  Control structures have critical influences on converter-interfaced
distributed generations (DG) under unbalanced conditions. Most of previous
works focus on suppressing active power oscillations and ripples of DC bus
voltage. In this paper, the relationship between amplitudes of the active power
oscillations and the reactive power oscillations are firstly deduced and the
hierarchical control of DG is proposed to reduce power oscillations. The
hierarchical control consists of primary and secondary levels. Current
references are generated in primary control level and the active power
oscillations can be suppressed by a dual current controller. Secondary control
reduces the active power and reactive power oscillations simultaneously by
optimal model aiming for minimum amplitudes of oscillations. Simulation results
show that the proposed secondary control with less injecting negative-sequence
current than traditional control methods can effectively limit both active
power and reactive power oscillations.
