<p>A quasi-resonant-integral compensation ADRC is applied in electrolytic capacitor-less (ECL) drive systems to suppress DC bus voltage fluctuations and enhance the dynamic performance of ECL drive systems. A quasi-proportional resonant filter is incorporated into the disturbance observation channel of the extended state observer for DC bus voltage, minimizing the second-order harmonics in the estimated DC bus voltage disturbance. Consequently, the second-order harmonics of the grid reference current are effectively diminished, which not only minimizes the voltage fluctuation of the DC bus but also improves the power quality of the grid. Furthermore, the perturbation integral term is expanded into a new state variable, and the observer gain is reconstructed to compensate for the accumulated perturbation variations into the total perturbation of the DC bus voltage. This mechanism results in a high perturbation estimation accuracy of the DC bus voltage with low bandwidth gain. Thus, the ECL drive system exhibits good dynamic performance under dynamic operating conditions. Experimental results validate the effectiveness of the proposed control strategy.</p>