Capacitor Current Differential Feedback Active Damping to Enhance Robustness of LCL-Type Inverters under Weak Grid Conditions

초록

Capacitor Current Feedback (CCF)-Active Damping (AD) is widely adopted due to its effective attenuation of LCL filter resonance and its straightforward implementation. However, CCF-AD suffers from potential instability issues arising from digital control delays. Specifically, CCF-AD forms a frequency-dependent virtual impedance, and digital control delays can cause the virtual resistance to become negative in frequency ranges above one-sixth of the sampling frequency (fs/6). These negative virtual resistances can introduce unstable poles into the open-loop transfer function, leading to Non Minimum Phase (NMP) behavior in the control loop. NMP systems require strict stability conditions, which are difficult to satisfy under weak grid conditions where the grid inductance can vary significantly. As a result, the control loop can easily become unstable under variations in grid inductance. To address this issue, the Positive Virtual Resistance Region (PVRR) must be extended. This paper proposes extending the PVRR up to one third of the sampling frequency (fs/3) by inserting a differentiator into the capacitor current feedback path. A detailed mathematical analysis of the proposed method is presented in the z-domain, and the theoretical results are validated through experimental verification.

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