High-power, resonant DC/DC converter for integration of renewable sources

This paper studies practical design aspects of a high-power DC-DC converter which might be employed with renewable power sources like large offshore wind farms. The component selection and losses are studied for a 6.8 kV/100 kV, 5 MW step-up DC/DC converter. It is concluded that inverter-grade thyristors offer overall advantages because of ability to operate at high switching frequencies. The converter operates at all zero current switchings and therefore an increase in the operating frequency has minimal effect on the efficiency. The detailed converter modelling at switch level indicates that the converter efficiency will be around 96-97%. Better efficiencies are possible with phase-control thyristors, but this would come at the expense of significant increase in the size and weight of the passive components. The dimensions and weight of the crucial passive components are calculated and the largest air-core inductor may weigh around 610 kg. The converter responses are simulated for reference step changes and the detailed steady-state curves are analysed. The converter is further tested for the worst case fault conditions. The simulations indicate that the converter will not propagate fault currents, if the converter components are adequately selected.