Research On Quality Improvement Method Of Output Voltage Waveform For Four-leg Inverter

As an important part of the distributed generation technology,the microgrid has attracted extensive attention.The three-phase inverter which is the key interface circuit in the microgrid,plays a decisive role in power quality.However,the unbalanced load and nonlinear load will adversely affect the waveform quality of the inverter,causing output voltage to become imbalanced or distorted.In order to deal with the degradation of power quality caused by these loads,pulse width modulation(PWM)strategies and control methods of the four-leg inverter are studied in this dissertation.In order to compensate for the shortcomings of the three-dimensional Space Vector Modulation(3-D SVM)method,an improved 3-D SVM strategy is proposed.According to the conclusion of theoretical analysis,it is found that zero vector is the key to causing the high common-mode voltage.Therefore,only six voltage vectors without common-mode voltage and a pair of complementary voltage vectors(pppn and nnnp)are used in the improved algorithm.The simulation and experimental results verify that the improved 3-D SVM algorithm not only can effectively reduce the output common-mode voltage of the inverter,but also can simplify the algorithm.In order to solve the problem of power quality degradation caused by loads,a closed-loop control system based on improved repetitive control algorithm is designed.In the synchronous rotating coordinate system,the mathematical relationship between the load and the output voltage is established.With the help of the repetitive controller,the performance of the control system to suppress periodic disturbance signals is improved.In addition,in order to save the data occupation,the even-harmonic repetitive controller is adopted in the d-axis and the q-axis,and the odd-harmonic repetitive controller is adopted in the 0-axis.Simulation and experimental results show that the repetitive control scheme can effectively improve the output power quality of the four-leg inverter under unbalanced load and non-linear load conditions.