| HVDC transmission technology is the best choice for long-distance power transmission.Due to its modularity,easy expansion and high current waveform quality,Modular multilevel converter(MMC)has become the mainstream topology in the field of HVDC transmission.At present,most of the HVDC projects are applied to the point-to-point power transmission mode.Therefore,MMC back-to-back operation is the basic mode of HVDC transmission.And stability is the primary requirement for the MMC back-to-back transmission system,because once failure occurs will make a great economic loss because of its extremely high voltage and power level.At present,the stability study of MMC back-to-back operation mainly focuses on the AC side when MMC is connected to the wind farm or ideal AC grid using impedance analysis methods.Few studies have analyzed the stability of the DC bus during MMC back-to-back operation.First of all,to make up for the lack of existing research this paper uses the harmonic linearization method to derive the DC side impedance model for MMC.This method can reflect the coupling of complex harmonics inside MMC and transform the system into a linear time invariant model.And in the modeling process of this paper,the impedance of the AC grid connected to the AC side of the two back-to-back converter stations and the impedance on the DC line are both considered in the model.Secondly,considering the role of the controller,this paper analyzes the phase sequence of the harmonic components under the small signal disturbance of MMC and clarifies the control effect of the controller on different signals in the dqcoordinate system.Thus,an accurate closed-loop DC side impedance model is established for back-to-back rectifier and inverter stations.Finally,impedance criterion reveals that when the controller parameters and hardware parameters of the two stations are not designed properly,resonance phenomena will occur on the DC bus.In this paper,the influencing factors of the DC-side impedance form of two MMC back-to-back stations are analyzed detailed,the effects of the controller parameters,MMC hardware parameters,and the grid impedance connected to the MMC on the MMC DC side impedance form are clarified respectively.The optimal controller parameters and hardware parameter design guidelines for MMC back-to-back transmission system are presented.The models built in this paper and the related stability analysis results were verified using simulation models built by Matlab/Simulink or experiments. |
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