The high penetration of renewable energy generation (REG) systems influences the dynamics of power systems. Accurate transient simulations and dynamic models are indispensable for research on the dynamic behavior of REG systems. A large-scale REG system consisting of numerous grid-connected inverters consumes large amounts of computation time, so a dynamic aggregated model is needed for efficient simulation. The aggregated model established by coherent equivalence fits the detailed model well, and thus it is widely used in the aggregation of large-scale dynamic systems. Presently, the key challenge of the application of the coherent equivalence method is to identify inverters which have similar dynamic characteristics. The Hamilton principle expressed by Hamilton's action has a simple form and indicates the comprehensive dynamic characteristics of inverters. Thus, this paper proposes a coherent equivalence method of inverters based on Hamilton's action. The proposed method is applied to a wind farm of 16 permanent magnet synchronous generator (PMSG) -based wind turbines for demonstration. Compared with the existing equivalence method without considering coherency, the proposed method shows advantages in effectiveness and accuracy.

Source-side virtual synchronous machines (VSGs) and load-side VSMs (LVSM) are gradually utilized together in the microgrid to provide virtual inertia and damping. However, instability occurs in the islanded microgrid system due to the interaction dynamics between the VSG and the LVSM, which has been investigated in this article. At first, the dq -frame impedance models of the VSG and the LVSM are established and compared. It is revealed that the d-d channel impedance of LVSM behaves the negative resistor with a V-type magnitude in the low-frequency range, which easily interacts with the d–d channel impedance of the VSG and leads to instability of the system. Thus, the inductor current feedforward control and the additional voltage feedback control are proposed for the VSG to reshape its impedance. It diminishes the impedance magnitude and generates the active impedance of the VSG. In this way, the low-frequency interaction between the VSG and the LVSM can be mitigated. Besides, the proposed control preserves the dynamic performance of the system. Finally, simulations and experiments verify the effectiveness of stability analyses and the proposed suppression method.

Transient angle instability and fault overcurrent are two serious problems affecting the safe operation of the virtual synchronous generator during grid faults. However, the two aspects are mostly treated as two independent problems in existing studies. In this article, taking both transient angle instability and fault overcurrent into consideration, a two-stage transient control collaboratively utilizing virtual impedance (VI) and switching PI is put forward. In stage one, the VI is switched to the boundary value determined by the iteration algorithm to avoid fault overcurrent. In stage two, the proportional loop or integral loop in switching PI, which can embed virtual damping or virtual torque into the traditional swing equation, respectively, is activated to prohibit two types of transient angle instability induced by VI. By adaptively tuning VI and activating different loops, fault current limitation and transient angle stability are guaranteed simultaneously under different fault degrees. Furthermore, inertia support is maintained, and transient performance is improved. The proposed control is also capable for a multi-inverter system. Simulation and experimental tests validate the effectiveness of the proposed control.

In islanded microgrids with complex feeder networks, distributed generation units (DGs) and loads can be freely connected to any node, leading to changes in the feeder impedance between nodes and the microgrid structure. However, traditional harmonic sharing control methods encounter difficulties with microgrids with complex topologies. Additionally, existing techniques essentially require constructing additional harmonic virtual impedance, possibly resulting in conflict between voltage quality and harmonic sharing effectiveness. To address this issue, this paper proposes an adaptive harmonic impedance reshaping method based on a consensus algorithm. The proposed method adaptively reshapes the inverter harmonic impedance by introducing harmonic current feedforward compensation control and impedance reshaping factor adaptive control, which can achieve accurate harmonic sharing while greatly improving the voltage quality of the microgrid. The proposed method avoids constructing additional harmonic virtual impedance and can be applied to microgrids with complex feeder networks and variable structures without acquiring prior microgrid configuration knowledge. Moreover, the proposed method is a fully distributed control, where the microgrid central controller (MGCC) is not required. Furthermore, the control parameter design method based on modeling and stability analysis is discussed in detail. Finally, simulations and experiments verify the feasibility and effectiveness of the proposed method.

For the distributed generation units (DGs) using virtual synchronous generator (VSG) control algorithm connected to the islanded microgrid with high penetration of nonlinear load, there is always a trade-off between harmonics sharing accuracy and voltage quality when using conventional harmonics sharing methods. To address this issue, an impedance-based adaptively reshaping method is proposed in this paper, which is based on harmonic current feedforward compensation control and impedance reshaping factor adaptive control, assigning a variable harmonic impedance reshaping factor for each VSG to improve the controllability of VSG harmonic impedance, thus simultaneously enhancing nonlinear load sharing and voltage quality. Moreover, a droop-based methodology for impedance reshaping factor tuning is presented to adaptively reshape harmonic impedance for each VSG without acquiring the line parameters in advance. The proposed method also enjoys the advantage of avoiding introducing additional virtual harmonic impedance, and thus it is immune from the inverter closed-loop gain throughout the dominant harmonic frequencies. Furthermore, the stability analysis and the key control parameter design are discussed in detail. Finally, the effectiveness of the proposed method is verified by simulation and experimental results.

Virtual synchronous machine (VSM) based high voltage dc systems enhance the inertia of the power system. However, the dynamic interactions between the VSM-based rectifier station, the inverter station, and the grid could induce the system oscillation, which has been investigated in this article. At first, the hybrid ac–dc impedance models of the VSMs considering the coupling between the ac and dc dynamics are established. Then, the relationships between the dc impedance, dq -frame impedance, and the hybrid ac–dc impedance are presented. It is found that the dc impedance of the VSM-based inverter station and the d-d channel impedance of the dq -frame impedance of the rectifier station behave as negative resistors in the low-frequency range. Moreover, a five-dimensional impedance stability criterion based on the hybrid ac–dc impedance and generalized inverse Nyquist criterion is proposed to assess the system stability. The analysis results show that the low-frequency oscillation occurs when the grid short-circuit ratio of the rectifier station is small. Finally, the simulation and experimental results verify the impedance models and the stability criterion.