An Integrated Control of Enhanced-PLL and Synchronverter for Unbalanced Grid

Abstract

Synchronverter (SV) control for distributed energy resources (DERs) is popular due to its capability of synchronous generator behavior emulation. Under an unbalanced grid, additional control strategy along with a higher order phase-locked loop (PLL) is generally integrated to SV control, for the injected power to be free from double-frequency ripples. Thus, the enhanced-PLL (EPLL) structure comprising positive, negative, and zero sequence control units is integrated to the 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 presynchronizing 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 as follows: a) Integrating EPLL with SV yields a simple control structure, due to both being set in a natural frame; b) reduced number of parameters due to similarity between different sequence control units; c) simple implementation by using the low-pass filter of SV in the positive sequence control unit. Simulation and experimental results are presented to validate the presynchronization and ripple-free power injection by DER into an unbalanced grid for the proposed control.