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Transient angle instability and fault overcurrent are two serious problems affecting the safe operation of the virtual synchronous generator during grid faults. However, the two aspects are mostly treated as two independent problems in existing studies. In this article, taking both transient angle instability and fault overcurrent into consideration, a two-stage transient control collaboratively utilizing virtual impedance (VI) and switching PI is put forward. In stage one, the VI is switched to the boundary value determined by the iteration algorithm to avoid fault overcurrent. In stage two, the proportional loop or integral loop in switching PI, which can embed virtual damping or virtual torque into the traditional swing equation, respectively, is activated to prohibit two types of transient angle instability induced by VI. By adaptively tuning VI and activating different loops, fault current limitation and transient angle stability are guaranteed simultaneously under different fault degrees. Furthermore, inertia support is maintained, and transient performance is improved. The proposed control is also capable for a multi-inverter system. Simulation and experimental tests validate the effectiveness of the proposed control.

Due to the multifrequency coupling effect (m-FCE), precise modeling and effective stability assessment of an asymmetric grid-following converter (GFC)-based ac system are complex to achieve. Traditional small-signal modeling methods and recently reported order-reduction methods could establish multi-input and multioutput (MIMO) and equivalent single-input and single-output (SISO) models to characterize m-FCE effectively. However, revealing the physical implications of coupling components and the generation mechanism of m-FCE are still challenging. Meanwhile, although the amplitude and phase dynamics have been considered in the existing modeling, a concrete conclusion illustrating these dynamics’ impact on m-FCE is still missing. To fill the gaps, this article thoroughly considers the impacts of amplitude and phase dynamics and finds that phase dynamics form additional positive feedback loops to enhance m-FCE. Based on this cognition, an accurate complex circuit modeling framework is proposed. It reveals that m-FCE is generated by the admittance mismatching between conjugated responses and “chain reaction” among coupling components. The proposed method has two merits: 1) it is an SISO modeling approach and helps reduce analysis complexity and 2) its graphical form helps provide a deep physical insight into m-FCE. Simulations and experiments verified the correctness of the proposed method.

The influence of parameters on the damping characteristics in a multi-inverter-paralleled system is different from that in a single-inverter system, because of inverter interactions in a nonideal grid. To quantify the interaction degree and assess its impact on damping characteristics, this paper proposes a closed-loop interconnected model for multi-inverter-paralleled systems. The model can explain the impact mechanism of inverter interactions on system stability. Capacity expansion of inverter-based renewable distributed generation is taken as the background in this paper, and then, the proposed model is applied to assessing the influence of the collecting line length of the newly installed inverters on the overall system stability. The conclusion is that the damping of a multi-paralleled-inverter system in the medium frequency range degrades with the approaching of the lengths of the newly added line and the existing line. Therefore, to avoid damping degradation in capacity expansion scenarios, the length of the newly added line should be different from the length of existing lines or the parameters of new inverters should be readjusted. Simulation results are presented to verify the effectiveness of the proposed model and stability enhancement approaches.

Simplified weak grid models are widely used to emulate the nonideal power grid for the design and stability analysis of grid-connected inverters. However, the dynamic behavior of a real grid with multiple inverters is complex. In this article, an emulation method is proposed for multi-inverter integrated weak grids using an interaction-preserved aggregation model associated with grid impedance. First, the interaction among inverters in weak grids is analyzed using the state-space model and participation analysis. Then, a two-inverter aggregation is proposed for a large-scale system. This model can preserve the impact of parameters on the interaction modes (IMs) and predict the IM-related instability of multi-inverter systems. Thus, the two-inverter representation is applied to emulation to preserve interactions and to simplify the testing of multiple inverters. The emulation of the 16-inverter system is implemented on a hardware-in-the-loop (HIL) test bench with one practical inverter controller under test and the remaining parts integrated in the OPAL-RT real-time simulator. The test results show that the instability attributed to the interaction can be identified by the proposed emulation method.