Extended Tolerance of Capacitance Drifting for LCL-Interfaced Energy System Converter

Abstract

Active damping has been applied to LCL-filtered converter systems to ensure a stable current control. However, the aging effect and operational conditions may lead to a significant parameter drifting of the filter capacitance. This may result in a new resonant frequency beyond the designed active damping region. To address the issue, this paper analyzes the oscillation phenomena under deep capacitance degradations with the all-pass filter-based active damping. The analysis reveals that the oscillation at the –540°-phase-crossing will be firstly excited if the capacitance degrades gradually. This feature is then employed to extend the damping region against capacitance drifting. Moreover, a Discrete Fourier Transform (DFT) unit with limited calculation points is adopted to monitor the resonance around the –540°-phase-crossing point, by doing so, DFT calculation can be simplified to light the calculation burden of the processor. Once the monitored resonance exceeds the threshold, an adaptive Proportional Integral (PI) controller will be enabled to adjust the proportional gain of the current controller to maintain the operation. Real-time Simulation tests are performed, and the results verify that the proposed method can ensure the system stability.