Research on ADRC Plus with Repetitive Control Strategy for Three-phase Four-Leg Grid-connected Inverters

In grid-connected inverters applications, numerous non-ideal factors, such as disturbances and uncertainties, affect the performance of the control system, which can easily lead to harmonic increase fed into grid. To mitigate disturbances caused by the unmodeled portions and non-ideal characteristics of the inverter, a current control strategy based on active disturbance rejection control (ADRC) plus with repetitive control (RC) is proposed for LCL-type three-phase four-leg grid-connected inverters. To simplify the design process, the filter model is reduced from a third-order to a first-order. ADRC transforms the control structure of the inverter into the standard form of a series connection of integrators, and the portions deviating from the standard type are considered as total disturbances. A discrete extended state observer (ESO) is established to estimate the total disturbances in real time. In addition, a repetitive controller is employed to further reduce steady-state error, so that the current harmonics injected into grid can be further decreased. Finally, a 10kW three-phase four-leg Inverter model is built in Matlab, and the simulation results demonstrate that the adoption of ADRC plus with RC can significantly reduce the total harmonic distortion (THD)of the current fed into the grid, achieve zero steady-state error tracking, attain improved dynamic performance, and have good robustness.