Capacitor-current-feedback active damping has been widely used in LCL-type grid-connected inverters. However, the damping performance is deteriorated due to the negative equivalent resistance resulted by the digital control delays, and thus the grid-connected inverter is apt to be unstable under the grid impedance variation. To address this issue, this paper proposes the capacitor-current proportional-integral (PI) positive feedback active damping method, which can ensure a positive equivalent resistance almost within the Nyquist frequency, i.e., the full controllable frequency range. In theory, the proposed damping method can be implemented by feeding back the capacitor current through a PI function. However, the integral term will continuously accumulate the noise and dc bias arising from the feedback signal. To overcome this drawback, a more practical implementation solution is drawn in this paper. Furthermore, a straightforward design procedure is presented for the convenience of selecting the proper controller parameters. With the proposed damping method and its practical implementation, high inverter robustness against the grid impedance variation can be achieved. Experiments are performed on a 6-kW prototype and the experimental results are in well agreement with the theoretical expectations.

Feedforward schemes of the voltage at point of common coupling (PCC) have been widely used in grid-connected inverters to reject the current harmonics caused by the grid voltage distortion. However, in the weak grid, the PCC-voltage feedforward tends to destabilize the grid-connected inverters due to the effect of time delay. This stability issue is explicitly elaborated by the impedance-based stability criterion in this paper and then addressed with an improved feedforward scheme, which uses the grid voltage instead of the PCC voltage as the feedforward variable. Considering that the real grid voltage is unavailable for sensing, a method to extract the grid voltage from the sensed PCC voltage is put forward to implement the proposed scheme. By carefully arranging the sampling instants according to the duty cycle, a dual sampling mode is adopted to ensure an accurate extraction of the grid voltage. Finally, simulations and experiments are performed on a 6-kW single-phase grid-connected inverter, confirming that the proposed grid-voltage feedforward achieves a superior harmonic rejection ability and a strong stability under the weak grid condition.

Capacitor-current-feedback (CCF) active damping and point of common coupling (PCC) voltage feedforward can provide damping to the LCL-type grid-connected inverter. In general, they are individually adopted, and negative damping will occur in certain frequency range due to the digital control delay, leading to a non-minimum phase behavior. In this paper, a hybrid active damping that combines the CCF and unit PCC voltage feedforward is studied. It is found that when the CCF gain is properly designed, the positive damping range could sweep the entire frequency range with the variation of grid impedance. As a reward, the maximum profit of damping cooperation can be harvested, ensuring high robustness against both grid impedance variation and filter parameter fluctuation. The simulation and experimental results are provided to verify the effectiveness of the hybrid active damping.

This article takes a deep insight into the passivity-based design of LCL -filtered inverter with grid current control and capacitor current active damping. It reveals that although an optimal damping gain can theoretically ensure the passivity, it is susceptible to both the lagging phase of the current regulator and the fluctuations of filter parameters. After thoroughly assessing the impacts of these two factors, proper phase compensations are proposed to enhance the passivity. It is found that a phase-lead or phase-lag compensator is needed, depending on the relation of the one-sixth of the sampling frequency to the resonance frequency of inverter-side inductor and filter capacitor. The guidelines on the compensation selection and parameter design are provided. Simulation and experimental results are conducted to verify the theoretical analysis.

Capacitor-current active damping has been widely used in voltage source inverters with LC or LCL filters. However, the time delay poses an adverse impact on the damping performance, causing a negative-resistance behavior, and thus constraints the system stability. To address this issue, this letter proposes a compensation method, which eliminates the adverse impact by removing the time delay out of the capacitor-current loop. Consequently, the damping performance behaves as a constant positive resistance, and thereby the stable region of the damping gain is notably enlarged. Experimental results verify the effectiveness of the proposed method.

With the remarkable growth of renewable energy resources, the grid-forming (GFM) inverter with the function of grid voltage/frequency support attracts much attention. Due to the inverter-grid interaction, the stability of the GFM inverter is a critical issue. The passivity-based analysis approach, which was widely applied to the conventional grid-following inverter, has been proved to be promising. Yet, its application to the GFM inverter is still insufficient. To this end, this article conducts a comprehensive passivity-based analysis for the GFM inverter with single-loop voltage control. It finds that the two indices of the passivity-based stability criterion, i.e., the individual stability and the output impedance passivity, bring identical constraints on the voltage controller, and the passivity cannot be ensured with typical voltage controllers. To shrink the unexpected nonpassive frequency ranges, a generic grid-current feedforward scheme is explored, and the proper feedforward functions compatible with different voltage controllers are derived. With the proposed scheme, the passivity can be guaranteed up to the Nyquist frequency. Finally, experimental results from a 6-kVA prototype are provided to verify the theoretical analysis.