Synchronverter (SV) control or also known as virtual synchronous generator (VSG) control has emerged as a popular method for distributed energy resources (DERs), to emulate response of a synchronous generator. However, the injected power by DERs into an unbalanced grid has high double-frequency ripples for a standard SV control. Thus, the enhanced-PLL (EPLL) structure, consisting of positive, negative and zero sequence control units, is integrated to standard SV in the proposed control, which results in ripple free power injection by DER into an unbalanced grid. Further, the proposed control is capable of successful pre-synchronization, i.e., synchronization of DER voltage to grid voltage, before connecting DER to the grid. Other significant benefits of the proposed control over existing SV based methods for unbalanced grid are integrating EPLL with SV yields a simple control structure, due to both being set in a natural frame and reduced number of parameters due to similarity between different sequence control units. Simulation and experimental results are presented to validate the pre-synchronization and ripple free power injection by DER into an unbalanced grid for the proposed control.
In case of grid connected DERs, grid voltage variation within normal range causes undesired transients in the DER current and earlier works suggest the incorporation of virtual impedance or virtual inductance with the SV scheme. However, this method has drawbacks of partial current transient reduction and adverse impact of high virtual impedance on system stability. As the reactive power loop (RPL) of SV scheme dictates the transient performance during grid voltage variations, an improvement to the RPL is also presented.