Design of SSUDE Nominal Model for Stability Improvement of Grid-Tied Inverters

The uncertainty and disturbance estimator (UDE) with a time delay filter has been widely used for LCL grid-tied inverter systems for its satisfactory harmonic rejection performance. However, traditional separate-structure UDE (SSUDE) is often designed with a first-order nominal model, which is significantly different from the actual third-order system, thereby reducing system stability. Thus, this article proposes an advanced SSUDE control strategy based on a nominal model design. By designing the nominal model into a third-order model closer to the actual inverter system, the proposed approach can achieve a broader range of selectable filter bandwidths. Then, the system stability criterion is proposed based on the small-gain theorem, and the stability is analyzed through this criterion. The stability analysis results indicate that it is easier to obtain a range of stable parameters for the proposed strategy under critical conditions. Finally, the proposed strategy is validated through simulations and experiments.