| Modular multilevel converter(MMC)has been the preferred choice for high voltage direct current(HVDC)transmission engineering applications due to its unique modular construction and working performance.MMC solves the static and dynamic voltage balance problem caused by the series connection of IGBT devices in voltage source converter structure,and shares the advantages of smooth system output waveform,low harmonic content and low switching loss.In this thesis,the control strategy of MMC-HVDC system,the control of sub-module capacitance voltage balance and the circulation suppression strategy of MMC are studied.Firstly,the basic topology of MMC-HVDC system is introduced,the working principle of MMC is analyzed,and the mathematical model of MMC in three-phase abc stationary coordinate system and two-phase dq synchronously rotating coordinate system is established.Afterwards,the selection principle of sub-module capacitor and arm reactor which are the main device of MMC are elaborated.Secondly,in order to solve the problem that the current inner loop controller in the double closed-loop PI control based on dq synchronously rotating coordinate system requires the systematic mathematical model to realize the feed-forward decoupling compensation and that the parameters of the auto-disturbance rejection controller are difficult to tune,the second-order linear active disturbance rejection control strategy of MMC-HVDC which realizes the complete decoupling control of active and reactive power is proposed.The controller has many advantages such as fast response speed,strong anti-disturbance ability,and independence of the mathematical model of controlled object.Meanwhile,the capacitance voltage balance control algorithm is improved combined with the nearest level modulation strategy to reduce the switching frequency of bridge sub-modular in the traditional sorting algorithm.The simulation results verified the effectiveness of the proposed control strategy and the improved capacitive voltage balance control algorithm.Finally,in order to solve the problem that the existing PI controller does not extract the second harmonic current component and depends on the exact mathematical model of the circulating current,a new circulating current suppressing strategy of MMC is proposed.The second-order harmonic components of the arm circulating current is accurately extracted by using the filter based on the second-order generalized integrator.Then,the circulating current suppressing controller based on linear auto-disturbance rejection control which is independence of the mathematical model of circulating current and has less design parameters is adopted to restrict the second-order harmonic components accurately.The simulation results showed that the designed controller can significantly reduce the amplitude of the circulating current. |
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