This paper investigates the low-frequency resonance issue of multi-parallel grid-connected inverters from the perspective of admittance. Due to the effect of software phase-locked-loop (SPLL), the output admittance of grid-connected inverters with an LCL filter contains the negative damping at the low-frequency region. And with the number of parallel inverters increasing, the resonant frequency of the impedance network composed of grid-connected inverters and inductive grid impedance is decreased. When located at the low-frequency region, the negative damping of the output admittance will easily lead to system instability. In order to eliminate the negative damping and damp the resonance, the feed-forward of grid voltage is introduced to the current command value. Furthermore, the minimum theoretical value of feed-forward compensation coefficient is derived on the basis of minimum effects on other frequency characteristic of the output admittance. Finally, the experimental results verify the correctness of theory analysis and the effectiveness of negative damping compensation method.
