| The high voltage direct current based on modular multilevel converter(MMC-HVDC)transmission projects are developing in the direction of high voltage and large capacity,so that the number of sub-modules(SMs)that need to be cascaded in the bridge arm is increasing,which brings two aspects of practical problem.On the one hand,the floating capacitors in the bridge arm of the converter increase substantially,which increases the difficulty of capacitor voltage balancing control significantly and brings challenges to the safe and stable operation of the converter;on the other hand,the switching devices such as IGBT and diode in the converter multiply,which reduces the speed of electromagnetic transient simulation of MMC-HVDC detailed model greatly,and brings inconvenience to engineering simulation debugging.Based on a comprehensive review of the topology,operation principle,mathematical model and steady-state control system of MMC,this paper studies the voltage balancing method of large-scale capacitors inside the MMC and the efficient electromagnetic transient simulation model of cell-hybrid MMC.Aiming at the computation complexity and high switching frequency existing in conventional voltage balancing method of high-level modular multilevel converter,a novel voltage balancing method based on insertion priority of sub-modules is proposed.Instead of sorting the voltage values of SMs,the allowable voltage distribution band(AVDB)is applied to constrain the voltage distribution of SMs.The insertion priorities of each SM in current control cycle are determined by comparing the voltage values of SMs switched IN or OUT in last control cycle with the upper and lower limits of AVDB respectively,and the appropriate number of SMs are selected to insert.Furthermore,the quantity relationship of SMs among each priority is analyzed,the basis for determining the upper and lower limits of the voltage dispersion threshold is given,and the adaptability of the novel voltage balancing method to the hybrid MMC is discussed.The simulation results show that the proposed method can significantly reduce the switching loss of the converter and the calculation time of the voltage-balancing control when maintaining high capacitor voltage consistency,whether in steady or transient process.In addition,it is worth mentioning that the proposed method has good applicability to a variety of cell-hybrid MMCs.Aiming at the large-scale and long-time calculation existing in the electromagnetic transient simulation for the detailed model of cell-hybrid modular multilevel converter(CH-MMC)applied in AC-DC hybrid grid,an efficient simulation model of CH-MMC based on electric charge equalization of sub-module capacitors is proposed.Firstly,the equivalent circuits of the half-bridge and full-bridge SMs in normal operation and blocking state are analyzed respectively.Based on the numbers of half-bridge and full-bridge submodules in normal operation and blocking state,the full-state equivalent circuits of half-bridge valve segment and full-bridge valve segment are derived,and the efficient simulation model of CH-MMC is obtained by connecting the full-state equivalent circuits of two valve segments in series.Then,in order to improve the valve-level control of the model,a voltage balancing method is proposed based on the proposed efficient simulation model,and a three-stage charging strategy is introduced.Compared with the detailed model of CH-MMC,it is found that the proposed efficient simulation model can maintain high accuracy under various operation conditions,and greatly reduce the actual running time of electromagnetic transient simulation.Further,it can be applied to electromagnetic transient simulation of large-scale AC-DC hybrid power grids. |
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