Hybrid Active Damping for LCL-Filtered Inverters Under Weak-Grid Conditions Using Dual-Current Feedback and Capacitor-Voltage Feedforward

Variations in grid inductance shift the resonance characteristics of inductor-capacitor-inductor (LCL) filters and can reduce the stability margins of digitally controlled grid-connected inverters. This paper proposes a hybrid active-damping strategy that combines grid-current feedback for current regulation, inverter-current feedback for active damping, and capacitor-voltage feedforward to modify the closed-loop dynamics. An open-loop model incorporating the LCL network, equivalent grid inductance, and digital control delay is derived for small-signal stability analysis. Resonance damping is evaluated through frequency-domain analysis and comparative simulations with grid-current-only control. Additional offline simulations examine current-reference tracking and operation under background grid-voltage harmonics, while real-time hardware-in-the-loop (HIL) tests evaluate steady-state current regulation and harmonic performance. Frequency-domain analysis shows suppressed resonance-related gain amplification and positive phase margins at the three evaluated grid-inductance values. Reference tracking is demonstrated in offline simulations, which also show maintained current regulation under the specified grid-voltage harmonics. In the steady-state HIL tests, three-phase grid currents remain bounded and periodic without pronounced resonance-induced oscillations. Across the three HIL cases, individual harmonics from the second to the fortieth order remain below 1% of the fundamental component, and grid-current total harmonic distortion remains below 1.2%. These findings demonstrate effective resonance damping and grid-current regulation under the investigated grid-inductance conditions.