Impedance Modeling and Sub/Super Synchronous Oscillation Stability Analysis of Modular Multilevel Converter under Virtual Synchronous Generator Control
Virtual synchronous generator(VSG)control has been widely used in new power systems.Its research mainly focuseson new energy,and the problems of sub/supersynchronous oscillation and grid-connected stability in the VSC-HVDC still need to be clarified.Limitations exist in the research on the impedance modeling of grid-forming modular multilevel converter(MMC)and the sub/supersynchronous stability problem.The characteristics of the sub/supersynchronous oscillation mechanism in real projects of grid-forming MMC cannot be characterized,and the oscillation suppression method is also immature.Therefore,it is essential to conduct sequence impedance modeling,grid-connected system stability analysis and control optimization for grid-forming MMC.This paper adopts the harmonic linearization method to establish the sequence impedance model of grid-constructed MMC and verifies the accuracy of the analytical model by frequency scanning.Secondly,the impedance characteristics and grid-connected stability are compared with the traditional grid-following MMC.Then,the control optimization design scheme based on virtual impedance is adopted to optimize the impedance characteristics of the grid-forming MMC and improve the stability of the grid-connected system.Finally,the effects of active control,reactive control,voltage outer loop,and current inner loop parameters on the impedance characteristics of grid-forming MMC areexplored.Results show that grid-forming MMC has better stability in weak grids.However,there is a risk of sub/supersynchronous oscillation in strong grids,and virtual impedance control can increase the equivalent output impedance of the converter,thereby enhancing the stability of the MMC grid-connected system.At the same time,the reactive power droop and voltage outer loop link play a decisive role in the impedance characteristics of the grid-forming MMC.Appropriately increasing the reactive power droop coefficient and the voltage outer loop integration coefficient is conducive to reducing the negative impedance damping interval and improving the stability of the grid-connected system.The following conclusions can be drawn.(1)In the network construction MMC sequence impedance modeling,the small signal of the reference phase angle generated by VSG control has little influence on the impedance characteristics of the system.Thus,the influence of VSG control can be ignored in the impedance modeling process.(2)Compared with traditional control methods,the impedance of the grid-forming MMC is inductive in the sub/supersynchronous frequency band,which is not easy to resonate with the inductive power grid.The impedance characteristics of MMC can be improved using the grid-forming control,and the grid stability of the MMC grid-connected system can be enhanced.(3)Although the grid-forming MMC has good stability under weak power grids,oscillation is risky under strong grids.The virtual impedance control can enhance the stability of the grid-connected MMC system.
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