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.

This paper analyzes the parametric effect of the dc-link voltage controller (DVC) on the transient stability of grid-forming (GFM) inverters. It is revealed that the decrease of proportional gain of DVC may introduce the shrinking unstable limit cycle in the phase space according to the bifurcation theory. The unstable limit cycle, which appears between the Hopf bifurcation and Homoclinic bifurcation, may lead to the loss of synchronism for GFM inverters with the grid even if the new equilibrium point is stable after the large disturbance. The parametric effect of the integral gain of DVC is also identified upon bifurcation theory, based on which, a parametric space is proposed for designing DVC to guarantee the transient stability of GFM inverters. The theoretical findings are validated by experimental tests.

This paper proposes a flexible saturation limiter (FSL) for dc-link voltage control (DVC) of grid-forming (GFM) inverters, to enhance transient stability under large disturbances. First, the impact of the conventional saturation limiter (CSL) of DVC on the transient stability of GFM inverters is analyzed. It is found that the CSL can enhance transient stability, yet it may 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 then 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 and simulation tests confirm the effectiveness of the method.