| Modular multilevel converter(MMC)have gradually become the preferred topology for voltage source converter based high voltage direct current due to its advantages such as low harmonic content and expansibility.However,MMC is a multi-frequency system.So,there is a risk of broadband instability in the power grid.There are reports of sub-synchronous and middle frequency oscillation when MMC connect wind farms as well as high frequency oscillation when MMC connect transmission lines.Therefore,small signal modeling and stability analysis of MMC as well as oscillation suppression measures are of great significance.The existing modeling methods either ignore the multi-frequency component of MMC or carry out multi-frequency expansion of the MMC time domain model.Linear time-periodic(LTP)time-domain method can be used to analyze the stability of MMC systems,since it can be modeled without carrying out complex frequency expansions and omitting the frequency component.Furthermore,most of the modeling does not take the effect of the time delay of controller into account.In this paper,LTP time domain method is used to study the system stability of MMC grid-connected system considering the time delay of controller.First,this paper introduces the topology and principle of MMC.And the original mathematical model of MMC is established.The main consideration of controller include vector current controller(VCC),circulating current suppression controller and phase-locked loop(PLL).Secondly,this paper deduces the LTP time domain model and the simplified model only considering the external characteristics of MMC.By comparing the analyzing results between the two mehods,it is concluded that the LTP time domain analysis method is more accurate in analyzing the stability of lower frequency band,and the accuracy of the two methods is the same in higher frequency band.Then,LTP time domain method is used to analyze the stability of the strong and weak power grid.The results show that the higher frequency oscillation caused by the large k_p parameter of VCC has no direct relationship with the strength of the power grid.And,compared with the strong power grid,the range of PLL and VCC parameters required for system stability is greatly reduced under the condition of weak power grid.And,the influence of the time delay of controller on system stability is analyzed.It shows that the optional range of controller parameters required for system stability decreases when the time delay increases.But the time delay does not affect the oscillation in lower frequency band.Then,this paper takes high-frequency oscillation as an example.The oscillation frequency span is large,and the delay of the controller is difficult to be determined.These characteristics bring difficulties to the design of suppression measures.The distribution and content of high-frequency components from the perspective of bridge arm energy is analyzed.It concludes that there are few high-frequency components in the dc side and circulation.Then,the effectiveness of the method of adding filter in the VCC terminal voltage feedforward and current input terminal is analyzed.And the oscillation risk can be reduced by increasing the inductance of the bridge arm,and the filter bandwidth debugging range can be improved by increasing the resistance of the bridge arm.Then,the operation flow of the suppression measures is summarized.Finally,the MATLAB/simulink and MMC experiment platform are used to verify the effectiveness of the suppression measures. |
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