Grid-forming inverters (GFMIs) are anticipated to play a leading role in future power systems. In contrast to their counterpart grid-following inverters, which employ phase-locked loops for synchronization with the grid voltage and rely on stable grid connections, GFMIs primarily employ the power-based synchronization concept to form the voltage. Hence, they can not only stably operate in regions of the grid characterized by low strength but also provide critical ancillary services to power systems, including voltage, frequency and inertia support. Several control strategies have been employed for GFMIs, making it crucial to comprehend their stability characteristics for the analysis of small-signal stability and low-frequency oscillations. This article examines the performance of GFMIs when equipped with four different control strategies, namely: droop-based GFMI, virtual synchronous generator (VSG)-based GFMI, compensated generalized VSG (CGVSG)-based GFMI, and adaptive VSG (AVSG)-based GFMI. The comparative analysis assesses the performance and robustness of these four control strategies across various operational scenarios in frequency and time domains. Initially, the impedance-based stability analysis method is employed to evaluate these control strategies across different case studies in terms of grid strengths, grid impedance ratios, the dynamics with/without virtual impedance and inner voltage and current loops, and variations in the inverter's operating points. Subsequently, time domain verification using the electromagnetic transient models are conducted for these case studies as well as to assess the power tracking capability of these control strategies in response to changes in power references. Finally, the robustness of these four controllers is explored against external grid disturbances, including grid frequency deviations, phase jumps, and voltage sags, considering varying levels of disturbance magnitudes in both weak and strong grid connections. In conclusion, the evaluation of these control techniques in various operational scenarios reveals their strengths and weaknesses, offering valuable guidance for the selection of the most appropriate control technique to suit desired practical applications.

The conventional virtual synchronous generator (VSG) is typically designed to meet certain operational and control requirements in the islanded mode. However, once the VSG is switched to grid-connected mode (GCM), the robust operation cannot be guaranteed under different grid conditions. It can lead to poor dynamic performance, especially in strong grids, such as significant oscillation, long settling time, and large overshoot. To improve the VSG performance in the GCM, this article first analyzes in depth the inherent coupling between the active and reactive power and its dependence on grid conditions, such as the short circuit ratio and the grid impedance ratio. Subsequently, an adaptive VSG (AVSG) control strategy based on online grid impedance estimation is proposed to ensure robust operation of the VSG considering both strong and weak grid conditions. This technique allows the operator to specify the desired settling time of the output power and damping ratio. To estimate the grid impedance in real time without additional hardware and reduce the associated impacts on power quality, an online event-based grid impedance estimation algorithm is embedded in the control loop of the AVSG. The simulation and experimental results indicate that, compared with the conventional fixed-parameters-based controller design method, the AVSG exhibits desired performance such as no oscillation, specified time duration for the settling time, and minimal overshoot regardless of the grid conditions.

In islanded ac microgrids, the mismatched impedances of the interfacing feeders between the inverters and the load bus cause poor reactive power sharing when the conventional frequency and voltage droop control technique is employed. Such operation endangers the whole microgrid reliability as it may lead to overloading certain inverters and, consequently, triggering protection relays and causing cascaded failure. Thus, this article proposes an accurate reactive power sharing strategy that considers the mismatched feeder impedances in islanded ac microgrids. It is based on the optimal tuning of the virtual complex impedance for each inverter. The proposed strategy has several advantages. First, it has a physical meaning as it establishes an explicit relationship between the mismatched values of the actual resistive–inductive feeders and the assigned values for the proposed virtual complex impedance to each inverter. Hence, further degradations in the microgrid voltages are prevented. Second, there is no need for prior knowledge about the actual feeder impedances in the design stage as they are estimated online from the available measurements. Finally, the proposed control is reliable and fault-tolerant as it copes with unexpected failures such as failure of (or sudden switching off ) some inverters and communication disruptions/delays. It ensures accurate power sharing even under the primary controllers after losing communication links with the secondary controller. The simulation and experimental verification results are presented to validate the performance of the proposed technique.

