A Simple Method for Passivity Enhancement of Current Controlled Grid-Connected Inverters

Grid-connected inverters with LCL filter work stable using active damping methods. However, the stability of the control system can be threatened easily by grid impedance variation. On the other hand, delay in digitally-controlled systems shrinks the stable region. Therefore, it is very probable that by variation of grid impedance, resonance frequency located in the unstable region. To address this problem, a simple but effective method based on converter-side current feedback is proposed in this letter. This method does not need any extra sensors for active damping, which increases reliability and reduces the cost. The proposed method expands the stable region and guarantees the stability of the inverter against the grid impedance variation. It is shown that the proposed method is effective in the case of multiparalleled inverters whiles normally coupling effect among inverters makes the stability condition worse. The effectiveness of the proposed method is verified through experimental results.