Wide Damping Region for LCL-Type Grid-Connected Inverter with an Improved Capacitor-Current-Feedback Method
This project has presented a stability analysis of a LCL type grid-connected inverter in the discrete-time domain. It has been found that even though the system is stable when the resonance frequency fr is higher than one-sixth of the sampling frequency (fs/6), an effective damping scheme is still required due to the potential influence of the grid impedance. With a conventional proportional capacitor-current-feedback active damping (AD), the valid damping region is only up to fs/6. This however is not sufficient in the design process for obtaining a high quality output current and the system can easily become unstable due to the resonance frequency shifting. Considering the resonance frequency design rules of the LCL filter, this project proposes an improved capacitor-current-feedback AD method.
The grid-connected power generation unit normally has a pulse-width-modulation (PWM) controlled voltage source inverter, which can inject regulated active and reactive power to the grid. In order to reduce the harmonics of the inverter output current, output filters are often required. The LCL-type filter is increasingly adopted due to its better attenuation of the switching harmonics, especially in high-power systems with lower switching frequencies.
In order to ensure robustness against grid impedance variation, an improved capacitor-current-feedback AD is proposed. The influence of damping coefficient R on damping performance is studied. Unlike the passive damping, due to the calculation delay and ZOH, the optimal damping is obtained at the actual resonance frequency. Further, an approximate calculation for the optimal R is also given. Theoretical analysis and experimental results have verified the effectiveness of the proposed method.
TOOLS AND SOFTWARE USED:
- MP LAB
Xiaoqiang Li, Xiaojie Wu, Yiwen Geng, Xibo Yuan, Member, IEEE, Chenyang Xia, and Xue Zhang, “Wide Damping Region for LCL-Type Grid-Connected Inverter With an Improved Capacitor-Current-Feedback Method”, IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 30, NO. 9, SEPTEMBER 2015.