I. Introduction
A grid-connected inverter is equipped with an LCL filter after its output [1]–[3]. This filter has a resonant characteristic and many active damping methods have been researched to suppress the resonant effects [4]–[16]. However, the inverter is often controlled digitally and there is a control time delay which is inevitable due to A/D conversion, computation times, and so on. Effects of the delay have been extensively analyzed [17]–[23]. This delay degrades the inverter performances and frequency characteristics which are different from designed ones. Especially the frequency characteristics around the resonant frequency of the filter are affected and they have large peaks and phase-shifts, which degrade stabilities and damping effects. It is one of the most important subjects how to compensate the delay and realize the designed characteristics [24]–[30].