A Robust Method for Controlling Grid-Connected Inverters in Weak Grids

Stable operation of grid-connected converters with the LCL filter is essential. Variations of the grid impedance and consequently, variations of the resonance frequency, can push the system toward instability. Also, delay that comes from sampling and PWM, could worsen the stability condition. Thus, the control system should be designed by considering these non-ideal factors. To fill in this gap, this brief presents an effective but simple method based on biquad filter, which enhances the system robustness and guarantees the system stability in a varying grid impedance condition. Besides, the suggested technique is free from employing any active damping mechanism, which in turn, increases the reliability and reduces the cost. Finally, experimental results are provided to show the accuracy of the analysis and verify the proposed method.