This paper proposes a novel unified mathematical model for the doubly-fed induction generator（DFIG） in the positive- and negative- synchronously rotating frames under generalized unbalanced grid-voltage conditions while deducing and analyzing in detail the stator output instantaneous active- and reactive-power of the DFIG under unbalanced grid-voltage conditions. On the basis of the model developed， the paper presents four kinds of selectable unbalanced control schemes for the purpose of enhancing the fault ride-through（FRT） capability of the DFIG wind power generation system under unbalanced grid-voltage conditions. The design principles of calculating the positive- and negative-sequence rotor current references for the four kinds of selectable unbalanced control schemes are thoroughly discussed. Meanwhile， the unbalanced control strategies based on dual- dq rotor current regulators are presented in the positive- and negative- synchronously rotating frames， which makes the positive- and negative- components of rotor current regulated independently and， as a result， the FRT capability of DFIG under small steady-state unbalanced grid-voltage conditions is capable of effective improvement. The correctness of the theoretical analysis and the feasibility of the unbalanced control scheme with the dual- dq rotor current regulators employed are validated by detailed simulation studies.