This paper proposes a modified uncertainty and disturbance estimator (UDE) structure, aimed to improve the ability to approximate and eliminate signals characterized by periodic behavior. The algorithm combines delay action with low-pass filtering, introducing high loop gain magnitude values at multiple frequencies of interest while retaining implementation feasibility by imposing finite crossover frequency, similarly to repetitive control. For a given control bandwidth, the modified UDE is shown to outperform the classical one in terms of disturbance rejection at the expense of slight phase margin loss. Simulations and experiments are given to support the feasibility of the proposed control structure, applying the revealed modification to the current control of an LCL inverter, connected to a distorted ac grid.