| Distributed renewable energy and energy storage systems play an important role in the construction of novel power system that is mainly connected to new energies,and the DC microgrid is the key technology for this construction.Isolated bidirectional DC-DC converters are the interfaces of the DC microgrid to realize the bidirectional energy interaction between the grid sources and grid loads.Hence,the performance improvement of bidirectional DC-DC converters is of great significance,which has attracted more and more attention from scholars and the industrial community both at home and abroad.Compared with the widely applied single-phase dual-active-bridge(1p-DAB)DC-DC converter,three-phase dual-active-bridge(3p-DAB)DC-DC converters have higher power density and great development potentiality.The main investigation works of this thesis are focused on modulation optimization and advanced control strategies of the 3p-DAB,and the innovative achievements of this paper can be summarized as follows.1.A novel optimal simultaneous pulse width modulation(SPWM)scheme is proposed,which consisites of two degrees of control freedom.The analytical expressions of the transmission power and the current stress under different modes of the proposed modulation are obtained by the segmented analytical method,and the global optimation of the proposed modulation strategy is realized by minimizing the current stress in the whole load range.2.A novel hybrid modulation strategy is proposed,which includes three degrees of control freedom,and its analytical expressions are simplified by introducing a new variable.The proposed modulation scheme combines the triangular current buck modulation,triangular current boost modulation,and trapezoidal current modulation to improve the system efficiency when the 3p-DAB operates at light load conditions or the voltage conversion gain far deviates from unity.Additionally,the single phase-shift modulation is utilized to enhance the reliability of the system.3.Based on the hybrid modulation strategy proposed in this thesis,a novel moving-discreted-control-set model-predictive-control(MDCS-MPC)is proposed,and its moving step of the moving-discreted-control-set is auto-renewed by the presented adaptive step mechanism.In the proposed methodology,the steady-state error and stability of the system are investigated,and the weight factors of the cost function of the MDCS-MPC are determined by utilizing the phase portraits theory and frequency domain analysis,which improve the dynamic performance of the system.4.A model-free moving-discreted-control-set predictive-control(MF-MDCSPC)is proposed for the 3p-DAB DC-DC converter.To remove the issues of the performance of the MDCS-MPC depending on the accuracy of the applied model and its system robustness,a novel data-driven model for the 3p-DAB DC-DC converter is proposed in the proposed control scheme,and an adaptive forgetting factor recursive least square(AFFRLS)algorithm is developed to identify the parameters of the proposed data-driven model.In addition,a multi-layer recursive model is presented with the historical input/output data to estimate the parameters of the proposed data-driven model in the prediction horizon.On the basis of the proposed control scheme,an improved adaptive step mechanism is presented to enhance the transient performance,and the influence of the control parameters in the AFFRLS algorithm on the performance of the MF-MDCSPC is also analyzed. |
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