Integrated Modulation Of Dual-parallel Neutral-point-clamped Converters For Wind Power Applications

To meet the requirements of large power capacity for high power applications,the parallel operation of three-level converters has been widely adopted to increase the current rating,system redundancy,and reliability.However,conventional parallel operation modes suffer from the following issues.First,the system performance under synchronized paralleling is the same as that for individual converters,especially in terms of the output current ripples.Although the interleaved paralleling can improve the output harmonic performance,large high-frequency circulating current inevitably appears,which may impact system efficiency and switching losses negatively.Second,the neutral point balance and circulating current control cannot be achieved simultaneously,because there is only one degree of freedom(i.e.the zero-sequence component of space vectors),meaning that there are always tradeoffs between neutral point balancing and circulating current adjustment.Therefore,it is necessary to investigate into the new parallel operation mode that can comprehensively improve system performance.The following contents will be covered in this study.The fundamental theory and specific implementation for the integrated modulation of parallel three-level converters are to be studied.The validity of modulating dual parallel converters as a whole system should be investigated on the basis of the mathematical model and theoretical analysis of the multi-level output capability.In addition,a systematic approach is proposed,including the switching sequences design criteria,the distribution of five-level states to parallel converters,and the practical implementation on common-used digital signal processors.Furthermore,the performance in terms of output harmonics,switching transitions,and circulating current is verified and compared for various parallel operation modes.The common mode voltage suppression and elimination methods are developed based on the concept of integrated modulation.The relationship between common mode voltage and the five-level space vectors is revealed through mathematical deduction,and the integrated modulation strategies with reduced and eliminated common mode voltage are proposed for the parallel operated three-level converters,including the vector space division,switching sequence design,and dwell time calculation formula.The merits and drawbacks of system performance are verified in comparison with conventional three-level modulation methods.The neutral point balance and circulating current control strategies are proposed for the various integrated modulation of parallel three-level converters.The effect of diverse switching sequences on the neutral point voltage balance is analyzed,and the corresponding balancing algorithms can be proposed considering the available degree of freedom.In addition,the mathematical model and the generation mechanism of circulating current are analyzed to reveal the adverse influence of circulating current on the bridge current ripples and system efficiency.The differential and common mode circulating current suppression methods are developed based on the coupled inductor and additional freedom of control,respectively,for the integrated modulation.The coordinated control strategies for modular parallel three-level converters are developed.First,a decoupled control of neutral point voltage balance and circulating current is proposed through separated freedoms of control,which are not available for traditional parallel operation modes.Second,the best modulation strategy is selected in accordance with the unique requirements of the grid-side and machine-side converters in a back-to-back power conversion system.Therefore,not only the coordinated control of neutral point balance and circulating current can be achieved,but also the best performance for grid-and machine-side converters could be guaranteed.Finally,a back-to-back wind power conversion prototype with modular parallel three-level converters is built.Based on the laboratory platform,comprehensive experiments are conducted to verify the correctness of theoretical analysis and the effectiveness of relevant methods.Under various parallel operation modes and modulation processes,the system performance in terms of current quality,switching loss,circulating current and dynamic response is verified and compared accordingly.