Coordinated Control Strategy Of MMC Under Unbalanced Condition

Modular Multilevel Converter(MMC)is a cascading multi-modular voltage inverter that utilizes the most recent two-level,three-level inverter architecture Level modulation(NLM)modulation method,the increase in the number of levels makes the output voltage waveform close to the modulation wave,and the waveform quality is high.It has the advantages of low dynamic voltage sharing,easy expansion of voltage level and high reliability.It has broad application prospects in long-distance and large-capacity transmission,island power supply,voltage distributed generation,urban distribution network expansion,and power quality control.It is the technical trend of large-capacity HVDC transmission.At present,the control technology of MMC-HVDC for grid voltage balance is relatively mature.However,the negative sequence current generated by the system network side will cause the MMC circulation to contain higher harmonics,and the active and reactive power will appear twice under unbalanced voltage.Fluctuations,DC bus voltage fluctuations and other issues.In order to solve these problems,this paper first analyzes the topology of MMC and the working principle and power characteristics of MMC when the grid voltage is unbalanced,and studies the coordinated control strategy of grid voltage imbalance.When the grid voltage is unbalanced,the problem of suppressing the negative sequence current and suppressing the power fluctuation needs separate control,and a coordinated control strategy of current/power is proposed.The working mechanism and power of MMC are analyzed,and the overall control scheme of MMC is designed,including circulating harmonic suppressor and current/power coordination controller.The effectiveness of the proposed control scheme is verified by simulation.1)Compare the topology and operation principle of the three common MMC sub-modules.The three-phase MMC topology based on the half-bridge sub-module and related mathematical models are analyzed.The mathematical equations between the various electric quantities in the model are established from the fluctuation of the sub-module capacitance voltage.Since the electric quantities are interconnected,mathematics is used.The method can quantitatively analyze the phase-to-phase circulation,and propose a circulation suppression strategy and verify the simulation.2)Firstly,the internal characteristics of MMC when the grid voltage is unbalanced are studied,including the sub-module capacitor voltage fluctuation caused by the negative sequence current and the quantitative analysis of the output voltage ripple of the upper and lower bridge arms and the phase-to-phase circulation.Secondly,the positive and negative sequence components of the grid voltage are extracted,and according to the power calculation formula,the characteristics of the power asymmetry of the upper and lower arms of the MMC under different control targets are revealed.Finally,the above analysis was verified by simulation on PSCAD software.3)When the grid voltage is unbalanced,the problem of suppressing the negative sequence current and suppressing the power fluctuation needs to be controlled separately,and a coordinated control strategy of current/power is proposed.A proportional reduced-order vector control method is proposed for the current-side harmonic problem of the grid side,which reduces the main 5th harmonic component of the grid-side current.In order to prevent current distortion and ensure stable operation of power electronic devices,a low-voltage current limiting link is designed.Finally,the effectiveness of the proposed control scheme is verified by simulation.