In grid-connected inverter systems, frequency-domain passivity theory is increasingly employed to analyze grid-inverter interactions and guide inverter control designs. However, due to difficulties in meeting sufficient passivity-based stability conditions at low frequencies, passivity theory often falls short of achieving stable system specifications. This article introduces an extended frequency-domain passivity theory. By incorporating a weighting matrix, an extended stability condition is derived. Compared to conventional passivity-based stability conditions, the proposed theory significantly reduces conservativeness and is more suited for analyzing grid-inverter interactions and guiding inverter control design. Theoretical analyses, numerical examples, and experimental results are provided to validate the effectiveness of the proposed methods.

This paper investigates the phenomenon of reverse active power in grid-forming voltage source converters during grid voltage recovery events. The study demonstrates that this issue originates from two primary aspects: the operating point during the fault steady state and the control dynamics. The operating point analysis reveals that maintaining a certain level of active power transmission during the fault steady state is necessary to keep the operating point within the positive-power region; otherwise, the operating point may shift into the negativepower region. Furthermore, the control dynamics cause the phase angle of the modulation voltage vector to rotate clockwise, which reduces the power angle and exacerbates the power reversal issue. Theoretical findings are validated through both simulation and experimental results.

Active damping is crucial for the stable operation and dynamic performance of grid-forming (GFM) voltage source converters (VSCs). This paper presents a comparative analysis of active damping methods used in the synchronization control of GFM-VSCs. These methods are classified into frequency-difference-based, active-power-based, and q-axis-voltage-based damping approaches. The comparison is based on three key metrics: reference tracking performance, inertial response capability, and stability impact. The advantages and drawbacks of each approach are analyzed, and practical design guidelines are proposed, including hybrid configurations that integrate power and voltage feedforward strategies. The effectiveness of the comparative evaluation is validated through both simulation and experimental results.

This paper presents an analytical framework to evaluate the damping contributed by inner control loops in gridforming voltage-source converters. First, an impedance model is developed to characterize the dynamics of three types of inner loops, with the control-shaped resistive component indicating the damping for synchronous oscillations. Then, inner-outer loop interactions and interaction-induced oscillations are evaluated using the complex torque coefficient, with the damping torque used for stability assessment. The framework offers two benefits: (i) it yields intuitive physical insight into inner-outer loop interactions and oscillation mechanisms; and (ii) it enables innerloop parameter tuning using electrical damping torque with minimal dependence on outer-loop operating points. The method is exemplified for virtual-admittance and current-control inner loops, where both synchronous and sub-synchronous oscillations are analyzed and mitigated. Time-domain simulations and hardware experiments validate the approach and its findings.

This article enhances impedance-based modeling generality and stability assessment flexibility for multi-parallel GFM inverters by proposing a unified perunit scaling method and a configurable decentralized stability condition based on passivity theory.

This paper investigates the transient performance of grid-forming (GFM) converters during power reference regulation. It is revealed that implementing fast power control induces a rapid fast voltage source behavior, which, however, compromises the GFM capability. Furthermore, the decoupling control of active and reactive power responses leads to larger voltage magnitude fluctuations, thereby sacrificing the ability to maintain a stiff voltage source. To address those trade-offs, an adaptive control strategy is proposed, which is activated only during power reference regulation to achieve fast and decoupled power control. The GFM converter, utilizing the proposed control method, successfully achieves fast power regulation while minimizing the power coupling. Both simulation and experimental results validate the theoretical analysis.

The main purpose of this article is to elaborate on the limitations of using frequency-domain passivity theories in analyzing grid-inverter interactions within the low-frequency range. It primarily covers three levels of limitations: 1) the limitations and selection criteria of two kinds of passivity index, 2) potential conflicts between different passivity index tuning methods, and 3) the relationship between the frequency range of negative passivity index and system stability robustness. The findings suggest that caution should be exercised when applying passivity theory, particularly in the low-frequency range.

This paper evaluates the transient performance of grid-forming (GFM) converters considering the high-switching-frequency characteristics of power devices. It is found that the disturbance suppression performance of the current control is compromised with higher switching frequencies, while employing the voltage feedforward control can improve it. Consequently, two issues of GFM converters during the low-voltage ride-through are presented, i.e., the current overshoot during the fault occurrence and the transient overvoltage during the fault recovery. Simulation and experimental results confirm theoretical analyses.

This letter revisits vector voltage control (VVC) and finds that by introducing a P-Eq droop into the q-axis voltage reference, a conventional phase-locked loop (PLL) can effectively substitute the power synchronization control for replicating the power-frequency (angle) dynamics of grid-forming (GFM) inverters. Consequently, a simple GFM control strategy that retains the traditional PLL and VVC is proposed. In contrast to recently reported PLL-based GFM approaches, the method eliminates the need for virtual admittance control and offers higher stability robustness across a wide range of grid shortcircuit ratios (SCRs). The proposed scheme is validated through experimental results spanning SCR values from 1.2 to 9.7.