Resonant Characteristics Analysis and Suppression Strategy of Multiple Parallel Grid-connected Inverters with LCL Filter

The multiple parallel inverters have more than one resonant frequency influenced by many factors, which tends to result in stability and power quality problems. This paper develops the multiple input multiple output model of the grid-connected inverters system using the closed-loop transfer function. The influence factors of resonant characteristics are analyzed with the developed model. The analysis results show that the resonant frequency is closely related to the number, type and composition ratio of parallel inverters. To suppress resonance, a scheme based on virtual impedance is presented where the virtual impedance is only emulated in the vicinity of resonance frequency. The proposed scheme needs one inverter with virtual impedance control which reduces the design complexity of other inverter controllers. Simulations and experimental tests are carried out on two single phase converters-based setup. The results validate the correctness of the model, analytical results and resonant suppressing scheme.