This paper presents an active disturbance rejection control strategy for LLCL power conversion systems based on repeated resonance extended state observer (RRESO) to ensure the stability of the system under various disturbances (such as grid background harmonics, grid voltage unbalance, filter parameter drift, etc.). The control strategy is mainly composed of repeated resonance extended state observer, pre-filter and PR control. The ability of the observer to observe the disturbance is enhanced by repeating the control link, and the observed disturbance is compensated in the feedforward link of the control signal. The pre-filter is used to extract the positive sequence component information of the grid voltage without harmonic pollution in the disturbance observations. PR control ensures the system's sinusoidal reference tracking capability. This control approach guarantees stable operation of the system with savings on voltage sensors and reduced system cost. A simulation using Matlab/Simulink verifies the control method, and the results demonstrate the accuracy of the theoretical analysis and feasibility of the control approach presented in this paper.