Modeling and Attenuation of Common Mode Resonance Current for Improved LCL-Type Parallel Inverters in PV Plants

This article addresses the common-mode (CM) resonance problem in parallel inverters with separate dc inputs and improved LCL filters. Unlike single inverter system, parallel system presents a more challenging picture. Especially when the grid impedance and parasitic capacitance are considered, the parallel inverter interactions introduce more complex coupling relationships. In this situation, intricate CM resonances are excited at various frequencies, which is different from the resonance characteristics of a single inverter. Therefore, the control methods based on the single inverter system is insufficient in the parallel system and cannot behave as expected. To solve this issue, the CM resonance suppression problem in parallel system with separate inputs and parasitic parameters is being considered and analyzed for the first time. Firstly, the Thevenin's CM equivalent model of the parallel system is established. Compared with a single inverter model, the increase of CM loops triggers another two CM resonance peaks, which further deteriorates the system stability. Then, the internal and external CM excitations between each inverter are analyzed. The results show that the CM current of each inverter is coupled with the CM excitation sources of all inverters and the resonance characteristics are affected by the number of inverters. Furthermore, the influence of grid impedance and parasitic capacitance on the CM resonance characteristics of the system is analyzed. Accordingly, a CM resonance controller and a control parameter design method are proposed. By damping the CM excitation sources, the CM resonance suppression and CM current attenuation can be achieved simultaneously.