Pre-synchronization Control Method of Virtual Synchronous Generator with Alterable Inertia

Virtual synchronous generator (VSG) is considered to be a promising grid-connect method for distributed generators (DGs) due to its inertial response and grid support ability. Just the same as conventional synchronous generators, the VSG should be regulated in synchronism with the point of common coupling (PCC) before grid-connection to avoid inrush current. In this paper, a practical pre-synchronization control method for the VSG in microgrid is proposed. The proposed method gets the voltage information of the PCC with a phase locked loop (PLL), and then regulates the VSG output voltage in accordance with the PCC voltage in amplitude, frequency and phase position by regulating the corresponding reference values and the feedback of phase-difference between the VSG and the PCC. Besides, an alterable inertia is introduced in the synchronization process of the VSG after it connects to the grid to reduce the power and frequency oscillations in the case when offset exists. Simulation results proved the proposed method to be feasible and effective.