcoming the practical problem of traditional virtual impedance methods requiring measurement of system impedance,and achieving decoupling between active and reactive power in the steady-state of the system.At the same time,in order to reduce the steady-state deviation of frequency and bus voltage caused by droop control,an event-triggered secondary control strategy for the microgrid is established to compensate and restore the frequency and voltage in the primary control,in order to improve the power supply quality of the system.MATLAB/Simulink software is used to simulate and compare the system before and after the introduction of the control strategy,verifying the effectiveness of the proposed strategy.Finally,a parallel physical model of two three-phase inverters is built,and the hardware circuit and component parameters are designed.With TMS320F28335 as the control core,the control software design of the system is completed.Combined with hardware debugging,experiments are conducted on this platform and the results are analyzed,verifying the effectiveness and feasibility of the proposed control strategy in improving power distribution and power quality. |
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