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Stability Enhancement of Inverters in Grid-Connected Microgrids Using FIR Filter

Under weak grid conditions and in microgrids, the gird impedance is often large and has a highly varying nature. In such a scenario, stabilizing grid-connected inverters with an LCL output filter could be a challenging task. The control delay that comes from sampling and PWM may contribute to worsen this condition. In particular, the mentioned delay causes a critical frequency at one-sixth of the sampling frequency (fs/6), which may jeopardize the converter's stability in the case of grid impedance variations. Also, the coupling effect among parallel converters, which is a likely scenario in microgrid applications, is another factor that may threaten the system stability. To deal with these challenges, this article proposes an LCL filter resonance damping method that employs a finite-impulse response filter to amend the delay effect. In this way, the system robustness is enhanced and the system keeps its stability irrespective of grid impedance variations and interaction between inverters. Experimental results show the correctness of the theoretical conclusions and confirm the efficiency of the suggested technique.