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A Virtual RLC Active Damping Method for LCLType GridConnected Inverters
Yiwen Geng, Yawen Qi, Pengfei Zheng, Fei Guo, Xiang Gao 
Abstract 
Proportional capacitor?current?feedback active damping (AD) is a common damping method for the resonance of LCLtype gridconnected inverters. Proportional capacitor?current?feedback AD behaves as a virtual resistor in parallel with the capacitor. However, the existence of delay in the actual control system causes impedance in the virtual resistor. Impedance is manifested as negative resistance when the resonance frequency exceeds onesixth of the sampling frequency (fs/6). As a result, the damping effect disappears. To extend the system damping region, this study proposes a virtual resistor?inductor?capacitor (RLC) AD method. The method is implemented by feeding the filter capacitor current passing through a bandpass filter, which functions as a virtual RLC in parallel with the filter capacitor to achieve positive resistance in a wide resonance frequency range. A combination of Nyquist theory and system closeloop polezero diagrams is used for damping parameter design to obtain optimal damping parameters. An experiment is performed with a 10 kW gridconnected inverter. The effectiveness of the proposed AD method and the system¡¯s robustness against grid impedance variation are demonstrated. 
Keyword 
LCL filter, active damping, system delay, virtual RLC, grid impedance 
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