A high efficient medium voltage step-up DC/DC converter with zero voltage switching (ZVS) and low voltage stress for offshore wind energy systems

Conventional wind farms have a medium voltage AC (MVAC) grid that collects the power from the individual wind turbines and one or two transformer substations that facilitate high voltage AC (HVAC) power transmission. Since the output voltage of a wind turbine is too low for effective power transmission, it is stepped-up to a medium voltage level through individual transformers to facilitate short distance power transmission to the substation for further voltage boost. The AC transformers are bulky and typically located in the immediate vicinity of the turbines and in off-shore wind farms, marginally rated transformers are usually installed in the turbine´s nacelle. In order to provide efficient long distance power transmission from the wind farms, HVAC links are now being replaced by high voltage DC (HVDC) links where the bulky transformers are replaced by DC/DC step-up converters. In this paper, a new medium voltage step-up resonant DC/DC converter is proposed for high power wind turbines in offshore wind energy systems. The proposed converter features zero voltage switching and low voltage stress across the power transistors. Four strings of switch pairs are connected in series in the proposed converter so that the voltage stress across each power transistor is equal to only a quarter of the input DC voltage. The circuit´s operating principles will be discussed in this paper. Simulation results and experimental results on a proof-of-concept prototype are provided to highlight the merits of the proposed converter.