The Reasearch On Control Technology Of Four-Leg Inverter Under Unbalanced And Nonlinear Loads

With the continuous depletion of fossil fuels and the increasingly serious environmental pollution problems,clean renewable energy sources have been widely used,which has changed the traditional pattern of power generation,power supply and usage of AC power.DC power is often converted to AC power through inverters and used for load.However,in the actual production and life,there are unbalanced loads and non-linear loads.If they are not controlled,unbalanced voltages and multiple harmonics will be generated,affecting the power quality.For this purpose,appropriate control techniques must be used to counteract the transformer is controlled to achieve three-phase output voltage balance and reduce harmonic components so as to meet the power system requirements for power quality.The main reason for the unbalanced three-phase voltage output of the inverter is that the three-phase load is unbalanced.For this reason,the selected topology is a three-phase four-leg inverter,which has the ability to have an unbalanced load.The mathematical model based on the stationary coordinate system and the rotating coordinate system was established,which laid a good theoretical foundation for the study of follow-up control methods.Through analysis and comparison,the three-dimensional space vector modulation(3D-SVPWM)strategy in the coordinate system is determined,which has the advantage of achieving a simple and simple calculation process.A composite control method using voltage control loop and fuzzy quasi-proportional resonance control is established.Unbalanced three-phase voltage output is caused by unbalanced load.In this paper,the symmetrical component method is used to decompose asymmetric components into positive sequence,negative sequence and zero sequence.The sequence components are converted into DC signals through coordinate conversion and decoupling,PI controller realizes the double closed-loop control of voltage and current in the rotating coordinate system.Because the negative sequence component and the positive sequence component rotate in opposite directions,a dual synchronous rotating coordinate system is established.For a nonlinear load,the components in the rotating coordinate system contain harmonics of different frequencies and cannot be controlled by the double synchronous rotating coordinates.Therefore,a fuzzy quasi-proportional resonance control method is added to the PI control of the voltage outer loop.The quasi-proportional resonance control method plays a role in suppressing harmonics.The fuzzy control method can change the control parameters in real time according to different loads.Their combination can meet the power quality requirements under different loads and real-time varying loads,and improve the steady-state performance of the inverter.Under the environment of Matlab/Simulink,the simulation models of the main circuit,positive sequence,negative sequence,zero sequence,modulation strategy,double closed loop,coordinate transformation and other modules were established.The open-loop simulation analysis of the adopted three-dimensional space vector modulation strategy was carried out.Then,the closed-loop simulation method is implemented by the double closed-loop control method.The simulation results show that the control strategy and control method used in this paper can achieve the three-phase output voltage balance of the three-phase four-leg inverter under unbalanced or non-linear load conditions.The purpose of the smaller harmonic distortion rate.