Research On Two-stage LCC Inverter And Its Control Strategy In Low Voltage Microgrid

In view of the low voltage microgrid characteristics and the growing problem of nonlinear factors and three-phase unbalanced load,the research of its control mode and inverter system is significant.In this paper,the two-stage inverter topology structure system is analyzed,and the former level completed DC voltage transform by Buck-Boost circuit,following the fluctuation influence of power supply voltage to accomplish the voltage adjustment,to meet inverter’s follow-up work,and the backward inverter adopts LCC(inductor-capacitor-capacitor)inverter circuit to transform the front-end DC/DC output voltage into alternating current that could satisfy the requirement of grid.Firstly,by the method of state space averaging to analyze and modeling for Buck-Boost circuit,which can get the linear transfer function of AC small signal,considering the system’s instability,the proportional integral compensator was designed,which makes the control system can effectively suppress the low frequency disturbance and quickly follow the given voltage with the input voltage and load step disturbances.Then,established voltage type LCC inverter system,whose output voltage wave only determined by fundamental voltage wave of the input,and which has good output voltage quality under the condition of nonlinear load and unbalanced load.Based on the basic good qualities of LCC inverter system,mainly studies its voltage control strategy in this paper.Aiming at the problem that the open loop control strategy of LCC inverter has a large overshoot and can not restrain the disturbance of input voltage,a real-time hysteresis control strategy of output voltage is adopted to effectively reduce the start-up overshoot and realize the rapid regulation of voltage disturbance.Through the analysis of LCC inverter with three-phase unbalanced load and nonlinear load,according to the system model of LCC inverter,and a compensation chain is introduced to offset the influence of nonlinear load.Aiming at the problem that the output voltage amplitude is decreased due to the dead time effect in the modulation process,the elimination strategy of the dead time effect is designed by analyzing the working mode of the single phase bridge arm at the dead time.Besides,on the basis of introduce the principle of the traditional Pf-QV droop control,the PV-Qf droop control strategy is proposed for the low voltage microgrid,which is based on the coordinate control of frequency and voltage with the virtual impedance.When the inverter supply to the grid,a grid open-loop control strategy is designed,the simulation results show that the open-loop control is feasible in ideal condition,but for the existence of disturbance,the proportional resonant control strategy is designed to realize synchronous grid for LCC inverter system,and the control system can track sinusoidal signal of 50 Hz with no phase deviation,and has effect on other harmonic attenuation.Finally,the microgrid simulation model with two LCC inverters is built in MATLAB/Simulink,and the simulation analysis is carried out under the island mode and grid connected mode.In the island mode,verified the improved droop control strategy can achieve the power allocation and smooth switching.In the grid connected mode,the proportional resonant controller is designed,verified the control can realize the following of grid current to the given current and ensure the unity power factor operation.In addition,the grid connected current of LCC inverter is compared with that of LCL inverter,and the steady-state error and harmonic distortion rate of LCC inverter are less than LCL inverter,further demonstrated the excellent characteristics of LCC inverter.