Research On Repetitive Control Method Applied To Grid-connected Inverter With LCL Filter

With the widespread application of the distributed power generation system, moreand more attention is being paid to the problems associated with the power quality ofgrid-connected converter. The grid-connection of inverter will be affected by many factors,especially by the periodic disturbance. In order to suppress the periodic disturbance, in thecontrol link, with a delay time going through the feedforward as the delay time of theperiod of power grid base wave, the repetitive control model is constituted by adopting therepetitive control technology based on the internal model principle. Thus, the static errorfree tracking for the fundamental wave signal of grid current can be realized. In practice,however, the delay time in the digital control must only be an integer multiple of thesampling period. Once the actual grid frequency is changed, the ratio of the samplingfrequency of system to the frequency of grid could not be maintained to an integer and theperformance of steady-state tracking and harmonic suppression capability of the systemwould be significantly reduced for the repetitive control technology.An modified repetitive control system with the parameters of the finite impulseresponse (FIR) filter as the internal model parameter, and a new design method for FIRfilter parameter are proposed to overcome the drawbacks of the conventional repetitivecontrol technology. As the designed FIR filter has both linear phase frequency andlowpass filtering characteristics in the medium and low frequency bands, it can simulatethe non-integer delay links. Therefore, the resonant frequency of the FIR filter can alwaysbe consistent with that of the fundamental and harmonic waves, and thus the modifiedrepetitive control system will have nearly ideal repetitive control effect at the medium andlow frequency bands. Based on the above theoretical analysis, simulations studies arecarried out by using the MATLAB software to verify the effectiveness of the modifiedrepetitive control method.Finally, an experimental platform of three phase grid-connected inverter (VSI) withLCL filter, which is controlled by DSP, is set up for verifying the modified repetitivecontrol technique proposed in this paper. The experimental results show that the system not only can effectively reduce the steady state tracking error and eliminate the harmonicwave of the grid-connected current under the balanced or the imbalanced systems of three-phase, but also can maintain the control system to keep the ideal control effect under theconditions of varied frequencies. As a result, it makes up the defect of insufficient adaptivecapacity of the conventional repetitive control systems.