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This paper investigates the control of doubly-fed induction generator (DFIG) based wind farms for compensating voltage unbalance in weak networks. A DFIG system model containing the generator and its back-to-back converters suitable for analyzing system operation under unbalanced conditions is developed. A control strategy for compensating grid voltage unbalance using DFIG systems is proposed. The negative sequence current injected into the transmission line for the rebalancing control can be provided by either the grid-side or the rotor-side converter. Various methods for coordinating these two converters and their impact on system operation are analyzed. The capabilities of the converters with a DFIG system for negative sequence current compensation are also discussed. The validity of the proposed control strategy is demonstrated by Matlab/Simulink simulations. With the proposed strategy, DFIG based wind farms can provide voltage unbalance compensation for the connected weak grid to improve the performance and stability of the whole wind energy system.