Analysis and Dynamic Performance Improvement of Grid-Connected Voltage–Source Converters Under Unbalanced Network Conditions

The energy sector is moving toward an extensive utilization of distributed and renewable energy resources. Such resources are usually interfaced to power grids via voltage-source converters (VSCs). Due to the increased penetration level of VSC-interfaced resources, the utilization of interfacing VSCs to support host grids under unbalanced conditions (e.g., due to grid voltage unbalance, unbalanced load conditions and unsymmetrical faults) becomes essential. However, detailed dynamic analysis and systematic design procedure to enhance the dynamic performance of grid-connected VSCs equipped with grid-support controllers are not reported in the literature. To fill in this gap, this paper presents a detailed small-signal model and analysis of the dynamics of a grid-connected VSC equipped with the recently developed balanced positive-sequence control and positive/negative-sequence control methods to support the grid under unbalanced conditions. The effects of the short-circuit ratio, angle of the ac system impedance, and phase-locked-loop parameters on the transient behavior of the VSC are thoroughly studied and characterized. Furthermore, to improve the dynamic performance of grid-connected VSCs, a simple yet effective current-control-based compensator is developed to mitigate possible instabilities associated with the low-voltage operation. Comparative simulation and experimental results validate the theoretical analysis and the effectiveness of the proposed compensation scheme.