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The ride-through capability of a doubly-fed induction generator under three-phase balanced voltage sags is examined, under the condition that the machine should have the capability of real and reactive power control during the external fault. Mathematical formulae for the peak rotor fault current and the required rotor voltage output under vector control are derived. Moreover, the DC link dynamics are incorporated into the analysis and it is shown that they can have a decisive impact on fault behaviour of the machine during voltage sags. Combined, a design methodology for the rotor side voltage source inverter aiming to achieve a ride-through capability at the lowest cost is described. Simulation results in PSCAD/EMTDC show very good agreement with the theoretical analysis.