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For doubly-fed induction generator-based wind energy conversion systems (DFIG-based WECSs), the output voltage of rotor-side converter (RSC) has to be lower than the de-link voltage due to its buck circuit topology. For this reason, under severe grid faults that cause large electromotive forces in the generator, the short circuit current can not be suppressed effectively. To evaluate the control limit of the RSC, this paper proposes an optimal control-based approach that takes this circuit constraint into account and produces quantitative results. Firstly, the low voltage ride-through problem is reformulated as a rotor current suppression optimal control problem with constrained controls. Then the Pontryagin's minimum principle is applied to find the optimal control. Using the proposed approach, the control limit of the RSC, which is represented by a collection of maximum short circuit current surfaces, is thoroughly evaluated over the operation area. The obtained results could be helpful for manufacturers to assess and improve existing controller and converter designs on a quantitative basis.