Research On Three-phase Four-leg Converter Based On Repetitive Control

In order to cope with the shortage of fossil energy and environmental pollution,the country strongly advocates the use of solar energy,wind energy and other clean and nonpolluting energy,as a device to realize the conversion of electrical energy and the interface between the load and the grid,the converter has also been vigorously developed.With the widespread use of unbalanced and nonlinear loads,various converter topologies adapted to the four-wire system have been proposed.The three-phase four-leg converter adds an additional leg to the traditional three-phase three-leg converter,and the midpoint of this leg is connected to the neutral point of the load to provide a zero-sequence current path,which enables the converter to maintain higher power quality under unbalanced and non-linear conditions.Therefore,the modulation and control method of three-phase four-leg will be studied deeply in this thesis.In this thesis,a generalized carrier modulation method is proposed to solve the problem of complex matrix operation such as a large number of sector judgment and duty cycle calculation of three-dimensional space vector modulation（3D-SVPWM）method for threephase four-leg converter.This method can make the traditional sinusoidal pulse width modulation method equivalent to continuous 3D-SVPWM method and common discontinuous pulse width modulation method through specific third harmonic injection,and has the ability of overmodulation strategy;On this basis,this thesis also proposes a minimum switching loss modulation method considering the loss of the fourth leg to minimize the switching loss of the system under different power factor angles.Secondly,this thesis establishes the mathematical models of the three-phase four-leg inverter in the stationary and rotating coordinate frame systems,and compares the control strategies of the inverters in different coordinate systems.In the dq0 rotating coordinate system,the positive-sequence,negative-sequence and zero-sequence components of the output voltage are separated and then double closed-loop control is carried out respectively.In addition,in view of the problem of large harmonic content of inverter output voltage under nonlinear load,this thesis adds a repetitive control on the basis of traditional proportional-integral control to form a composite control strategy of proportional-integral and repetitive control,which further reduce harmonic content of inverter output voltage.Then,the fourth leg of the three-phase four-leg rectifier and three-phase four-leg inverter are combined into a common leg to form a three-phase seven-leg double conversion uninterruptible power supply.The carrier modulation method using the rectifier inverter joint modulation is proposed to compromise the influence of the same zero-sequence component on both.Under the condition of unbalanced load,the secondary pulsating voltage on the DClink is filtered by an adaptive notch filter to reduce the harmonic content of the input current of the converter.Finally,an experimental prototype of a low-power three-phase four-leg converter is built,and the modulation methods and control strategies proposed in this thesis are verified by the simulation and experimental results...