An increasing intake of grid-connected inverters could change the characteristics of low voltage networks including the equivalent grid impedance. This can impact the overall performance of the inverters, thus it becomes necessary for grid-connected inverters to estimate the grid impedance online. However, there are some challenges when it comes to grid impedance estimation using grid-connected inverters. These include the estimation accuracy and the associated power quality issues that are mainly related to the amplitude, frequency and period of the injected disturbance into the grid. To address these challenges, a novel online event-based grid impedance estimation technique for grid-connected inverters is proposed in this paper. This technique combines an active grid impedance estimation technique, based on variations of the inverter's output power, and the continuous monitoring of the positive-sequence amplitude of the PCC voltage. The main advantage is that the estimation technique only performs a deliberate perturbation of P and Q if there is a certain change in voltage magnitude related to grid impedance change. This significantly reduces the occurrence of required PQ variations, thus minimizing the power losses and the impact on power quality. Simulation and experimental results of a grid-connected inverter system validate the performance of the proposed technique.

This article presents a comprehensive comparison of three control strategies used for grid-following inverters (GFLIs). The first strategy is the phase-locked loop (PLL)-based vector current control (VCC), and the other two PLL-less controllers are the voltage-modulated direct power control (VMDPC) and the linear-parameter-varying power-synchronized control (LPV-PSC). The VCC relies on the PLL to synchronize with the grid frequency to control the exchanged real and reactive power with the grid, which may result in instability issues under weak grid conditions. To prevent this, the VMDPC and LPV-PSC are proposed recently as PLL-less approaches to overcome the difficulties of the VCC in weak grid conditions. The performances of the VCC, VMDPC, and LPV-PSC are comprehensively reviewed in this article, considering the operations of the GFLIs under both strong and weak grids. In each operation mode, the provided tests investigate: 1) step response of active and reactive power, 2) frequency jump in the grid voltage source, 3) phase jump in the grid voltage source, and 4) voltage sag in the grid voltage source. In addition to evaluating the three controllers in the time-domain, the frequency-domain impedance-based stability analysis based on the generalized Nyquist criterion is also considered, which confirms the time-domain findings in terms of accurate predictions of the stable/unstable operations of the GFLIs that are equipped with these controllers. It is found that, compared to the conventional VCC and the VMDPC, the LPV-PSC has promising performance under various test conditions in both strong and weak grids, which may be a future solution for weak-grids connection of the GFLIs.

Grid-connected inverters play an important role in the integration of renewable energy sources such as solar and wind. However, due to the unneglectable grid impedance value seen by the inverters at the point of common coupling (PCC), especially in the weaks and resistive low voltage distribution networks, there is an inherent strong coupling between active and reactive power flow. This power coupling causes significant power quality problems including 1) voltage fluctuation of the common AC bus resulted from high penetration of intermittent renewable generation systems, 2) non-optimal control of neither power flow nor power factor of the delivered power by inverters to the common AC bus, and 3) unintended/uncompensated transmission losses, where the flow of active power through the transmission/distribution lines will cause unintended reverse reactive power flow from the grid-side and vice versa with the reactive power flow. To solve these issues, this paper proposes an adaptive mechanism for droop-based grid-connected inverters to decouple the power flow by compensating the associated unintended active and reactive power losses flowing through the transmission line (or any desired segment of it). This control strategy relies on modifying the power command provided to the frequency and voltage droop loops by considering the effects of both the transmission line resistance and inductance components on the power flow between the inverter and the grid. It uses only the local current and voltage measurements to first perform an online estimation of the transmission line resistance and inductance and then to calculate the proposed adaptive power terms. The performance of the proposed control is validated in MATLAB/Simulink and HIL experiment for a 350 kW droop-based grid-connected inverter system. The proposed control strategy can be utilized to provide ancillary services to the grid such as accurate frequency and voltage support at the location of interest.