Research On Control Technology Of Four-leg Inverter Under Unbalanced Load

With the continuous depletion of fossil energy,the problems of energy crisis and energy pollution have gradually worsened.and the efficient use of renewable energy by distributed generation is an important part of solving the problems.As the key equipment connecting the power grid and distributed generation system,the inverter is connected with a wide variety of loads.If they are not controlled,the output voltage quality of the inverter will be affected.Therefore,it is of practical significance to select appropriate topology and control strategy to improve the load capacity and ensure that the inverter outputs symmetrical three-phase voltage under unbalanced load.The traditional three-leg inverter can only be used to output three-phase symmetrical voltage under balanced load,but can not be used under various unbalanced load conditions.Therefore,this paper adopts the topology of three-phase four leg inverter,and the added fourth leg can provide a flow path for unbalanced current to realize the output three-phase symmetrical voltage.According to the coordinate transformation in vector control,through the formula derivation and theoretical analysis of the mathematical model of four-leg inverter,the second derivative model of three-phase load voltage in dq0 rotating coordinate system is obtained,which lays a theoretical foundation for the design of controller later.The modulation strategy has a great impact on the output performance of the inverter.This paper introduces the three-dimensional space vector modulation based on the introduction of the two-dimensional space vector modulation strategy.Through the detailed introduction and comparison the three-dimensional space vector modulation algorithms in the abc and αβγcoordinate systems,so the three-dimensional space vector modulation in abc coordinate system is selected,and the open-loop simulation model and experimental prototype of the system is built.The simulation and experimental results verify the correctness of the modulation strategy and the ability of the four-leg inverter with unbalanced load.Open-loop control can not meet the control performance requirements of the inverter system.On the basis of analyzing and comparing the control strategies of four-leg inverters at home and abroad,an integral sliding mode controller based on vector control is proposed,the detailed design process of the controller is given,and the stability of the system is verified according to Lyapunov stability theory.The closed-loop simulation model of four-leg inverter system is built in Matlab / Simulink environment and compared with PI voltage and current double closed-loop controller.The simulation results show that the four-leg inverter can output three-phase symmetrical voltage under various unbalanced loads,and the system shows good dynamic and static performance under the conditions of system parameter perturbation,load mutation and DC voltage jump,In addition,aiming at the chattering phenomenon caused by the sliding mode control itself,an improved integral sliding mode controller based on sigmoid function is further proposed,which uses sigmoid function instead of symbolic function.The simulation results show the effectiveness of the proposed improved method.There are 62 pictures,10 tables and 65 references in this paper.