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A discrete feedforward integral variable structure control or DFIVSC for the uninterruptible power system (UPS) are presented, which is tracking a sinusoidal AC voltage with specified frequency and amplitude. The DFIVSC algorithm combines feedforward strategy with the conventional variable structure control. The control function is derived to guarantee the existence of a sliding mode. The experimental results illustrate that the proposed approach gives a significant improvement on the tracking performances when compared with some existing control methods, like the IVSC and MIVSC strategies. Moreover, it can achieve the requirements of robustness and can supply a high-quality voltage power source in the presence of plant parameter variations, external load disturbances and nonlinear dynamic interactions.