Impact of Virtual Flux reference frame orientation on voltage source inverters in weak grids

This paper investigates how different approaches to grid synchronization and reference frame orientation can improve the control performance of a voltage source inverter operating in a weak grid. It is first shown how voltage-sensor-less operation based on Virtual Flux allows for flexibility in how to choose the reference frame orientation of the control system. The different possible orientations are then explained, and the potential improvement in stability and performance of the control system is discussed. If is further explained how the choice of reference frame orientation can be used to select a point in the grid seen from the converter, where reactive power flow or power factor can be explicitly controlled. The dynamic response and stability limits of an inverter operating in a weak grid have been investigated by simulations for verifying and illustrating the influence of different reference frame orientations.