The grid-forming and grid-following controls are adopted in three-phase voltage source converters according to different grid conditions. However, the basic operation principle of the two kinds of control are different and will leads to the instability of the grid-connected system through various paths. In this paper, the energy function model of the grid-forming and grid-following controlled converters are established and compared in detail. The influence of system parameters is studied, and the large-signal stability boundaries are derived through a general method. Moreover, compared to the grid-forming controlled converter, the grid-following converter will lose stability by exhibiting a varying damping coefficient transient. The equivalent damping coefficient can be used as a criterion of whether the grid-forming/following control is suitable for the weak/strong grid condition. The simulation and experiment results show the difference of the transient process and validate the control criterion for different grid strength under large-signal disturbance.

Active power oscillation is observed when two or more parallel synchronverters undergo load fluctuations, potentially impacting the safe operation of the synchronverters. This paper provides a fundamental analysis for the power oscillation mechanisms, and proposes a new control strategy for suppressing power oscillation especially the oscillation excitation is proposed. It is found that power oscillation is inherently caused by weak damping and large inertia of the synchronverters under the condition of undesirable instantaneous active power sharing. Then the conditions for reducing the oscillation excitation of synchronverters which is caused by undesirable instantaneous active power sharing is deduced. The new control strategy effectively suppresses the power oscillation by adding a virtual damping element for enhancing the system damping and a virtual reactance for reducing the oscillation excitation to the conventional control scheme without changing the basic dynamic and steady characteristics of the synchronverters. The theoretical analysis and the proposed oscillation suppression control techniques are validated with an extensive hardware-in-loop simulation study.

As a flexible and reliable way for distributed energy consumption and integration, the converters-dominated microgrid has attracted more and more attention recently. Owing to the low inertia and high nonlinearity of power converters, the islanded microgrid under large-signal disturbances is easily suffer to transient stability problems. To support the stable operation of microgrid, grid-forming converters are extensively investigated and applied. In this paper, an improved equal area criterion (IEAC) is proposed for the transient stability analysis of a simple microgrid equipped with both grid-following and grid-forming converters. Firstly, a simplified second-order model of the microgrid is established, which contains a nonlinear damping term relying on the power angle. Then, the limitation of conventional equal area criterion (EAC) for transient stability analysis is revealed, which completely ignores the unfavorable influence of negative damping and may lead to erroneous judgement. To fill this gap, an improved equal area criterion is proposed to derive the transient stability conditions for islanded microgrid, which could improve the estimation accuracy of stability boundaries. Moreover, impacts of system parameters and controller parameters on transient stability are studied, which provides useful guidelines for the parameter optimization of grid-forming and grid-following converters. Eventually, simulation and hardware-in-loop experiments are conducted to verify the effectiveness and superiority of the proposed method.

A two-layer optimal consensus-based distributed control strategy is proposed for the coordinated operation of networked microgrids (MGs). Several networked MG operation objectives are optimized using the proposed control strategy, including frequency/voltage regulation, proportional active power sharing, and smooth MG islanding/reconnection operation. The global objective function of networked MGs is decomposed into a series of local objectives assigned to participating distributed energy resources (DERs). In the decomposed optimization, each control layer is realized in a distributed manner in which DER state variables reach consensus along with optimizing their assigned local objective functions. Each control layer in the two-layer control strategy is formulated as an optimal consensus problem in which the optimality and asymptotical stability of the system equilibrium point for each control layer are demonstrated. The effectiveness of the proposed control strategy is validated in a modified IEEE 33-bus distribution system using the PSCAD/EMTDC platform.

With the increasing penetration of renewable energy generators, the stability issues of grid-tied converter systems become much more important. However, due to the high nonlinearity and varying damping of converter systems, conventional transient stability analysis methods are not applicable, which may bring to conservativeness or misjudgment on stability assessment. In this paper, the transient stability of grid-tied converter systems with varying damping is investigated to provide stability boundary estimation. Firstly, considering the frequency mutation of the phase-locked loop (PLL) caused by various perturbations, a modified swing equation model of grid-tied converter systems is built, which greatly improves the mathematical model accuracy under transient disturbances. To evaluate the impacts of varying damping and frequency mutation, an iterative equal area criterion (ITEAC) method is proposed with the iterative calculation of the accelerating and decelerating area, which renders stability boundaries with high accuracy. Moreover, the impacts of controller and system parameters on stability boundaries are quantitatively analyzed. Eventually, simulation and hardware-in-loop experiments are performed to verify the effectiveness and superiority of the proposed ITEAC.

Microgrid has been rapidly developed for the integration of distributed renewable energy, owing to its superiority of flexible operation and high reliability. Without frequency support from the main grid, the microgrid under islanded operating mode is more likely to lose stability in the presence of large-signal disturbances. However, previous studies mainly focus on weak grid conditions and ignore the interaction effects between different-type converters. Therefore, this paper investigates the transient synchronization stability of an islanded AC microgrid considering dynamic interactions between grid-forming and grid-following converters. Firstly, a simplified nonlinear model of the islanded microgrid is established, which reveals the nonlinear interaction and damping impact on system stability. Based on the passivity principle and LaSalle’s theorem, an explicit stability criterion is formulated to reveal the instability mechanism. With the proposed stability criterion, the impacts of different system parameters are analyzed, which points out the direction of parameters optimization to enhance stability. To validate the effectiveness of the proposed method, simulations and experiments are carried out.