This paper proposes a design of the three-parallel converter system and grid-connection technique for wind turbine systems using a PMSG. The back-to-back converter of the wind turbine system is directly connected to the grid so that both the power devices and the filters are needed to have large power ratings. The three-parallel converter configuration and LCL-filter can reduce the required power ratings of the devices and filters. However, the three-parallel converter can cause circulating currents. These circulating currents can be suppressed by sellecting proper inner inductance at each leg. Second, grid connection techniques are proposed. The resonance caused by the LCL filter is compensated by an active damping method using power theory without additional power loss. The decline of the power quality caused by the unbalanced and distorted grid voltages is also compensated with an additional compensation algorithm. The simulation and experimental results show that the proposed system and compensation methods are effective for the wind turbine systems.
