Stability and resonance analysis and improved design of N-paralleled grid-connected PV inverters coupled due to grid impedance

Abstract

Paralleled grid-connected PV inverters with LCL filter are coupled due to the grid impedance. The inverters may arise harmonic resonance and even be unstable because of the variety of grid impedance. In this paper, the linearized control model and the equivalent impedance model of N-paralleled grid-connected PV inverters have been established, through which the relationship between system stability and harmonic resonance has been analyzed in detail. When the inverters are in critical stable state, there will be series resonance due to the impedance matching, and the impedance matching frequency is the system cut-off frequency. In order to reduce the harmonic in the grid side current and guarantee the inverter always with enough stability margin, a “strong” grid-connected inverter has been proposed, and the step-by-step design procedures have also been presented. Finally, the experiment have verified that the proposed inverter can always output high quality current, and has high adaptability to the grid impedance.