Due to the limited overcurrent capability of semiconductor components, the grid-forming (GFM) inverters require additional overcurrent protection schemes. This article analyzes the interaction between the circular current limiter and different types of virtual impedance from the perspective of transient stability. The established unified equivalent circuit model reveals the mechanism of current limitation by the circular limiter, which is based on both the feedforward-virtual impedance (FVI) method and embedded-virtual impedance (EVI) method. Although with the same current-limiting effects, the power-angle characteristics of these two methods are different. The dominant factor influencing transient stability is demonstrated by the analysis of power-angle trajectory. This study observes that the EVI method exhibits superior transient stability compared to the FVI method under the same parameters. Further, a parameter-switching approach based on the EVI method is proposed, which aims to achieve an extended fault ride-through duration. Finally, theoretical analysis is validated through simulations and control hardware-in-the-loop experiments.

With the increasing penetration of wind power, the line-commutated converter-based high-voltage direct-current (LCC-HVDC) system in sending AC grid faces a potential risk of sub/super-synchronous oscillations (SSSOs), which threatens the stability of the power system. The impedance-based method is an effective way to analyze and suppress the SSSOs. However, since the effects of each control part in LCC-HVDC on the stability of sending AC grid are not yet well-investigated, there is a lack of theoretical guidance for implementing the impedance reshaping strategy for LCC-HVDC. Therefore, this paper establishes and analyzes the modular impedance model of LCC-HVDC, to elucidate the contribution of each control part and the interaction between different control loops. On this basis, an impedance reshaping strategy based on phase-locked angle feedforward (PAF) is proposed to improve system stability by attenuating the interaction. Compared to the existing virtual impedance-based method, the proposed PAF-based method addresses its possible failure in suppressing the SSSOs of sending AC grid. The theoretical analysis and experimental results validate the aforementioned conclusions and the effectiveness of the proposed impedance reshaping strategy.

To address the issues of diverse energy supply demands and power fluctuations in integrated energy systems (IESs), this study takes an IES composed of power-generation units, such as wind and photovoltaic units, along with various energy-storage systems, including electrical, thermal, and hydrogen storage, as the research subject. A collaborative control strategy is proposed for the IES, which comprehensively considers the status of diverse energy-storage systems like battery packs, thermal tanks, and hydrogen tanks. First, a mathematical model of the IES is constructed. Then, a dual-layer collaborative control strategy is designed, considering different operating modes of the IES, which includes a multi-energy-storage power allocation control layer based on second-order power-frequency processing and distribution and an adaptive adjustment layer for adjusting power-frequency coefficients based on adaptive fuzzy control. Finally, MATLAB is used to simulate and validate the proposed strategy. The results indicate that the collaborative control strategy based on variable-frequency coefficients optimizes the allocation of fluctuating power among multiple energy-storage systems, enhances the stability of bus voltage, reduces the deep charge and discharge time of battery packs, and extends the service life of battery packs.

Unbalanced voltages have a significant impact on power system security. Inverter-based resources (IBRs) show great potentials in managing the unbalanced voltages. However, the mitigation of voltage unbalance factor (VUF) through IBRs is challenged by the nonlinearity stemming from the product of voltage and current, and square root operation considering active power constraints. In this letter, a VUF reduction model considering active power constraint is established and solved analytically by the Karush–Kuhn–Tucker (KKT) conditions. Compared with the existing unbalance optimal methods, the proposed method shows a better performance in VUF reduction. Moreover, the solution of the proposed method is no iteration required.

The single-phase rooftop photovoltaic inverters connected to the three-phase four-wire weak grid have received plenty of applications, which also brings in small-signal stability issues. In order to analyze the small-signal stability accurately, the article established the small signal model and the impedance model of the system considering the phase to phase effect (PPCE) and frequency coupling effect to accurately describe the relationships between the small signal perturbation signals firstly. Then, the quantitative relationships between the strength of PPCE and phase-locked loop bandwidth and neutral inductance are revealed by the defined ratios of the off-diagonal elements amplitude to the diagonal elements amplitude in the established impedance model. Furthermore, the positive-sequence, negative sequence and zero sequence single in single out (SISO) impedance models of the three-phase four-wire system are established by the proposed extended loop decomposition method to quantify system stability margin intuitively. And the effect laws of phase-locked loop bandwidth, transmission power and neutral inductance on system stability are also analyzed quantitatively by the proposed SISO impedance models. The validity of theoretical analysis is further verified through the experimental results.

With the advent of large-scale electronic transportation, the construction of electric vehicle charging stations (EVCSs) has increased. The stochastic characteristic of the charging power of EVCSs leads to a risk of destabilization of the DC distribution network when there is a high degree of power electronification. Current deterministic stability analysis methods are too complicated to allow for brief descriptions of the effect of probabilistic characteristics of EVCSs on stability. This paper develops a probabilistic small-signal stability analysis method. Firstly, the probabilistic information of the system is obtained by combining the s-domain nodal impedance matrix based on the point estimation method. Then, the probability function of stability is fitted using the Cornish–Fisher expansion method. Finally, a comparison experiment using Monte Carlo simulation demonstrates that this method performs well in balancing accuracy and computational efficiency. The effects of line parameters and system control parameters on stability are investigated in the framework of probabilistic stability. This will provide a probabilistic perspective on the design of more complex power systems in the future.