The paper suggests a method aimed to improve the performance of dynamical systems with periodic uncertainties and disturbances (AC power converters are typical representatives of such systems) utilizing frequency domain disturbance observer (DOB) based control structure. In such control arrangement, DOB filter is the key component and should be designed so that its magnitude (phase) is as close to unity (zero) as possible at frequencies where the total uncertainty and disturbance exists. The methodology for designing DOB filters is well established mostly for systems with slow-varying uncertainties/disturbances and has been barely extended to systems with periodic excitation (such as grid-connected or stand-alone AC power converters). In this work, systematic enhancement of DOB filters designed according to the classical methodology is suggested based on appropriate mixing with series multi-resonant term. The proposed approach allows attaining improved periodic signals rejection capability while significantly sacrificing neither DC gain nor crossover frequency. Quantitative design guidelines are provided and possible trade-offs are revealed. The proposed methodology is validated by application to current control of LCL-filter based inverter connected to a distorted AC grid. Feasibility and performance of the suggested approach are well-supported by simulations and experiments.

In this brief, extension of known rules for designing optimized single-resonant ac current regulators to multiresonant current regulators parameter selection for single-phase grid-connected photovoltaic inverters is proposed. The proposed generalization is made possible by utilizing multiplied rather than summed resonant terms within the controller. A set of phase-margin oriented design guidelines is established, explicitly defining each controller parameter to corresponding figure of merit. Simulations and matching experimental results are shown to verify the proposed approach.

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.

The study suggests a method aimed to improve the performance of practical grid‐connected converters connected to distorted mains, utilising total uncertainty and disturbance estimator (TUDE) based control structures with time‐delay filter. Such a TUDE filter enhances disturbance rejection performance by possessing near‐unity magnitude and near‐zero phase at integer multiples of the base frequency, imposing repetitive‐control‐alike behaviour. However, it is well‐known that in repetitive control, disturbance rejection at mid‐frequencies, were non‐periodic disturbances and noise may exist, is deteriorated. This study presents an enhancement, allowing to retain good disturbance rejection at mid‐frequencies within the low‐frequency portion of control bandwidth (relevant in practical systems) while trading‐off the performance in the high‐frequency control bandwidth part. The proposed methodology is verified by application to output current control of LCL‐filter based inverter feeding distorted AC mains. The validity and performance of the proposed methodology are well‐supported by simulations and experiments.

This Letter generalises frequency‐domain oriented design guidelines for optimised single‐resonant three‐parameter AC current regulators for grid‐connected converters. Unlike in previously established set of design guidelines, possible practical restrictions imposed by relatively low‐switching frequency are taken into account in the proposed methodology. In addition, resonant term bandwidth is selected according to the desired robustness to fundamental frequency variations. As a result, a closed set of three non‐linear equations is created. The solution maximises the loop gain crossover frequency for a given phase margin while respecting frequency robustness and resonant gain restrictions, set a priori.