Research On Inverter Parallel Control Strategy Based On Virtual Impedance

Renewable energy power generation has attracted more and more attention,and distributed power generation and microgrid technology using renewable energy has gradually become a hot spot at home and abroad.By implementing parallel operation of multiple micro power inverters in the micro grid,the overall capacity and reliability of the micro grid can be improved.This article starts the research with the parallel operation of two inverters under an island,mainly researching and analyzing the distribution of its output power,system circulation and output voltage quality issues.First,a brief introduction to the control strategies often used in microgrid inverters is given,and the principle of droop control strategies for microgrid inverters is mainly analyzed,and the expressions of its output power and droop control are obtained.Establish the main circuit model of a single microgrid three-phase inverter,and carry out coordinate transformation on the mathematical model,and design the voltage and current double closed-loop controller,using the filter capacitor voltage as the voltage outer loop,and the filter inductor current as the current inner loop.Double closed loop control of the loop.The operation simulation analysis of a single inverter shows that the system model can run smoothly and stably.The second is to analyze the power distribution of the parallel operation of micro power inverters,the deviation of the output current of the two inverters and the relative deviation of reactive power.And the research discusses the influence of line inductance,active power droop coefficient and reactive power droop coefficient on the system in the system.In order to adjust the line impedance characteristics of the low-voltage microgrid,the output current feedback is introduced to form a virtual impedance loop,and the voltage feedback is compared with the voltage output by the droop controller to obtain the changed reference voltage.Analyze the influence of virtual impedance on system impedance and the influence of virtual resistance on system stability,and get the critical value of virtual resistance to its corresponding line resistance.From the comparison of the simulation results,it can be seen that the system can operate normally and stably,the distribution deviation of the inverter output power of the virtual composite impedance method is reduced,and the system circulation current is also reduced.Finally,the feedback control of the output voltage amplitude and the voltage on the power line are led to the reactive power control loop,and a new relationship between the reactive power and the output voltage amplitude is designed,and a voltage compensation item is added to compensate for the line impedance.Voltage drop,and strengthen the ability of output voltage to track changes in reference signal value.An improved droop control method based on virtual complex impedance is proposed to achieve reasonable load distribution and circulation suppression.Introduce virtual complex impedance,and increase the voltage amplitude feedback and voltage compensation in the voltage/reactive power control to reduce the reactive power distribution error caused by the inconsistent line impedance.The proposed method can realize the reasonable distribution of power,improve the suppression effect of circulating current,and the voltage of the common connection point can be kept near the expected value.Then a system model of two three-phase inverters operating in parallel is constructed.The simulation results show that the control method used can make the system run smoothly and stably,achieving the expected purpose,and the control method is feasible and effective.