Abstract
This paper presents a small-signal model of modular multilevel converter (MMC) based on the motion equation concept in the dc voltage control (DVC) timescale (around 10 Hz). The relations between the active/reactive powers and the phase/magnitude dynamics of the internal voltage vector are developed with considering the submodules (SMs) capacitor voltage ripple. With the proposed model, the stability mechanism of the DVC timescale in an MMC-HVdc system can be well interpreted with the equivalent inertia and damping coefficients. It is found that the fundamental frequency ripple in the capacitor voltage of SMs will result in new oscillation modes in DVC timescale when compared with the conventional two-level voltage source converter. Furthermore, the coupled relationship between the reactive power and phase dynamics of internal voltage is revealed, which has great effect on the system stability property. Then, comparative studies via eigenvalues analysis show the proposed model can hold the main behaviors of concern, and the correctness of the proposed model is verified by comparisons with detailed time-domain simulations and experimental tests. Finally a single MMC connected to infinite-bus grid is taken as an example to validate the feasibility of the proposed model for dynamics analysis.
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