In this paper, we find that multiple attractors may coexist with the stable equilibrium point in an islanded microgrid containing identical grid-forming converters and a ring power network, which was once believed to be impossible in previous study. The existence of such an attractor separates the microgrid into several clusters, each of which is internally synchronized, but desynchronized with each other. We define a cluster coherence metric to measure the internal coherence of each cluster. Finally, full-circuit cycle-to-cycle simulation is provided for verification.

In conventional power systems dominated by synchronous generators, transient stability was found to be less influenced by the secondary control (e.g., automatic generation control). However, due to the flexibility of control and communication of grid-connected converters, more advanced control schemes such as the virtual-impedance-based distributed secondary control can be applied to multi-converter islanded microgrids, which may have a significant impact on the microgrid’s transient stability. In this paper, a simplified model considering the essential synchronization dynamics is established, and the metric named microgrid stable basin is proposed to measure the impact of critical secondary control parameters on the transient stability. Finally, laboratory measurement is provided to verify the theoretical findings.

The increasing integration of inverter-based resou- rces is a prominent trend in power systems, which may increase the risk of large-scale blackouts. This paper proposes a complex network-based cascading failure model that incorporates the failure mechanism of power electronics-based (PE-based) nodes. This model facilitates the evaluation of cascading failure vulnerability in power electronics-penetrated (PE-penetrated) power systems, accounting for the impact of an increasing penetration level of inverter-based resources. Focusing on generation nodes with inverter-based resources (PE-based nodes) that may deviate from their stable operating limits during cascading failure and disconnect from the grid, we introduce a novel mechanism to characterize their failure behavior. Through a case study conducted on the IEEE 39-bus system, the model demonstrates its ability to accurately replicate the salient features observed in two real blackout events. Furthermore, simulation outcomes from the IEEE 118-bus system indicate the substantial impact of PE-based node failure on the cascading failure process. These results also underscore the effectiveness of utilizing the proposed model to prevent underestimating power outage risks within PE-penetrated power grids. This work emphasizes the critical importance of considering the penetration of inverter-based resources across different scenarios to safeguard the resilience of evolving power grids.

The widely deployed grid-following converters (GFLCs) may lose synchronization under grid disturbances, presenting a critical challenge to the power electronics penetrated power grid. While some methods have been proposed for analyzing the transient synchronization stability, their models tend to assume an ideal current loop. Using a full-order system model, this paper demonstrates that the nonideal current loop signif-icantly weakens the stability of GFLCs. Specifically, the basin of attraction gets decreased and more sensitive to parameter changes. The complex bifurcation behavior behind these changes is revealed via the numerical continuation. Accordingly, a design principle is derived for enhancing the transient synchronization stability.

In this paper, we report for the first time that in an islanded microgrid comprising a number of grid-forming converters, several hidden attractors may exist, including periodic orbits, quasi-periodic orbits, and chaotic attractors. The system operating at a stable equilibrium point may be brought to one of these hidden attractors under some transient disturbance. Then, the system will exhibit sustained oscillation, and the grid-forming converters will cease to synchronize. Full-circuit cycle-by-cycle simulations are provided to verify these findings.

Under some transient disturbance, the grid-forming voltage source converter may lose its synchronization with the grid, inducing low-frequency oscillation in the instantaneous power, current and angle. The physical origin of such oscillations is found to be a homoclinic bifurcation in this paper. Before the system runs into a homoclinic bifurcation, a stable equilibrium point and a stable periodic orbit co-exist. When a large disturbance is applied, the system exhibits a periodic orbit, which manifests itself as low-frequency oscillation. Moreover, no periodic orbit exists after the homoclinic bifurcation. On the basis of bifurcation analysis, boundaries of stable operation are derived in the parameter space which serve as practical design guidelines to avoid oscillation.