Output Admittance Passivation for Grid-Side Current Controlled LCL-Type Inverters using Capacitor-Voltage Feedforward

The passivity theory offers an effective way to assess the stability of grid-connected inverters. The system stability is generally ensured if critical grid resonances fall within regions where the output admittance of the inverter acts passively. In this paper, a simple capacitor-voltage feedforward scheme is proposed to reshape the output admittance of the grid-side current-controlled LCL-type grid-connected inverter. With the proposed method, the output admittance is turned passive for all frequencies from 0 Hz up to Nyquist frequency, which can not only ensure the stable operation of the inverter in presence of either inductive or capacitive grid but also avoid the design constraints of an LCL filter, i.e., f(r) > f(s)/6. Finally, experiments are performed to verify the theoretical expectations.