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.

The undesired nonlinear operation of phase-locked loops (PLL) is one of the main causes of transient instability in grid-connected converters. However, the stable operating region of PLL, knowing which is helpful for protection purposes, is hard to find due to the nonlinear characteristics of the transient process. In this paper, the nonlinear characteristic of the PLL control loop relevant to the grid-connected converter operation is identified. The averaging method is applied to derive a time-domain expression for the PLL operation under disturbance. Based on the analytical expression, the transient response and related stability criterion are established, and the PLL design is improved while targeted to focus on various AC grid faults or disturbances. The results, applicable to transient stability enhancement, are verified by circuit simulations and experiments.

Three-phase voltage source converters (VSCs) are commonly used as power flow interface in ac/dc hybrid power systems. The ac power grid suffers from unpredictable short-circuit faults and power flow fluctuations, causing undesirable grid voltage dips. The voltage dips may last for a short time or a long duration, and vary the working conditions of VSCs. Due to their nonlinear characteristics, VSCs may enter abnormal operating mode in response to voltage dips. In this paper, the transient response of three-phase VSCs under practical grid voltage dips is studied and a catastrophic bifurcation phenomenon is identified in the system. The converter will exhibit an irreversible instability after the dips. The expanded magnitude of ac reactive current may cause catastrophic consequence for the system. A full-order eigenvalue analysis and a reduced-order mixed-potential-theory-based analysis are adopted to reveal the physical origin of the large-signal instability phenomenon. The key parameters of the system are identified and the boundaries of instability are located. The bifurcation phenomenon and a set of design-oriented stability boundaries in some chosen parameter space are verified by cycle-by-cycle simulations and experimental measurement on a practical grid-connected VSC prototype.

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.

Voltage-source converters are commonly employed as rectifiers for providing a regulated dc voltage from an ac power source. In a typical microdistribution system, the power grid is nonideal and often presents itself as a voltage source with significant impedance. Thus, power converters connected to the grid interact with each other via the nonideal grid. In this paper, we study how stability can be compromised in a system of interacting grid-connected converters, which are used typically as rectifiers. Specifically, the stable operating regions in the selected parameter space may shrink when grid-connected converters interact under certain conditions. We consider the effect of both source (grid) impedance and transmission line impedance between converters, and derive bifurcation boundaries in the parameter space. A small-signal model in the dq-frame is adopted to analyze the interacting system using an impedance-based approach. It is shown that the system of interacting converters can become unstable. Moreover, results are presented in design-oriented forms so as to facilitate the identification of variation trends of stable operation boundaries. Experimental results verify the instability phenomenon.