Research On Harmonic Disturbance Problem And Circulating Current Suppressing Strategy For MMC In Medium-low Voltage Flexible Dc Distribution Network

Compared with the traditional voltage source converter(VSC),the modular multilevel converter(MMC)has the advantages of high quality of output waveform,low switching loss,and low static and dynamic voltage-sharing requirements of IGBT,it is a kind of VSC topology which is suitable for constructing the medium-low voltage flexible DC distribution network.However,the AC side connected to the MMC of the DC distribution network contains abundant harmonic disturbances and is prone to the frequency offset of the power network,which will lead to new features of causes and manifestations of the circulating current problem in MMC.Therefore,it is necessary to study the harmonic disturbances and the circulating current suppressing strategy for MMC under the background of medium-low voltage flexible DC distribution network.In this paper,the typical topology of the medium-low voltage flexible DC distribution network and the basic operation principle of MMC are introduced.On this basis,to study the conduction mechanism and rule of the AC harmonics via MMC,the steady state analysis method of MMC is adopted to deduce the expressions of the circulating current and voltage fluctuation of the bridge arm excited by the harmonic current of AC load and the background harmonic voltage of AC system in MMC,and discuss the relationship between the circulating current path and the AC harmonic property.Meanwhile,considering the factor of multiple harmonic disturbances,the superposition characteristics of different AC harmonics via MMC and its influences on DC side ripple contents are analyzed.On the basis of the above study,to solve the circulating current tracking accuracy problem of the traditional quasi proportional resonant(PR)controller under the frequency offset of power network,the MMC circulating current suppressing strategy based on adaptive quasi-PR control is proposed.Firstly,based on the algorithm of adaptive window width discrete fourier transformation(AWWDFT),an adaptive quasi-PR controller is put forward.The controller obtains the fundamental frequency of the real-time power network by AWWDFT algorithm,and combined with the law of harmonic transmission,the adaptive tracking of circulating current frequency is realized.Then,a circulating current closed-loop control model of adaptive quasi-PR is constructed.In addition,the circulating current controller is designed in ??0 static coordinate system,and its parameters are designed in the discrete domain.Finally,the Matlab/Simulink simulation results show that the MMC circulating current suppressing strategy based on adaptive quasi-PR control can effectively improve the tracking accuracy under power network frequency offset,which has a better effect on circulating current suppression.