High frequency wind energy conversion system for offshore DC collection grid — Part II: Efficiency improvements

This is the second part of a two-part paper dealing with a high frequency wind energy conversion system for series-connected DC wind turbines. In part I, the efficiency of the AC–AC converter and high frequency transformer was assessed and compared for three different converter topologies: the matrix converter, indirect matrix converter and back-to-back converter. The conversion system with matrix converter presented the highest efficiency, however the three converters had comparable efficiencies. In this part II paper, improvements are presented in order to increase the operating frequency of the transformer without deteriorating the efficiency of the system. The loss analysis methods presented in part I are used to evaluate the benefits of the suggested modifications. Two modulations adapted for the considered high frequency conversion system are explained which keep the efficiency high while allowing for a significant increase in the transformer operating frequency. Using the most efficient modulation, the comparison of the three AC–AC converters is extended to wind speeds below nominal. The results show that the gap between the efficiency of the matrix-type converter and the back-to-back increases when the wind speed decreases. A multi-winding transformer design is suggested in order to reduce the volume and losses.