Flexible reference frame orientation of Virtual Flux-based Dual Frame Current controllers for operation in weak grids

This paper discusses voltage sensor-less grid synchronization and control of three-phase Voltage Source Converters under unbalanced conditions. First, it will be shown how the concept of “Virtual Flux” can be used for grid synchronization in unbalanced conditions. For this purpose, a method for separate estimation of positive and negative sequence Virtual Flux components will be reviewed and analyzed in combination with Dual Frame Current Controllers for separate control of Positive and Negative Sequence current components. Then, it will be shown how the presented approach allows for flexibility in choosing the point of grid synchronization for the Virtual Flux estimation, and that this flexibility can be utilized to improve the operation and stability of the converter in power systems with high impedance. Simulation results are presented to show the influence on the dynamic response and on the limits for stable operation of the converter when different points of reference frame orientation are selected.