Stability Improvement of Converter-side Current Controlled Grid-Connected Inverters

Grid-connected inverters with LCL filter work stable using active damping methods. However, stability can be threatened easily by grid impedance variation. Particularly, delay in digitally controlled systems has a negative effect on the stable region of grid-connected inverters and shrinks this region. Therefore, by variation of grid impedance, it is very probable that resonance frequency located in the unstable region. To address this problem, a delay compensation method based on converter-side current feedback is proposed in this paper, which tackles the negative impact of the delay. The proposed method expands the stable region using passivity-based stability and guarantees the stable operation of inverter against grid impedance variation. Also, it does not need any extra sensors for active damping, which reduces the cost and increases the reliability. It is shown that the proposed method is effective in case of multi-paralleled inverters whiles normally coupling effect among inverters makes the stability condition worse. The effectiveness of the proposed method is verified through various case studies.