Controller parameter optimization of LCL-type grid-connected inverters for passivity margin enhancement

•Admittance shaping based on capacitor voltage feedforward and control delay reduction.•Passive admittance of LCL-type grid-connected inverter with desired passivity margin.•Controller parameter design problem is formulated as a bilevel optimization problem.•Nested black widow optimization algorithm is developed. The conventional passivity-based controller design of LCL-type grid-connected inverters can ensure the stability of the inverter-grid system, but cannot guarantee sufficient stability margin. Harmonic resonance caused by insufficient phase margin at the intersection of inverter output admittance and grid admittance can degrade power quality. To address this issue, this article proposes an admittance shaping method based on capacitor voltage feedforward and control delay reduction to passivate the output admittance up to the Nyquist frequency with satisfactory passivity margin for both inverter-side and grid-side current-controlled LCL-type grid-connected inverters. It is found that the controller parameter design problem for making the inverter comply with the passivity margin criterion can be formulated as a bilevel optimization problem. A nested black widow optimization algorithm is developed to solve this bilevel optimization problem, which can automatically design a passive output admittance with desired passivity margin for both inverter-side and grid-side current control by optimizing the gain of current regulator and capacitor voltage feedforward and the control delay of the corresponding paths, so that the system has sufficient phase margin regardless of inductive grid conditions. Experimental results verify the effectiveness of the proposed method.