Research On Three-phase Fuel Cell Grid-tied Power Generation System Based On Model Predictive Control

Fuel cell has become the preferred choice for distributed generation due to its high power generation efficiency,no pollution to the environment and less installation restrictions,and are widely used in grid-tied power generation systems.The three-phase fuel cell grid-tied power generation system usually adopts a two-stage topology.The front stage dc-dc converter is used to boost the low output voltage of the fuel cell,and the latter stage dc-ac inverter is used to convert the dc-link voltage into ac voltage and connect it to the three-phase grid.For the research of three-phase fuel cell grid-tied power generation system,it is often considered that it operates in ideal balanced grid,and the control objectives are stable dc-link voltage and high quality grid current.However,in practice,the three-phase grid also shows unbalanced non-ideal characteristics,and the control target increases the suppression of low frequency ripple of fuel cell output current.Due to unbalanced grid voltages,the output power of the ac-side will produce a pulsating component of twice the grid frequency,and the current distortion of the ac-side increases.Due to the dc-ac power coupling,the output current of the fuel cell contains low frequency ripple,which will affect its output and service life.Therefore,for the three-phase proton exchange membrane fuel cell(PEMFC)grid-tied power generation system,only considering the ideal situation is insufficient,but also need to consider unbalanced grid voltages.To this end,this paper carried out a series of studies,the main research work is as follows:(1)The topology and control algorithm of two-stage three-phase PEMFC grid-tied system are determined.The topology and control algorithm of the front stage dc-dc and the latter stage dc-ac converters of the three-phase PEMFC grid-tied system are studied in depth,and the two-stage topology of the boost circuit and the three-phase full-bridge inverter circuit is determined.The traditional PI and PR control algorithms are compared and analyzed.Model predictive control(MPC)is selected as the control algorithm in this paper due to the advantages of no parameter tuning,fast dynamic response and strong robustness.(2)The improved MPC algorithm model of boost converter and three-phase full-bridge inverter is established.Aiming at the problem that the voltage vector of boost converter has little influence on the tracking accuracy,the concept of virtual vector is combined with the control of boost converter to improve the tracking accuracy of MPC algorithm.Aiming at the problem of ac-side power quality decline,the traditional MPC,MPC based on optimal duty cycle and three vector MPC are analyzed in detail,and the simulation model is established to analyze the control effect.The control effect,calculation amount and control complexity are compared in detail,and the MPC based on the optimal duty cycle is selected as the control algorithm in this paper,which improves the tracking accuracy of grid current.(3)Then,considering the ideal grid and unbalanced grid voltage,an applicable active control strategy based on power decoupling is proposed.The simulation model of three-phase PEMFC grid-tied power generation system is built in MATLAB/Simulink,and the improvement effect of the proposed control strategy and control algorithm is verified.The steady-state output results of the traditional control algorithm and the improved MPC algorithm are compared under balanced and unbalanced grid voltage.In the ideal case,the improved MPC algorithm can better realize the stable control of dc-link voltage and high quality grid current.In the case of unbalanced grid voltage,the proposed control strategy automatically realizes power decoupling.Without large dc-link capacitor and additional decoupling circuit,the fuel cell output current ripple is eliminated,and the total harmonic distortion(THD)of grid current is reduced,and the dynamic response performance is better.Finally,the experimental prototype of the three-phase PEMFC grid-tied system is designed,and the effectiveness of the proposed control method is further verified by experiments.