The future electric grid is supported by a vast number of smart inverters interfacing with distributed energy resources at the edge. These inverters’ dynamics are typically characterized as impedances, which are crucial for ensuring grid stability and resiliency. However, the physical implementation of these inverters may change significantly from inverters to inverters and may be kept confidential. Existing analytical impedance models require a complete and precise understanding of system parameters. They can hardly capture the complete electrical behaviors when the inverters are performing complex functions. Online impedance measurements for many inverters across multiple operating points are not scalable. To address these issues, we present InvNet, a machine learning framework to systematically evaluate the effectiveness of data-driven methods for modeling inverter impedance patterns across a wide operation range, even with limited impedance data. Leveraging transfer learning, the InvNet can extrapolate from physics-based models to real-world ones and from one inverter to another with very limited data. This framework demonstrates machine  learning as a powerful tool for modeling and analyzing black-box characteristics of grid-tied inverter systems that cannot be accurately described by traditional analytical methods, such as inverters under model predictive control. Comprehensive evaluations were conducted to verify the effectiveness of the InvNet in various scenarios. All data and models were open-sourced.

This paper presents an analytical approach to explore the damping effect of inner loops on grid-forming converters. First, an impedance model is proposed to characterize the behaviors of inner loops, thereby illustrating their influence on output impedance shaping. Then, based on the impedance representation, the complex torque coefficient method is employed to assess the contribution of inner loops to system damping. The interactions among inner loops, outer loops, and the ac grid are analyzed. It reveals that inner loops shape the electrical damping torque coefficient and consequently influence both synchronous and sub-synchronous oscillation modes. The virtual admittance and current control-based inner-loop scheme is employed to illustrate the proposed analytical approach. The case study comprises the analysis of impedance profiles, the analysis of damping torque contributed by inner loops under various grid strengths, and the comparison between dq-frame and αβ-frame realizations of inner loops. Finally, simulation and experimental tests collaborate with theoretical approaches and findings.

The phase-locked loop (PLL) is a commonly used synchronization control method for grid-tied inverters. The PLL-synchronized inverters tend to have poor stability robustness with weak grid interconnections, especially when the PLL is designed with a high control bandwidth. To tackle this challenge, this paper proposes an enhanced q-axis voltage integral damping (Q-VID) control, which not only stabilizes PLL-synchronized inverters in weak grids but also lifts the restriction on PLL bandwidth. This superior feature enables inverters to operate stably in ultraweak grids and with a superior transient response. Experimental tests confirm the performance of the method with 400-Hz PLL bandwidth under a short circuit ratio of 1.28 of the grids.

For grid-forming (GFM) converters, the dc-link voltage control (DVC) can directly generate the frequency that is used for the grid synchronization. However, this method suffers from low-frequency oscillations (LFO). To tackle this challenge, this paper proposes a robust synchronization control for GFM converters. In the approach, the DVC and the q-axis voltage feedforward control loops are connected in parallel for the synchronization purpose. Compared with the conventional single-loop DVC scheme, the proposed synchronization approach can dampen the LFO. Compared with conventional cascaded or paralleled DVC and active power control schemes, the proposed synchronization approach can avoid the trade-off between LFO and synchronous resonance issues. Experimental tests of GFM converters are performed in rectifier and inverter modes. The results confirm the effectiveness of theoretical analysis and the performance of the proposed control approach.

The power-synchronization control has been proven effective to enhance the stability of voltage-source converters (VSCs) in weak grids. This method can be realized with the dc-link voltage control for VSCs with regulated dc-link dynamics, which leads to the dc-link voltage-synchronization control. This paper analyzes the small-signal stability of dc-link voltage-synchronized VSCs, considering the effect of constant power load/source at the dc link. It is found that low-frequency oscillations may arise with dc-link constant-power dynamics. To dampen the oscillation, an active damping control method based on the active power feedforward (PFF) control is proposed. A notch filter tuned at the synchronous frequency is embedded in the PFF loop to avoid the adverse effect on synchronous oscillation. Design guideline for the PFF control parameters is formulated. Experimental tests of VSC operating in the rectifier and the inverter modes are performed. The results validate the theoretical analysis and the effectiveness of the control approach.