Advanced control of three-phase full-bridge converter in microgrids
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This thesis focused on application and control of three-phase full-bridge converter in renewable energy system (RES). Current challenges and application of the converter are reviewed, novel concept of smart converter and novel microgrid topology with modular design are proposed. Various kinds of control strategies are comprehensively researched and proposed to improve the performance. Through review of power electronic interface (PEI) and control strategies in RES, it is concluded that three-phase full-bridge converter is generally utilized for AC/DC bi-directional conversion and could realize DC/DC conversion with high frequency transformer. Novel topology of microgrid is proposed with modular structure and wireless communication ability by using three-phase full-bridge converter modules, which is extremely reliable, expandable and flexible. The control modes, the control strategies for each module and system control strategy are discussed. Meanwhile, the smart converter concept is proposed for RES to improve the utility power quality and system stability. For ancillary services and advanced functions, control strategies of three-phase full-bridge AC/DC converter play an important role. Firstly, the conventional switching table based direct power control (STDPC) and model predictive based direct power control (MPDPC) are compared. Though MPDPC has advantages such as fast dynamic response and lack of modulator, the mutual influence of control objectives is obvious and switching frequency is high. Thus novel advanced multi-functional MPDPC for improving both the dynamic and steady state performances simultaneously is proposed. Not only the novel control can improve the steady-state performance while impeding switching frequency increment, but it can also eliminate mutual influence between active and reactive powers during dynamic instant. Due to finite number of vectors, the single vector selection based method still bears with variable switching frequency, relatively higher power ripple and spread spectrum nature of harmonics. Therefore, the three-vector based conventional predictive duty cycle control (PDCC) uses prediction value of power slope and square error minimization method to calculate duration time of adjacent two none-zero vectors, which are selected based on sector information. However, incorrect vector selection cannot be avoided and negative duration time exists. Improved PDCC is presented to solve this issue by reselection of none-zero vector and recalculation of duration time, while the complexity and computation burden increase significantly. Finally, reversible PDCC method is proposed, which does not recalculate the power slope and duration time, it reduces the control complexity significantly while achieving better dynamic and steady performance. However, there are still some inherent disadvantages of above mentioned methods. Firstly, the mutual influence inherits. Secondly, the calculated negative duration time cannot be wholly eliminated. Thirdly, the computation burden is quite high. Lastly, it limits the method to three vectors based approach only. Therefore, model predictive based vector selection is proposed to avoid incorrect vector selection, simplified duration time calculation method without power slope calculation is proposed, the dynamic and steady-state performance are further improved. Two vector based simplified model predictive duty cycle control (SMPDCC) is proposed with comparison of conventional two vector based approach, improved space vector modulation based DPC (ISVMDPC) is presented. Three vector based approaches with mutual influence elimination and simplified calculation are proposed. Comprehensive comparisons of each methods with simulation and experimental results are conducted.
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