Virtual synchronous generator (VSG) can provide inertia support for a more renewable-energy-integrated power system. However, there are power couplings between active power and reactive power in weak grid. To solve this issue, the dynamic models of active power and reactive power are established first to better reflect the characteristics of power coupling. Then, the feedforward decoupling method using virtual power angle and virtual voltage is designed under the nominal system, which can suppress the power couplings well. Furthermore, considering the variation of line impedance, a power decoupling strategy based on extended state observers (ESO) is proposed, which can estimate the power couplings as the internal disturbances of the system and output compensations for the feedforward decoupling method. Consequently, the robustness of the proposed strategy is improved greatly, and good decoupling performance can be achieved under different line impedance conditions. Finally, the effectiveness of the proposed method is verified by both simulation and experimental results.

Model predictive control (MPC) methods have attracted growing attention in modular multilevel converter (MMC) systems due to their fast dynamic response, straightforward principle, and ability to control multiple objectives with appropriate cost functions. This article proposes an arm-current-based MPC strategy for MMCs to achieve comprehensive control of ac-side current, circulating current, and submodule (SM) capacitor voltages under unbalanced grid conditions. By directly obtaining the optimal arm voltage according to the discrete time model of arm current, the computational complexity is significantly reduced and is independent of SM numbers. Meanwhile, the tedious cost function evaluation and weighting factors optimization can be avoided. Furthermore, with the arm voltages regulated separately, the proposed method is able to generate $2{N} + 1$ output levels, contributing to enhanced total harmonic distortion (THD) performance and circulating current suppression ability. In addition, the calculation method of the arm current reference is presented, especially for the dc-side current reference, the capacitor voltages can be maintained at the rated value by adjusting the dc current reference. Finally, the validity of the proposed strategy is demonstrated under balanced and unbalanced grid conditions by both simulation and experimental results.

Capacitor voltage ripple suppression is significant for hybrid modular multilevel converters (MMCs) to reduce the capacitance, footprint and cost. However, the existing voltage ripple suppression methods are mainly intended for half-bridge submodule (HBSM)-based MMCs, and not applicable to the hybrid MMC due to its special configuration of mixed half-bridge and full-bridge submodules (FBSMs). Therefore, this article aims to reduce the capacitor voltage ripple for hybrid MMCs under boosted ac voltage conditions. The analysis of charging and discharging behaviors of FBSMs and HBSMs with boosted ac voltage is conducted first to obtain the maximum energy ripple in SM capacitors, which is equivalent to peak-to-peak value of voltage ripple. Then, the effect of modulation index and power factor angle on the capacitor voltage ripple is investigated in detail to find the possible ways to suppress the fluctuation. Consequently, the optimal modulation index that minimizes the capacitor voltage ripple is derived. Furthermore, this article proposes a circulating current injection method to suppress the capacitor voltage ripple under various operating points. In this method, the capacitor voltage ripple amplitude is taken as the direct suppression object to ensure the effect of voltage ripple minimization. Finally, the validity of the proposed method is proved under different operating conditions by simulation and experimental results.

With the increasing penetration of wind power, the transient synchronization stability of doubly fed induction generator (DFIG)-based wind turbines during grid faults has become a critical issue. While conventional fault ride-through methods like Crowbar protection can ensure safety, they compromise system controllability and worsen grid voltage conditions. Virtual resistance demagnetization control has emerged as a promising alternative due to its simple structure and effective flux damping. However, its impact on transient synchronization stability has not been revealed in existing studies. To fill this gap, this paper presents a comprehensive analysis of the transient synchronization stability of DFIG systems under virtual resistance control, introducing a novel fourth-order transient synchronization model that explicitly captures the coupling between the virtual resistance demagnetization control and phase-locked loop (PLL) dynamics. The model reveals the emergence of transient power and positive damping terms induced by the virtual resistance, which play a pivotal role in stabilizing the system. Furthermore, this work theoretically investigates how the virtual resistance and current loop’s proportional-integral (PI) parameters jointly influence transient stability, demonstrating that increasing the virtual resistance while reducing the integral gain of the current loop significantly enhances synchronization stability. Simulation results validate the accuracy of the model and the effectiveness of the proposed analysis. The findings provide a theoretical foundation for optimizing control parameters and improving the stability of DFIG-based wind turbines during grid faults.

Due to possessing full-bridge submodules (FBSMs), hybrid modular multilevel converter (HMMC) can operate under over-modulation conditions. Meanwhile, the 2nd-order circulating current injection (CCI) is used to reduce the capacitor voltage fluctuation. However, its optimal design suitable for different over-modulation conditions remains a challenging issue. And the varied charging and discharging characteristics of FBSMs and half-bridge submodules (HBSMs) increase analysis complexity. This paper analyzes capacitor voltage fluctuation characteristics and designs optimal injected 2nd-order CCI under different over-modulation conditions. First, a balanced operating range is given, and two capacitor voltage fluctuation types are defined. Maximum capacitor voltage fluctuation is also calculated. On this basis, the optimal 2nd-order CCI is proposed for SM capacitor voltage fluctuation reduction under varied over-modulation conditions. Moreover, the impact of the hybridization ratio, the power factor angle and the modulation index on capacitor voltage fluctuation and its reduction method is discussed in detail. Simulations and experiments validate the analysis and the proposed method.

Grid-connected inverters are widely used to interface renewable energy and energy storage resources into power grids. However, the impedance of the power transmission line is relatively large in weak grids, which can affect the stability of grid-connected inverters. In order to improve the stability of inverters in weak grids, a robust loop correction method is proposed in this paper. First, a reference model with good stability is designed. Then, a high-pass filter is used to obtain the high-frequency components of the reference model response, and a low-pass filter is used to obtain the low-frequency components of the actual system response. The outputs of the two filters are added together to form the actual feedback current. Consequently, this method can correct the frequency characteristics of an inverter in high frequency range, so that the inverter still has a good stability margin in a weak grid. The effectiveness of the proposed method is verified by both simulation and experimental results.