The virtual synchronous generator (VSG) was proposed to emulate a synchronous machine's dynamics when integrating power electronic converter-based distributed energy resources to the power grid. However, the VSG's synchronization stability during grid faults is not fully explored. The underlying mechanism of loss of synchronization (LOS) still needs to be further revealed due to VSG's nonlinear characteristics. In this paper, a step-by-step analytical method based on combining the linear and nonlinear models is proposed to analyze VSG's dynamic behaviors during a large disturbance. The relationship between the linear and nonlinear models is first brought to light, showing that the linear model is suitable for qualitative analysis to give an intuitive physical insight. Simultaneously, the latter is adopted for quantitative analysis to assess stability after a grid fault. Moreover, to avoid the conflict of the synchronization stability and the inertia response, a transient damping method is added in the active power control loop, which can simultaneously improve the synchronization stability and frequency stability. Design guidelines are also proposed to identify the parameter of the transient damping with different inertia requirements. Finally, the experimental results verify the analytical method and the theoretical analysis.

The synchronization stability of the virtual synchronous generator (VSG) under grid fault is an important issue for maintaining stable operation in the power system. Existing work has pointed out a low-pass filter (LPF) with a sufficiently low cutoff frequency in the reactive power control loop (RPCL) can improve the transient stability. Yet, the underlying mechanism was unknown. Moreover, as a key index of VSG and precondition of synchronization stability, the frequency response is rarely studied. In this article, based on the linearized model for qualitative analysis, combined with the nonlinear model for quantitative analysis, the underlying mechanism of improving synchronization stability using an LPF in the RPCL is revealed. Furthermore, to avoid increasing the system order and solve the conflict between transient stability and frequency response, an improved synchronization stability method is proposed by feedforwarding the frequency difference between the VSG and grid to the RPCL. The frequency response is also acquired based on the combined linearized and nonlinear model, which shows that the frequency feedforward method can further enhance the frequency stability. How to design the coefficient of the frequency feedforward path with different inertia requirements is also presented. Finally, this method is verified by experimental results.

T he virtual synchronous generator (VSG) control of a grid-connected converter is an attractive interfacing solution for high-penetration renewable generation systems. Unfortunately, the synchronous resonance can appear due to the power control loops, which are usually damped by adopting a virtual resistance (VR). However, the effects of VR on the transient stability of the VSG are rarely studied. In this article, a virtual point of common coupling and a virtual power angle concept are proposed to represent the mathematical model of the VSG with VR damping. Based on the model, the transient stability is further analyzed using the phase portrait and the attraction regions of the nonlinear system. It reveals that the VR has negative and different impacts on the transient stability compared with the real grid resistor. In order to keep the VSG with VR to work normally during the grid voltage sag, an enhanced transient stability method, by reducing the active power commands when the grid fault is detected, is introduced. A design-oriented analysis and the parameter design with different VRs are also presented. Finally, the theoretical analysis is verified by the experimental results.

A transient damping method (TDM), introducing the frequency of a virtual synchronous generator through a high-pass filter (HPF) to the power reference, is usually adopted to stabilize the system against disturbances. However, different from a small-signal stability perspective, this letter reveals that the gain of HPF should not be too large due to the transient stability issues. Thus, an optimal parameter design method of the TDM is proposed for the improvement of transient stability. The experimental results validate the theoretical findings.

Low-frequency oscillations may occur in a virtual synchronous generator (VSG), resulting from an insufficient equivalent damping ration, which may be a serious stability issue for VSG. Thus, an additional damping ration is expected to be provided without affecting droop behaviors and the inertia response of VSG. To that end, an analytical approach based on combining a damping torque model and a small-signal model is developed to investigate the damping behaviors of the VSG. The relationship between the two models is brought to light, indicating that the damping torque model is suitable for qualitative analysis to give the contribution of different feedback loops to the system damping level. Subsequently, the latter is adopted for quantitative analysis to illustrate the damping ration of the system. Furthermore, an additional damping torque method is proposed, which can provide an extra damping ration for the system by increasing the equivalent positive damping torque and guaranteeing that the original droop characteristics and inertia are not compromised. Besides, the proposed control method is implemented by inserting an auxiliary branch into the power control loops of VSG. Experimental results verify the theoretical analysis and the effectiveness of the proposed additional damping method.

The power-synchronization control (PSC) method of grid-connected converters, which is based on the active power orientation, is a promising solution for high-penetration renewable power generation systems. Unfortunately, synchronous frequency resonance (SFR) is inevitably found in PSC-controlled converters, especially when connected to inductive networks, which seriously damages the stability of the system. The relatively small equivalent resistance of the inductive network is regarded as the main cause of SFR, and a virtual resistor (VR) is usually employed to dampen this kind of oscillation. However, another important reason for SFR is the nonminimum phase effect of the loop gains, which is discussed in this article. Considering the dynamics of the grid equivalent inductance, the small-signal model of PSC-controlled converter is established. By this, the mechanism of SFR and the nonminimum phase effect are revealed. Furthermore, a conjugate poles elimination control method based on auxiliary branches introduced into the power control loops is proposed to suppress SFR, which can also broaden the control bandwidth compared with the VR method. Finally, the theoretical analysis is verified by simulations and hardware-in-loop experimental results.