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 paper, 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. INDEX TERMS Modular multilevel converter (MMC), impedance model, oscillation suppression, time delay, optimal design.
The paper proposes an approach to transmit electric power system dynamics in the SCADA. With the prevalent application of digital substation automation system, it is feasible for the remote terminal units (RTUs) to collect phasors within a substation. However, limited communication capacity remains the bottleneck that prevents SCADA from transmitting system dynamics. This paper proposes to compress dynamics data with curve fitting in the RTUs and reconstruct the dynamics in the SCADA server for reducing communication demand. Dispatchers in the control center can thus get system dynamics with a delay of several seconds. Simulation result shows that for a power system under disturbance with short-circuit that once occurred and was cleared, the SCADA can approximate the original dynamics with satisfying precision using limited degree polynomial fitting. The approach is highly scalable and adaptable, and can be implemented on existing communication infrastructure with a few software modifications. The approach has extensive application potential.
The stability of Modular multilevel converter (MMC) itself is the premise of analyzing the stability of MMC cascading with other cells, so this paper addressed the small-signal stability of MMC with dc voltage control mode which is the common operation mode of grid-side MMC in high-voltage direct current (HVDC) systems. First, the small signal model of MMC with open-loop control mode and dc voltage control mode while considering the dc voltage controller and current controller are established with the harmonic state-space (HSS) modeling method. Subsequently, eigenvalues and participation factors are analyzed to estimate the stability of MMC system and the influence of controller parameters, especially the proportional coefficients of dual-loop controllers, which can improve the design efficiency of control system. Finally, time-domain simulations in MATLAB/Simulink are provided to verify the accuracy of HSS small signal model and the effectiveness of the theoretical stability analysis.
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