Frequency and Voltage Stability Analysis of Grid-forming Virtual Synchronous Generator Attached to Weak Grid

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.