Virtual synchronous generator (VSG) control technique is widely used for the grid-friendly integration of renewable energy. However, when VSG is attached to a weak grid, it is prone to voltage and frequency instability, which aggravates the voltage quality and frequency quality at the point of common coupling (PCC), and in turn, pollutes the supplying quality for the loads. On the one hand, focusing on voltage stability, this paper proposes the impedance modeling for VSG in the synchronously inertial reference coordinates (SIRC) to measure the voltage stability of grid-tied VSG and identify the interaction between VSG and the weak power grid. Studies show that weak grid strength can lead to a wide-band oscillation of VSG. On the other hand, focusing on frequency instability, the motion equation has been put forward to illustrate the oscillation and dynamic deviation of frequency. The relationship between synchronizing ability of VSG and grid stiffness is discussed. Besides, this paper proposes an inertia-damping-strengthened control method to improve frequency stability. Finally, the theoretical analysis is compared with simulations and experiments.

Inertia and damping are the core index of the estimation of power system stability. Existing analysis to the support of inertia and damping are focused on the improvement of system stability as well as parameters tuning of virtual inertia and virtual damping but rarely involved in the quantitative calculation of level of inertia and damping under a certain condition. Based on this background, this article puts forward a unified calculation method to identify the equivalence of inertia and damping under various ratios of R_g / X_g . In addition, the quantity of improved inertia and damping support enabled by virtual inductance control are estimated by the derived calculation model of equivalence. The calculated mathematical model of equivalence of inertia and damping is well verified by the overlapped eigenvalues distribution. Besides, the negative synchronous power will inevitably influence the aperiodic loss of synchronization (ALoS) of power system caused by the large virtual inductance. Therefore, a solution should be derived to make a compromise between oscillation damping and ALoS by tuning the value of virtual inductance. Finally, the simulation and experiment results in various cases verify the effectiveness of the proposed equivalent damping and inertia calculation method as well as virtual inductance control.

Virtual synchronous generator (VSG), recognized as a grid-friendly control technique, contributes to providing inertia and damping support from the grid-tied power electronic systems. However, similar to the conventional synchronous generator, the VSG is vulnerable to wideband oscillations and can even become unstable when the system is subjected to a variety of disturbances. To tackle this issue, this article proposes a magnitude-phase motion equation (MPME) for grid-forming VSG attached to a weak grid. Besides, a feedback analytical method is introduced to identify the frequency dynamic stability, judged by the sign of ( dω / dt )/ δω . Besides, the rate of change of frequency (RoCoF) and frequency nadir (FN) are evaluated by the two indices, i.e., dω / dt and δω . In addition, the relationship between the low-inertia-RoCoF-FN-related stability and the weak-damping-oscillation-related stability is illustrated by the feedback analysis. Moreover, in the framework of MPME, a reactive-power feedforward (RPFF) decoupling control strategy is proposed for the enhancement of stability and robustness performance of the VSG system. Finally, the proposed modeling and the RPFF decoupling control strategy are demonstrated with the experiments.

Frequency stabilization is the premise of guaranteeing grid-friendly integration of virtual synchronous generator (VSG). Based on that premise, this article, focused on frequency stability, establishes the small signal model of grid-forming VSG system. At first, the mechanism of frequency oscillation occurring in system of active- and reactive-power coupling is analyzed by the defined feedback effect factor in this article. Besides, a new dynamic model is established for identifying dynamic interaction of voltage magnitudes and frequency by means of feedback effect. The analytical results of established models agree with that of eigenvalue analysis. Furthermore, a new virtual inductance control strategy is proposed to mitigate the unstable oscillation of frequency and powers, enhance damping performance, and improve stability margins. Unlike the conventional virtual inductance control, which is reliable on dual-loop control framework, the proposed virtual inductance control in this article is based on principle of energy conservation and can be applicable for the grid-forming inverter without inner dual-loop control structure. Finally, the proposed modeling as well as virtual inductance control method is experimentally verified.

Virtual synchronous generator (VSG) has been a grid-friendly integration control technique for the integration of grid-connected inverters. However, the emulated inertia and damping of VSG control technique can also be used in the field of dc systems. In this article, a virtual synchronous control is proposed to dampen the wideband oscillation of dc voltage in a dc microgrid. The proposed control strategy contributes to maintaining the synchronous operation of dc converter with the network. Besides, the relationships among damping, inertia, wideband oscillation, rate of change of voltage (RoCoV) as well as DC voltage nadir (DCVN) are studied. It is concluded that the RoCoV and DCVN are similar as the oscillation frequency and fluctuation ranges of poorly-damped oscillation, respectively. A unified concept is proposed by connecting the oscillation-related stability with the inertial transient response originated from the imbalanced powers/mismatched currents. Besides this, the inertia plays the same role as damping because the inertia contributes to maintaining the original state and damping to impeding further change. A new feedback analytical method is proposed to clarify the important role of RoCoV and DCVN on the motion of dc voltage. Finally, the theoretical results are compared with simulations and experiments.

Voltage of DC microgrid is prone to oscillation, originated from three factors: 1) negative damping performance of the DC converter; 2) interaction between the power converter and DC network; and 3) positive feedback of DC voltage control loop. Analogous to the relation between the force and velocity of motion, it derives the function relationship between DC current and DC voltage. Motion of DC voltage can be illustrated by the derived vectors since transfer functions between DC current and DC voltage has corresponding phase and gain at a specific frequency. It is found that it forms a positive feedback when the damping of DC converter is negative, which can destabilize DC-side voltage at the oscillated frequency. However, a negative feedback can stabilize the system and makes DC voltage attenuated. A virtual inertia (VI) control strategy is proposed for enhancement of damping performance and forming a negative feedback for the system. The proposed theoretical analysis is demonstrated by Star-Sim hardware-in-the-loop (HIL) experiments.