Research On Control Strategy Of Modular Multilevel Photovoltaic Grid-connected Inverter

With the vigorous developing of renewable energy and constant growing of power grid to high-voltage and high-power level, Modular Multilevel Converter has become a hot research topic in the field of photovoltaic power generation because of not being limited by the capacity of semiconductor power devices and being expanded easily. The study of PV Grid-connected Systems based on MMC is carried out in this paper, including inverter modulation technology, grid-connected current control tache and photovoltaic cell maximum power point tracking algorithm and the inverter fault-tolerant control strategy several aspects.Firstly, the mathematical model of MMC is established based on the operating principle of MMC, and the calculation formulas for parameter design of sub module capacitance and arm inductance is given in this paper by analyzing the fluctuation of capacitor voltage of the sub module and the components of the circulating current. Then the Nearest Level Modulation and Carrier Phase-shifted SPWM are compared by simulation.Secondly, the control system of modular multilevel photovoltaic grid connected inverter is designed. The doubled closed-loop control part in which the inner loop use Quasi-Proportional Resonant controller to track the fundamental frequency current and the power-feedforward control to improve the voltage tracking speed is introduced into the outer loop is designed. The power output mode of grid connected inverter is divided into two types, the maximum power output mode and the reactive power injection mode. Under non-standard conditions, the inverter is converted to the reactive power injection mode, which improves the utilization ratio of the inverter. Then a PV grid-connected system based on MMC is built in the Matlab/Simulink, and the above control parts are simulated and verified.Finally, the electrical characteristics under fault-tolerant strategy are analyzed based on the average switching model of MMC. MMC can not continue to run for the reason that the remaining sub module is not enough to support the DC bus voltage after its sub module failure. To solve this problem, an improved faulttolerant control strategy is proposed by only bypassing the fault sub module. By changing the capacitor voltage reference value and the carrier phase shift angle, the remaining sub module can support the DC bus voltage and maintain the main components of the circulating current, so that the MMC can continue to operate. Compared with the traditional fault-tolerant strategy, the improved fault-tolerant control strategy improves the output level number of MMC and reduces the harmonic distortion of output current. In addition, The maximum power point tracking control tache of fault tolerance is improved.Aim to solve the problem of long recovery time of DC bus voltage when the sub modules fault, the method of constant voltage tracking is presented to the maximum power tracking control of photovoltaic cells.Then the simulation to verify the advantages mentioned above is carried out in Matlab/Simulink.