Research On Control Technology Of Parallel Converters In Microgrid System

In islanding operation mode,the efficient and stable operation of the micro-grid with diversified Distributed Generation(DG)running in parallel is inseparable from the reasonable control technology of parallel converter,the droop control strategy that can realize the seamless switch between isolated island and grid-connected mode has been widely studied.However,in the resistive low-voltage microgrid,power is coupled with voltage and frequency respectively,so the traditional(P-f,Q-U)droop control strategy cannot be applied,the virtual impedance technology can improve the equivalent output impedance of the inverter,but it will affect the stability of the system.In parallel operation of DG with different power levels,how to coordinate load distribution and reduce the influence of line impedance mismatch on the accuracy of reactive power distribution is also the key to ensure the stable operation of micro-grid.Based on the above research background,this paper studies the stability of the parallel multi-inverter system with virtual impedance and the improvement of droop control to achieve reasonable power distribution.The main research works are as follows:Construct a microgrid structure that includes wind and light renewable energy sources.Research and analyze the mathematical models of wind and light distributed power sources,the working characteristics,and the front-end parts of their respective interface converters and their control methods.And build simulation models of photovoltaic and wind power systems in Matlab/simulink for simulation analysis.The simulation results show that the output dc voltage of the two power generation systems can be stabilized to meet the requirements of the inverter for the dc side voltage constant.Aiming at the grid-connected inverter based on traditional droop control,analyze its mathematical model in the dq coordinate system,design a double closed-loop decoupling control structure,and build a multi-inverter parallel model for power analysis.Adding voltage compensation to traditional droop control of reactive power can solve the problem of unbalanced reactive power distribution caused by line impedance mismatch,but the distribution accuracy is not high.The main reason is that when the droop coefficient is set to a fixed value,there is an inherent contradiction between the accuracy of reactive power distribution and the accuracy of voltage compensation.Too improving the accuracy of reactive power distribution will lead to the instability of public bus voltage.Adding virtual complex impedance to the double closed-loop structure improves the inverter's equivalent output impedance from resistance to sensitivity,realizes the decoupling of power,and improves the applicability of(P-f,Q-U)droop control.Simplify the multi-inverter parallel model considering virtual impedance into Thevenin's equivalent circuit,solve the impedance ratio transfer function for the equivalent internal resistance of the power supply and the equivalent load,and analyze the stability of the system according to the Nyquist stability criterion,The results show that the multi-inverter parallel system can remain stable after adding the virtual complex impedance.An improved droop control strategy with adaptive droop coefficient is proposed by analyzing the fixed droop coefficient.The droop coefficient can be adjusted continuously according to the reactive output when the reactive output is related to the droop coefficient,so as to improve the accuracy of reactive power distribution.Build a simulation model of a multi-inverter parallel operation system in Matlab/Simulink to verify the improved droop control strategy under different operating conditions.The simulation results show that the improved droop control strategy is feasible and effective.This paper designs a three-loop control structure with virtual impedance for the parallel connection of inverters with different power levels in the microgrid,which improves the applicability of the traditional droop control strategy in low-voltage microgrids;An improved droop control strategy with adaptive droop coefficients is proposed to improve the distribution accuracy of the reactive power output of each DG.