| The problem of huge and concentrated electricity consumption exists in megacities and metropolitan areas,which raises higher requirements for safe and stable operation of regional power grids.In order to enhance the power grid’s ability to cope with the challenges faced by economic development,we need not only construction on additional grids,but also improvement of the structure of the power system,and breakthroughs in transmission and distribution technologies,so that electric energy can be utilized more efficiently and efficiently.The unified power flow controller can solve the problem of excessive load of partial symbol and uneven distribution of power flow in transmission network.This paper focuses on the study of the transient process and control strategy of the unified power flow controller based on modular multilevel converter.First,the background and significance of studying the unified power flow controller are stated,then the research progress and current engineering situation of the existing UPFC is introduced,along with the disadvantages of the fault current limiting VSC-UPFC.The basic structure and working principle of UPFC based on modularized multilevel converter(MMC)are described.The operation mode of MMC sub module and the modulation mode of MMC are introduced.A hybrid modulation strategy is proposed,which can effectively reduce the harmonic component of the voltage injected to the grid from the UPFC series side converter when the modulation ratio is low.The operation characteristics of UPFC are analyzed.The relationship between the compensation range of output current of parallel side converter and output voltage of series side converter and the system parameters is studied.The relationship between active and reactive power of each node in UPFC power flow regulation is drawn.Secondly,a current limiting UPFC topology based on split inductance is proposed,the specific parameters of the split inductor are designed,and the design method of the size parameter of UPFC series transformer is introduced.The steady state control strategy of MMC-UPFC is studied.The basic principle of internal model control is introduced,and the robustness analysis is carried out.The internal model control link is introduced into the decoupling control strategy.The main parameters of internal model control are calculated in the case of MMC PSPWM modulation when the module number of bridge arm is 12.The power flow regulation effect of UPFC under internal model control is verified by simulation,and the validity of the internal model control strategy is proved to be applied to MMC-UPFC.Next,the non fault transient control strategy of MMC-UPFC is studied.Aiming at the unbalanced condition of the system,a SDR-SOGI based two step phase-locked strategy is designed,which is suitable for UPFC.SDR is used to extract positive sequence components,and SOGI is used to remove harmonics and lock phase.Simulation results verify the validity of the phase-locked strategy.Aiming at the low frequency oscillation caused by single-phase small disturbance,the UPFC damping controller is designed based on the analysis of the state model of single machine infinite bus system including UPFC.The effectiveness of UPFC damping control strategy to suppress low frequency oscillation is verified by simulation.Research on the transient process of MMC-UPFC AC side short-circuit fault is proposed.Considering the difference of the position of the short circuit fault points,the single-phase and three-phase short circuit fault is divided into four stages of two cases,each stage of fault mechanism are analyzed.Considering the leakage inductance of series transformer,the fault equivalent circuit of the system is analyzed.The fault equivalent circuit of the system and the path of system fault current are analyzed.The transient characteristics of short-circuit fault are concluted,and the fault handling flow suitable for the new topology is fomulated.The effectiveness of the topology in suppressing short circuit current is verified by simulation.Finally,the work done in this paper is summarized,and the direction of further research is stated. |
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