Research On Digital Control Technique For Three-Phase Three-Wire Inverter

According to the features of three-phase inverter, several aspects are studied systematically and deeply, such as good voltage regulation, dynamic performance and waveform characteristic, and so on. Based on the mathematical model, the digital control system of three-phase inverter is analyzed typically. The studies carry out in the thesis and the according results are shown as follow:(1) The mathematical models in abc and dq frame for three–phase inverter are established and analyzed. This thesis analyzes the coupling relationship of dq frame for three-phase three-wire inverter in detail, and compares the difference, such as with and without PI control. This thesis proposes and analyzes single-loop decoupling method, and testifies the feasibility and arability.(2) The causes of waveform distortion for three-phase inverter are analyzed in detail, such as the influence of SVPWM modulation, PWM over-modulation, switch-vector choice and SVPWM dead time effects. The thesis deeply analyzes the harmonic effects of unbalanced linear load and balanced nonlinear load, and the magnetic DC bias. Base on these, the control system design based on internal mode principle is the ultimate solution to improve output voltage waveform.(3) The amplitude-response Bode plot of the inverter is tested by using the frequency-response test to get the accurate model by using the second-order system approximation. Based on this model, the continuous domain design, indirect discrete domain design and direct discrete domain design are compared to analyze their characteristic differences. The simulations using the direct discrete domain design testify that, close loop design has good adaptability and strong robust performance. Theoretical discussion of the repeative controller are presented in detail, including the stability, convergence and steady-state error. In this thesis, the repeative controller design of this system is presented, especially using three compensator design method.(4) The stability ad steady-state error analysis about the parallel control system between PI control and repeative control are presented in detail, and then present that, in this condition, the target of the repeative controller should not be seen as the close-loop system including PI controller. At last, the simulation and experiment results of this whole three-phase three-wire inverter system are presented in detail to testify the reasonability and effectiveness.