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Harmonic Voltage Distortion Damping Method for Parallel-Connected LCL-Type Inverters in Islanded Operation

For parallel-connected LCL-type inverters in islanded operation, it is difficult to reshape the large harmonic impedance by the conventional capacitor voltage control due to the presence of grid-side inductor, which seriously deteriorates the voltage quality at the point of common coupling (PCC) under nonlinear loads. To address this issue, a harmonic voltage distortion damping method is proposed in this paper, which mainly includes a direct output voltage control and the parallel virtual-admittance control. First, the direct output voltage control converts the grid-side inductor into the equivalent output impedance of the inverter itself by changing the voltage feedback variable from capacitor voltage to output voltage, and then the harmonic impedance is reshaped from a large value to a small value with the P + Repetitive controller. However, it is found that the direct output voltage control fails to damp PCC voltage distortion with droop-frequency offset. That is because its voltage controller cannot guarantee the resonance bandwidth of main harmonic frequency, and then a slight droop-frequency offset can cause a large increase in the harmonic impedance. For this, the parallel virtual-admittance control is further proposed to limit the size of the output impedance to no more than that of the impedance in parallel, which effectively reduces the adverse effect of droop-frequency offset on the quality of the PCC voltage. Further, the parallel virtual-admittance control is simplified with the same harmonic voltage distortion damping consequent. The system stability conditions are analyzed in detail, and its proper control parameters are selected without complicated trial. Finally, the experimental results validate the proposed control method.