Protected Control Method for Power Conversion Interface Under Unbalanced Operating Conditions in AC/DC Hybrid Distributed Grid

The voltage source converters (VSC) are the key power interfaces between the individual ac grids and dc grids in the hybrid power system. When the ac grid is unbalanced, the grid currents are distorted and the harmonic is induced. Meanwhile, the oscillations in the active and reactive power are also increased, which may damage these interfaces. In order to ensure the safety of the converters and the grid facility, a flexible control strategy is proposed for the ac/dc hybrid grid in this paper. At first, the current harmonic for the VSC is theoretically analyzed under unbalanced conditions. Furthermore, the methods to separate the positive- and negative-sequence components of the ac grid voltages are summed up and analyzed. Based on the theoretical analysis, different protected control methods are compared. A new control method, which can flexibly control oscillations of the active and reactive power by introducing a parameter k, has been proposed. The estimation of maximum current in each phase is discussed; meanwhile, the relationship between the voltage ripple on dc link and the parameter k is given. Finally, the validity and advantages of the proposed method are verified by the simulating and experimental results.