Grid-forming converters (GFMCs) provide voltage support and other grid services, such as inertia and droop, in more-electronics power systems. The GFMCs with small droop coefficients or connected to the grid through a small impedance may have a fast power loop, which leads to serious conflicts with the inner ac voltage control loop, resulting in power oscillations or even instability. To address this problem, an extremely fast ac voltage loop should be expected for GFMCs. On top of that, GFMCs must operate well in both stand-alone and grid-tied conditions. Through detailed analysis, this article first reveals that the grid-tied operation of a dual-loop controlled GFMC with an LCL output filter features a much slower voltage control loop than that of the stand-alone mode. To improve the dynamics of GFMCs, we propose a generic voltage control scheme with a high-pass filter in the current feedback loop. The generic voltage controller has fast voltage tracking performances under both grid-tied and stand-alone operations, thereby enabling GFMCs to achieve better power regulation and grid service. The improvements in both voltage tracking and power control are verified via experiments.

The increasing penetration of renewable energy resources facilitates the carbon footprint reduction process yet reduces the power system inertia. As a result, the grid frequency and the rate of change of frequency (RoCoF) might probably go beyond the normal range, resulting in unexpected load shedding, generator tripping, and even frequency instability. To address this problem, grid-connected inverters are designed to participate in frequency regulation and provide the equivalent inertial support. Nevertheless, the inertia emulation effect is affected by the inverter synchronization dynamic and high RoCoF events may occur as the result of poor synchronization dynamics. In view of this limitation, a synthetic inertia control is developed in this article considering the synchronization dynamics. The synthetic inertia principles and control design guideline are explicitly provided. Finally, hardware experimental results of a scaled-down power system prototype are provided to validate the effectiveness of the proposed approach.

Grid-forming converters (GFMCs) prevail for their capability of providing voltage and frequency support in modern power grids with large integration of intermittent distributed power sources. As GFMCs behave as voltage sources, the current limiting of GFMCs under large disturbances requires special efforts. Furthermore, a reliable GFMC should also be capable of remaining synchronized with the grid during and after large voltage disturbances. To avoid the instability caused by phase-locked loops during low-voltage faults, GFMCs usually retain the active power synchronization under low-voltage faults when the current saturation is triggered. When voltage sags are severe, the transient stability highly depends on the fault duration. This article proposed a robust fault ride through strategy for GFMCs based on reactive power synchronization (Q-Syn) under voltage sags. The Q-Syn method ensures transient stability of GFMCs regardless of the fault duration or level. Furthermore, fast postfault recovery can be achieved with the Q-Syn method. The effectiveness of the proposed method is verified by experiments.

In islanded microgrids, the harmonic power of nonlinear loads is distributed among parallel voltage source inverters (VSIs) according to the effective harmonic impedances, i.e., the sums of VSI output impedances and grid impedances. Since grid impedances are unknown and could be mismatched, VSI output impedances are usually reshaped to ensure the harmonic power sharing accuracy. However, as conventional techniques only regulate VSI impedances in one dimension, only one degree of freedom (DOF) is provided for the impedance shaping. It is revealed that such manoeuvres can hardly fulfil the proper harmonic power sharing requirement for complex grid impedance situations. As a result, circulating harmonic currents will occur and produce additional power losses even if the total harmonic power has been accurately shared. To solve this problem, this paper proposes a two-dimensional impedance-shaping control, which can adaptively regulate VSI output resistances and inductances at the same time. The proposed control strategy requires no prior grid impedance knowledge and can eliminate the circulating harmonic currents for arbitrary grid impedances. Simulation and experimental results from an islanding microgrid prototype with three parallel VSIs are provided to validate the effectiveness of the proposed method.

Grid synchronization stability is an important research topic for grid-forming converters. To prevent instability and undesired oscillations, the small-signal modeling and stability analysis must be performed, which requires the line impedance information. Nevertheless, the line impedance may be unknown in most cases. As a consequence, the designed control parameters may fail to satisfy the stability requirement. To fill in this research gap, this article presents a geometrical approach to evaluate and improve the small-signal synchronization stability of grid-forming converters. The proposed analytical approach maps the line impedance variation on a two-dimensional plane and identifies the stability boundary in a geometrical way. As a benefit, the controller of grid-forming converters can be explicitly designed to achieve the robust synchronization stability. Finally, the feasibility of the proposed technique is verified by simulation and hardware experimental results.

Driven by the demands of carbon footprint reduction, the conventional fossil-based generators are gradually being replaced by grid-forming inverter. One crucial aspect of grid-forming inverter is synchronization stability, which is not only related to the inverter power-frequency (P-f) dynamic but also influenced by power grid characteristics as well as grid impedances. From the perspective of a local inverter, it is difficult to know the detailed information. As a result, severe oscillations and even the loss of synchronization may occur and pose a great threat to the power grid. To overcome this problem, a black-box approach is presented in this article. Initially, a frequency-domain identification approach is proposed to acquire the power grid characteristic by the inverter itself. Based on the measurement results, the P-f dynamic of grid-forming inverter is explicitly reshaped to ensure the synchronization stability. The proposed methodology does not require any prior power system knowledge or the trivial modeling process. Finally, the effectiveness of the proposed method is verified by hardware experimental results.