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.

Capacitor-current-feedback active damping has been widely used in LCL-type grid-connected inverters. However, the damping performance is deteriorated due to the negative equivalent resistance resulted by the digital control delays, and thus the grid-connected inverter is apt to be unstable under the grid impedance variation. To address this issue, this paper proposes the capacitor-current proportional-integral (PI) positive feedback active damping method, which can ensure a positive equivalent resistance almost within the Nyquist frequency, i.e., the full controllable frequency range. In theory, the proposed damping method can be implemented by feeding back the capacitor current through a PI function. However, the integral term will continuously accumulate the noise and dc bias arising from the feedback signal. To overcome this drawback, a more practical implementation solution is drawn in this paper. Furthermore, a straightforward design procedure is presented for the convenience of selecting the proper controller parameters. With the proposed damping method and its practical implementation, high inverter robustness against the grid impedance variation can be achieved. Experiments are performed on a 6-kW prototype and the experimental results are in well agreement with the theoretical expectations.

This article discusses the mathematical relationship between gridvoltage-modulated (GVM) direct-power control (DPC) and vector-current control (VCC) for three-phase voltage-source converters (VSCs). GVM-DPC is equivalent to VCC at the steady state, and it presents a superior transient performance by removing the need for the phase-locked loop (PLL). This means that GVMDPC solves the disadvantage of conventional DPC, such as poor steady-state performance. Moreover, GVM-DPC will reduce the computational burden in comparison with VCC due to the absence of Park transformation and PLL. Consequently, we can expect that the GVM-DPC method will be capable of plug-and-play for VSCs. Finally, the experimental results match the theoretical expectations closely.

Feedforward schemes of the voltage at point of common coupling (PCC) have been widely used in grid-connected inverters to reject the current harmonics caused by the grid voltage distortion. However, in the weak grid, the PCC-voltage feedforward tends to destabilize the grid-connected inverters due to the effect of time delay. This stability issue is explicitly elaborated by the impedance-based stability criterion in this paper and then addressed with an improved feedforward scheme, which uses the grid voltage instead of the PCC voltage as the feedforward variable. Considering that the real grid voltage is unavailable for sensing, a method to extract the grid voltage from the sensed PCC voltage is put forward to implement the proposed scheme. By carefully arranging the sampling instants according to the duty cycle, a dual sampling mode is adopted to ensure an accurate extraction of the grid voltage. Finally, simulations and experiments are performed on a 6-kW single-phase grid-connected inverter, confirming that the proposed grid-voltage feedforward achieves a superior harmonic rejection ability and a strong stability under the weak grid condition.

Capacitor-current-feedback (CCF) active damping and point of common coupling (PCC) voltage feedforward can provide damping to the LCL-type grid-connected inverter. In general, they are individually adopted, and negative damping will occur in certain frequency range due to the digital control delay, leading to a non-minimum phase behavior. In this paper, a hybrid active damping that combines the CCF and unit PCC voltage feedforward is studied. It is found that when the CCF gain is properly designed, the positive damping range could sweep the entire frequency range with the variation of grid impedance. As a reward, the maximum profit of damping cooperation can be harvested, ensuring high robustness against both grid impedance variation and filter parameter fluctuation. The simulation and experimental results are provided to verify the effectiveness of the hybrid active damping.