Power Balancing Investigation of Grid-Side Series-Connected Current Source Inverters in Wind Conversion Systems

In current source inverter (CSI)-based series-connected configurations, recently proposed for offshore wind energy conversion systems, a number of medium-voltage CSIs are connected in series to form a centralized dc/ac inverter to transfer offshore wind power to the grid. In previous studies, the series-connected CSIs are controlled based on assumptions that all CSIs are ideal and identical and the power is evenly distributed among them. However, in practice, such assumptions are invalid as tolerance exists. This study therefore works on the investigation of power distribution among series-connected CSIs considering the tolerance. First, the possible imbalance of power among CSIs is unveiled, analyzed, and quantitatively defined. Second, a combined control scheme is proposed, based on which both conventional control objectives, which are dc-link current control and power factor control, and an equal power distribution among CSIs are ensured simultaneously. Finally, simulations and lab-scaled experiments are provided.