| HVDC flexible transmission technology based on modular multilevel converter(MMC)plays an increasingly important role in the new power system with its advantages of highly modular integrated design,low switching loss,and flexible power control.However,according to the relevant reports in actual projects,it can be seen that the high-frequency oscillation problem of MMC grid-connected system is more prominent in comparison with other power electronic equipment,which seriously threatens the safe and stable operation of power grid.The existing research work mainly focuses on the influence of long time-delay and parameters of current controller for fundamental frequency on the high-frequency stability of MMC gridconnected system.On the whole,the system characteristics under high-frequency oscillation condition are not fully understood.In addition,the existing two suppression measures of highfrequency oscillation based on control optimization are widely used,but the system still contains the oscillation risk area under the whole frequency range.In view of the above shortcomings,this paper focuses on the small-signal modeling and analysis of high-frequency oscillation of MMC grid-connected system,and then puts forward new suppression measures.Firstly,the basic topology and modeling process of MMC grid-connected system are described.On the basis of the harmonic state space(HSS)theory,the HSS small-signal model considering the internal dynamic characteristics of MMC is established.Also,the truncation basis of infinite dimensional MMC-HSS small-signal model is created in combination with modal-participation-factor analysis,and the MMC-HSS small-signal truncation model suitable for high-frequency oscillation analysis of the system is obtained.Secondly,the influence of various parameters on the high-frequency stability of MMC gridconnected system is explored,a simplified model for analyzing the high frequency oscillation of the system is proposed,and the mechanism of high-frequency oscillation of the system is further analyzed with the simplified model.Based on the MMC-HSS small-signal truncation model,by using the method of eigenvalue-trajectory analysis,the effects of controller parameters,operating points and connected AC and DC network parameters on high-frequency stability of MMC grid-connected system are studied.Then,combined with the results of highfrequency modal-participation-factor analysis under different frequencies,the main state variables affecting the high-frequency stability of the system are extracted,and two simplified models suitable for analyzing the high-frequency oscillation of the system under different operating conditions are proposed,which are verified by the sweep curve.And,the proposed simplified models are verified by using the frequency-sweep characteristic curve of MMC.Furthermore,by employing Nyquist stability criterion,the mechanism of high-frequency oscillation is elaborated.Then,the characteristics of the two existing suppression measures to high-frequency oscillation based on control optimization are explored,the existing problems are identified and new suppression measures are proposed.Based on Nyquist stability criterion,the influence of existing suppression measures on the frequency characteristics of MMC is analyzed.It is pointed out that the existing suppression measures can not effectively improve the system stability under certain high-frequency oscillation conditions.In order to solve the system stability problem under the above specific high-frequency oscillation conditions,the suppression measure based on the composite-filter of proportional branch of PLL controller is proposed,and the proposed idea,parameter design method and suppression effect under different working points are described.Finally,the above results of theoretical analysis above are verified by MATLAB/Simulink time-domain simulation model and a physical experiment platform of three-phase MMC. |
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