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In the low-voltage islanded multi-bus microgrid (LVIMB-MG), the virtual synchronous generators’ (VSGs’) inertia constant and damping coefficient play an important role in low-frequency oscillations (LFOs). However, existing LFO analyses lack the consideration of microgrids’ required rate of change of frequency (RoCoF) and steady-state active power-frequency (P-f) characteristics when designing the VSG’s inertia and damping. With these practical considerations, this paper reveals that the interactions between VSGs can cause significant LFOs, posing a threat to the microgrid’s stability. In contrast, the LFOs related to the interactions between SGs are less pronounced. To this end, a distributed LFO damping (DLFOD) control is proposed to suppress the LFOs between VSGs from two aspects: the adjustment of the VSG’s phase angle mitigates the uneven instantaneous active power sharing, while the adjustment of the VSG’s active power setpoint enhances the mutual damping. Compared with existing methods, the proposed DLFOD control 1) pertains to multi-bus microgrids, 2) guarantees the RoCoF and steady-state P-f characteristics, and 3) possesses a linear structure. Furthermore, a dynamic switching variable is devised to handle the disconnection and reconnection of communication links or VSGs, which enables the DLFOD control to remain partially functional during these interruptions. Finally, the proposed method is evaluated through theoretical analysis, real-time simulations, and experiments.

The deployment of distributed renewable energy resources (DERs) in North America and numerous other countries has led to a consistent increase in the total capacity of generators connected to the distribution network. The inherent variability and unpredictability of DERs introduce new challenges for distribution system operation and planning, necessitating upgrades to distribution substations.

Three-level neutral point clamped (3L-NPC) converters are a favorable candidate for the grid interfacing converter stage in bipolar dc distribution systems. This article proposes the use of dual 3L-NPCs where, by virtue of connecting their ac ports in a differential fashion across a center-tapped transformer winding, two main benefits can be realized: 1) full bipolar voltage balancing capability that can accommodate any degree of imbalance between two dc poles, and 2) improved dc-side power quality in terms of reduced ripples in the pole voltages and currents. The proposed converter system also avoids the reliance on more complex zigzag transformer arrangements used in prior art. Detailed theoretical analyses of the pole voltage balancing and the ripple reduction mechanisms are conducted, with the results guiding the development of a suitable control strategy. The advantages of the proposed scheme and the correctness of the theoretical analyses are validated through both simulation and experimental results.

This article presents new design and control approaches for an input-series-output-parallel (ISOP) connected hybrid converter with partial power processing (PPP) capabilities, which comprises a CLLC converter and a dual-active-bridge (DAB). In this hybrid CLLC-DAB configuration, secondary-side bridges are shared between CLLC and DAB topologies to minimize the number of utilized switches. Incorporating the high efficiency of CLLC with the exceptional control flexibility of DAB, the hybrid configuration transfers the main power through CLLC while enabling DAB to handle partial power flow. The power distribution between CLLC and DAB is optimized, considering system efficiency, dynamic response, etc. Based on the resulting power ratio, system parameters are also optimized, including considerations for soft-switching of all switches in a wide load range, output power capacity, component tolerances, and overall efficiency. Furthermore, the output voltage can be accurately regulated by manipulating the phase shift angle of DAB. To enhance the system's transient performance in scenarios involving varying loads and input voltages, an adaptive fast dynamic response control strategy exhibiting robustness against variations in system parameters is also proposed. Finally, experimental results validate that the hybrid CLLC-DAB system, employing the proposed design methodology and control strategy, attains both high system efficiency and ultrafast dynamic response.