Power Decoupling Control For Single-phase Grid-tied PEMFC Systems With Virtual-vector-based MPC

When proton exchange membrane fuel cell(PEMFC)is connected to grid for power generation,due to the small voltage generated,it is necessary to apply the front-stage DC-DC converter to boost the low DC-link voltage to the high DC-link voltage,and then invert the DC energy to the 50 Hz AC energy through the back-stage full-bridge inverter.Therefore,the single-phase grid-tied PEMFC system is usually composed of two-stage structure.In a single-phase system,inherent low-order current pulsations are introduced into the system,which can have harmful effects on the fuel cell stack.For example,reducing the output voltage and output efficiency,a reduction in service life,and even accelerates the degradation rate of the membrane electrode of a proton exchange membrane fuel cell(PEMFC).In addition,DC/AC coupling power can cause distortion in the AC grid current,and the output side current of the fuel cell will have a high content of low frequency ripple(100Hz).This will not only result in the low output power quality of the power generation system,which is difficult to meet the grid connected power quality requirements,but also reduce the efficiency of the whole system.In this thesis,aiming at the adverse effects of low frequency current ripple on PEMFC and inverter control,a series of studies are carried out to reduce the low frequency current ripple of PEMFC and improve the grid connected power quality.The main research work of this thesis is as follows:(1)Aiming at the problem of inaccurate tracking caused by the limitation of the number of voltage vectors in the traditional model predictive control(MPC)algorithm,an improved harmonic-reduction-based MPC algorithm is proposed.According to the actual operation of the system,the virtual vector is constructed by dividing sectors,which increases the number of voltage vectors that can be selected in MPC algorithm.In one control cycle,real vectors,virtual vectors and zero vectors are used to control.Compared with the traditional MPC algorithm,this method has higher control accuracy and more accurate tracking,reduces the total harmonic distortion of AC current on the grid side,and ensures the high AC power quality on the grid side.(2)Aiming at the problem of low-frequency current ripple in single-phase grid-tied PEMFC systems,a low frequency current ripple suppression method based on power decoupling control is proposed.This method adopts the improved harmonic-reduction-based MPC algorithm,which can effectively suppress the low-frequency current ripple and ensure the excellent tracking effect and robustness of the system,so as to improve the efficiency of the system and prolong the working life.(3)The simulation experiment of the proposed method is carried out based on the simulation software MATLAB/Simulink.The simulation results show that the algorithm can not only eliminate the input current ripple and reduce the total harmonic distortion of AC current,but also significantly improve the transient performance of the system,and the efficiency and safety of the system are significantly improved.Finally,an experimental prototype is designed,and the experimental results verify the correctness and feasibility of the proposed method.