Abstract
Impedance-based stability criterion is effective for analyzing the oscillation phenomenon in modular multilevel converter-based high-voltage DC (MMC-HVDC) transmission system, and modeling MMC impedance is the critical step. However, few papers have modeled MMC impedance with comprehensive consideration of time delay and outer-loop control at the same time. The stabilization strategies to improve system stability are not considered during impedance modeling, and the influences of stabilization strategy on impedance are not analyzed mathematically, either. In this article, an accurate sequence impedance model of MMC is derived with the consideration of time delay and complete control loops. Based on this impedance model, the negative damping caused by time delay is pointed out and the weak stability of grid-tied MMC system is analyzed. Then a stabilization control strategy is proposed to suppress the system oscillation. In the meantime, taking the proposed stabilization strategy into account, impedance modeling is revised to study the influence of this strategy on MMC impedance. Furthermore, based on this revised impedance model, an optimal design method for controller parameters of the stabilization strategy is proposed to maximize the phase margin of the interconnected system. Finally, the effectiveness of the accurate impedance model, the proposed stabilization control strategy and optimal design method are verified by simulation results.
Highlights
Modular multilevel converter (MMC) has attracted a lot of research attention especially after the accomplishment of the Trans Bay Cable project in America [1]
The main contribution of this article is summarized as: (1) an accurate impedance model of MMC is established with consideration of power outer-loop control, time delay and stabilization control strategy; (2) the negative damping of impedance caused by time delay is pointed out and analyzed; (3) an optimal design method for controller parameters of the stabilization control strategy is proposed
In order to verify the correctness of the above analysis, the stabilization control strategy is added to the control system for simulation
Summary
Modular multilevel converter (MMC) has attracted a lot of research attention especially after the accomplishment of the Trans Bay Cable project in America [1]. The main contribution of this article is summarized as: (1) an accurate impedance model of MMC is established with consideration of power outer-loop control, time delay and stabilization control strategy; (2) the negative damping of impedance caused by time delay is pointed out and analyzed; (3) an optimal design method for controller parameters of the stabilization control strategy is proposed. Assuming that the equivalent switching frequency is high enough and the capacitor voltages of all submodules are balanced at all times [10], [25], [26], according to the symmetry of MMC, taking the upper arm of phase a as an example, the small signal model of MMC power stage can be obtained by harmonic linearization [27]–[30].
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