| The emergence of photovoltaic microgrid can help China achieve the double carbon goal and improve the reliability of system power supply.The photovoltaic microgrid is powered by a plurality of photovoltaic inverters in parallel.However,due to the different control methods and line impedance parameters of each inverter,a high circulating current will be generated in the photovoltaic microgrid.When the photovoltaic microgrid is in the island mode,the voltage of the AC bus will be separated from the control of the large power grid and coordinated by multiple inverters in the microgrid,which requires higher control requirements for parallel inverters.Therefore,it is of great significance to study the control of parallel inverter of isolated photovoltaic microgrid.Therefore,this paper focuses on the harmonic circulation problem of parallel inverters,aiming at restraining the circulation problem of photovoltaic microgrid in island operation mode,and through the research on the generation rule and physical mechanism of the circulation problem of parallel inverters,a feasible suppression scheme that can adapt to photovoltaic microgrid parallel inverters is proposed.The main research contents of this paper are as follows:First of all,this paper takes three-phase parallel inverter as the research object,establishes the mathematical model of a single inverter,and constructs the equivalent model of two parallel inverters,analyzes the mechanism of the circulation of parallel inverter system,and designs the voltage current double closed loop.Secondly,aiming at the fundamental wave circulation,the control mechanism and shortcomings of traditional droop control and conventional integral droop control are analyzed,and an improved integral droop control based on virtual resistance is proposed.The virtual resistance is used to make the equivalent impedance of the inverter system resistive in the fundamental wave domain,so that the active power is related to the amplitude of the output voltage,the reactive power is related to the phase angle of the output voltage,and the active and reactive power can be decoupled,Make droop control more accurate.On the basis of conventional integral droop control,the setting of the initial value of the inverter output voltage is introduced to reduce the impact current of the inverter during parallel operation and load shedding,so that smooth transition can be achieved during parallel operation and load shedding.The loop current of the parallel inverter during the transient process is suppressed,and the stability of the system is improved.Finally,aiming at the harmonic wave circulation,on the basis of analyzing the generation of harmonic circulation,the virtual harmonic impedance based on the comprehensive strategy and the improved active disturbance rejection control strategy are proposed.By generating negative virtual harmonic resistance in the system with large impedance and positive virtual harmonic resistance in the system with small impedance,the equivalent impedance of the system in the harmonic domain is kept equal;By designing linear state error feedback rate based on compensation disturbance and linear extended state observer with series correction,the improved linear ADRC can track and compensate harmonic disturbance more accurately.Finally,the feasibility of restraining circulation is verified by simulation. |
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