Internal Model-Based Disturbance Observer With Application to CVCF PWM Inverter

In constant-voltage constant-frequency (CV CF) pulsewidth modulation (PWM) dc-ac inverters, the lumped disturbance, including the parameter uncertainties, unmodeled dynamics, and nonlinear loads, will deteriorate the performance and cause high harmonic distortions, or even result in instability. Conventional disturbance observer (CDOB), in which a low-pass Q filter is adopted to estimate the lumped disturbance, cannot completely eliminate the harmonics. While the conventional repetitive control (CRC) can attenuate the harmonics by using the error signal of previous period(s), that may result in slow transient response due to nonperiodic components in the tracking error during the transient state. In this paper, an internal model-based disturbance observer (IM-DOB) is proposed and has been applied in a CVCF PWM inverter to improve the control performance. In IM-DOB, a novel IM-based disturbance estimation observer is proposed to estimate the disturbance both in the low-frequency region and at harmonic frequencies, while compensation filters are introduced to enhance the robustness and stability. The IM-DOB scheme, which fuses the merit of CDOB and CRC, is actually a two-degree-of-freedom scheme. The stability analysis and a design case are also given. Comparative experiments are conducted to demonstrate the effectiveness of the proposed scheme.