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.

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.

Transient stability issues of power converter systems have attracted much attention. Generally, the converter systems behave in high-order nonlinear dynamics due to multiple cascaded control loops, which become obstacles for transient stability analysis. Considering the dynamic coupling between active power and reactive power control loops, the grid-tied virtual synchronous generators (VSG) system is a third-order nonlinear system. Including the equal area criterion (EAC), many conventional methods are only applicable to second-order systems and cannot fully capture damping effects. To fill this gap, a novel iterative EAC method is proposed to handle the transient stability of the third-order VSG system. First, an implicit relationship from the output voltage to the power angle is derived by integral substitution. From energy aspects, the dynamic relations between voltage, power angle, and frequency are approximated by algebraic equations through iterative calculation. The impact of nonlinear damping is also fully captured quantitatively. Both conservatism and quantity of computation are improved compared with previous studies. Furthermore, the critical clearing angles under different disturbance forms and degrees are derived. In addition, some discussions about current limiters’ influence and different controller parameters’ influence are given. Simulation and hardware-in-the-loop experiments based on RT-Lab are conducted to verify the accuracy of the proposed method.

With the rapid development of distributed renewable energy generation and integration, stability issues of power converters systems have been widely studied in recent years. Many previous studies focus on the transient stability of a single converter connected to the weak grid. However, the nonlinear dynamics and stability mechanism of multiple converters systems remain unclear. In this article, a unified nonlinear model of the paralleled-converters system is established, which reveals the interactive power and interactive damping among multiple converters and the weak grid. To analyze the impacts of the interactive power and damping terms on system stability, a dual-iterative equal area criterion (DITEAC) is proposed with a dual-layer iterative calculation of the accelerating area and decelerating area. Compared with the previous studies, the proposed DITEAC method could deal with the nonlinear interactive damping and reduce the conservatism caused by the inequality scaling, which greatly improves the calculation accuracy of stability boundaries. The expansion feasibility of the proposed unified nonlinear model and DITEAC method to the N -converters system is fully discussed. Eventually, simulation and hardware-in-loop experiment results are presented to verify the effectiveness of the proposed method.

The transient instability mechanism and enhanced control for Grid-Forming controlled voltage source converter (GFM-VSC) during voltage sags has been widely investigated. However, the postfault performance are overlooked in the previous studies. In this paper, the postfault transient instability and windup problem of GFM-VSC with virtual inertia is first revealed based on established piecewise nonlinear model and power angle curve. Second, the impact of key parameters on postfault transient stability and windup phenomenon and the impact mechanism is analyzed, revealing that the conventional deceleration effect enhancement method increase the postfault transient stability, but increase the risk of windup problem. Third, the torque error and q -axis voltage feedforward based hybrid synchronization method is adopted to achieve transient stability enhancement and anti-windup. Different from the conventional global deceleration effect enhancement, the adopted control method can inject extra kinetic energy during first current saturation mode and extra damping during current unsaturation state, therefore, the global deceleration effect can be enhanced and the local deceleration effect in the first current saturation period can be deteriorated. Then, the anti-windup and transient stability enhancement is achieved. Finally, the analysis and effectiveness of the designed control strategy has been verified on a 500W experimental prototype.