This article reveals that the saturation block (SB) used with the power-angle control loop can enhance the transient stability of grid-forming (GFM) inverters. Three methods of using the SB in the power-angle loop are examined based on power-angle curves. It is found that all can enhance the transient stability of GFM inverters by either decreasing the acceleration area or increasing the deceleration area. At the same time, the threshold value selection of SB in the three schemes is also explained, considering the tradeoff between the transient stability and the response speed. A holistic comparison of the three schemes recommends the scheme with the SB used for power error adopted by the GFM inverters. Finally, experimental tests confirm the effectiveness and correctness of the analysis.

This article analyzes the parametric effect of the dc-link voltage controller (DVC) on the transient stability of grid-forming (GFM) inverters. It is found that the decrease of the proportional gain of DVC may lead to the loss of synchronism (LOS) even if the system is locally stable at the new equilibrium point (EP) after disturbance. Its physical insight is attributed to the unstable limit cycle (ULC) in the phase space according to the bifurcation theory. The GFM inverters whose operating point locates outside of ULC will lose synchronism with the grid. The decrease of the proportional gain of DVC will diminish the ULC, resulting in fewer operating points converging to the new EP and a higher risk of LOS. This bifurcation-based analysis provides a novel perspective for investigating the mechanism of parametric effect for the high-order nonlinear system, which is the article's main contribution. The parametric effect of the integral gain of DVC is also analyzed. A holistic parametric space partition for designing DVC is then drawn from the analysis, and a parametric configuration guideline is provided. The theoretical findings are validated by experimental tests.

This paper proposes an adaptive-output-voltage regulation (AOVR) based solution to alleviate the dc-link undervoltage for grid-forming (GFM) inverters. First, it is shown that large disturbances may cause the dc-link undervoltage for GFM inverters threatening the safe operation of the system. To tackle this issue, an AOVR-based control strategy is proposed, whose main idea is to reduce the output voltage of the GFM inverter and also speed up the dynamic process of the power angle by intentionally enlarging the frequency deviation of the active power control loop. By doing so, the dc-link output energy will reduce to avoid possible undervoltage. The parameter design of the proposed method is based on the overmodulation constraints, and an auxiliary module is configured for the low-voltage-ride-through (LVRT) requirements. The applicability of the proposed solution is checked when considering the current limitation. The pros of the proposed solution are also depicted when compared with state-of-art solutions. The effectiveness of the proposed control strategy is finally confirmed by the experimental tests.

This paper analyzes the influence of the saturation limiter used with dc-link voltage control (DVC) on the transient stability of grid-forming (GFM) inverters and proposes a flexible saturation limiter to alleviate its adverse impact. First, it is found that the conventional saturation limiter (CSL) can enhance transient stability. Yet, it would boost the dc-link voltage or even trigger overvoltage protection, especially when the threshold value of CSL is small. To tackle the adverse impact of CSL on dc-link voltage, a flexible saturation limiter is proposed, which only begins to limit the active power reference when the derivative of dc-link voltage turns from positive to negative (the dc-link voltage reaches its peak value at this instant). Finally, experimental tests confirm the effectiveness of the scheme.

Under unbalanced grid voltage faults, the output power oscillation of a grid-connected inverter is an urgent problem to be solved. In the traditional topology of inverters, it is impossible to eliminate power oscillation and simultaneously maintain balanced output current waveform. In this paper, considering the solvability of reference current matrix equation, the inherent mechanism of inverter output power oscillation is analyzed, and a modified topology with auxiliary modules inserted in series between the inverter output filter and the point of common coupling (PCC) is proposed. Due to the extra controllable freedoms provided by auxiliary modules, the inverter could generate extra voltage to correct PCC voltage while keeping balance of output current, so as to eliminate the oscillation of output power. Simulation and experimental results verify the effectiveness of the proposed topology.

This paper proposes an adaptive-output-voltage regulation (AOVR) based solution to alleviate the dc-link undervoltage for grid-forming (GFM) inverters. First, it is shown that large disturbances may cause the dc-link undervoltage for GFM inverters threatening the safe operation of the system. To tackle this issue, an AOVR-based control strategy is proposed, where the main idea is to reduce the output voltage of the GFM inverter and also speed up the dynamic process of the power angle by intentionally enlarging the frequency deviation of the active power control loop. By doing so, the dc-link output energy will reduce to avoid possible undervoltage. The parameter design of the proposed method is based on the overmodulation constraints, and an auxiliary module is configured for the low-voltage-ride-through (LVRT) requirements. The applicability of the proposed solution is also checked when considering the overcurrent limitation. The effectiveness of the proposed control strategy is finally confirmed by the experimental tests.