Research On Three-phase Four-leg Inverter Based On Improved Repetitive Control

Due to more and more asymmetric and nonlinear loads join in power grid, they have seriously influenced operational efficiency of power grid and enormously threatened power quality. Recently, a three-phase four- leg inverter topology has been proposed because of the merits of compacter structure, smaller bulk and higher efficiency in order to reduce adverse effects of asymmetric and nonlinear loads in the power grid. However, the control method for three-phase four- leg inverter becomes much more complex and difficult to achieve due to increase of switches. The main contents are below:(1) Mathematical models of three-phase four- leg inverter for three-phase balanced loads, unbalanced loads and nonlinear loads are established in stationary system and rotating coordinate system by using symmetrical component method and rotating coordinate transformation in order to accomplish decoupling control of voltage and current between the first three legs and the forth leg in rotating coordinate system.(2) Three dimensional space vector modulations(3D SVM) have been introduced in stationary ??? system and abc coordinate system. Meanwhile implementation process of 3D SVM algorithm in stationary abc coordinate system has been studied further deeply. The modulation strategy is able to avoid traditional ??? coordinate transformation. It makes the selection of switching voltage vectors and calculation of duty cycles easier, and reduces drastically complexity of modulation algorithm.(3) State space models of three-phase four- leg inverter for assumed loads and no load are established. The state feedback controller is designed by using principle of pole placement in order to achieve stability control of system. At the same time, the design of state observer is introduced in order to overcome the limitation of duty cycles by digital signal processor's delay time of sampling and calculating. According to transfer function of the inverter, distribution of expected poles and demand performance, the optimal parameters of PID controller are calculated. The two controllers combine with repetitive controller constitute two double loop control strategies for improving anti- interference of system.(4) Control processes of three-phase four- leg inverter for the three-phase balanced loads, unbalanced loads and nonlinear loads are simulated by MATLAB/Simulink. The results of simulation show that inverter dynamic characteristics and voltage output characteristics are good, under conditions of the loads varying suddenly and in case of the unbalanced or nonlinear loads.(5) Applying three-phase four-leg inverter in small wind power generation system, mathematical models of grid-side converter and wind turbine operation for unbalanced loads are established. As well as low voltage ride through(LVRT) capability of generators is researched when it occures suddenly three-phase short circuit faults and single-phase faults.