A grid-connected inverter is indispensable for photovoltaic power generation and smart grid systems, and it must be designed for stable operation. The impedance method based on the Nyquist criterion is often utilized to analyze the stability of grid-connected inverter systems. The impedance method is based on the eigen values of the product of the inverter output admittance and the line impedance matrices in the frequency domain. However, the frequency characteristics have so far been derived only for inverters with analog control systems. A new frequency analysis method for inverters with digital control systems is proposed in this paper. First, a stability analysis example for a three-phase LCL-type inverter controlled digitally is shown and the results are compared and validated with those by simulation using a Saber simulator. Finally, they are also compared and validated with experimental results digitally controlled by a DSP-based system.
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