Passivity-Based Design of Plug-and-Play Current-Controlled Grid-Connected Inverters

Stable operation of LCL filtered grid-connected inverters can be achieved using active damping. However, the stability can be threatened by non-ideal conditions such as delay in digitally controlled systems and grid impedance variation at the point of common coupling (PCC). In a grid-side current controlled inverter, the computational and pulsewidth modulation delays cause an unintentionally negative virtual resistance when the resonance frequency is higher than one-sixth of the sampling frequency <tex-math notation="LaTeX">(fₛ/ 6)</tex-math> . This paper proposes a delay compensation method to address this issue, which can expand the effective damping region up to Nyquist frequency <tex-math notation="LaTeX">(fₛ/ 2)</tex-math> . Also, it is shown that the negative real part of the inverter output admittance will make the system unstable under specific grid condition. Therefore, a PCC voltage feedforward method is proposed to cancel the negative real part of the inverter output admittance according to passivity-based stability. Thanks to the proposed methods, the inverter output admittance will be passive in all frequencies and can be connected to the grid regardless of grid impedance. It means that the inverter has robust plug-and-play functionality. The validity of the theoretical analysis and the effectiveness of the proposed approaches are verified using experimental results on a laboratory prototype.