DAH-FF approach to improve the current quality and stability of the LCL type grid-connected inverter

Full Feed-forward (FF) approach has been proved to be an effective method to suppress the output current distortion of the LCL type grid-connected inverter (GCI) due to grid voltage harmonics. Since most of the existing FF approachs are realized by digital control, a futher study of the digital control based (DCB) FF approach has been carried out in this paper firstly. It is shown that, there are two limitations of the DCB-FF approach: a) the DCB-FF approach does not play a role in suppressing current distortion at medium-high frequency range; b) the DCB-FF approach reduces the system phase margin at low frequency range and may cause the system instability problem at weak grid case. In order to solve the above problems, a digtal-analog hybrid based full feed-forward (DAH-FF) approach has been proposed in this paper. This DAH-FF approach not only can suppress the GCI current distortion at the medium-high frequency range, but also can improve the system stability when the GCI connects to weak grid. The concept, design principle and implementation method of the DAH-FF approach have been discussed carefully in this paper. Finally, the proposed DAH-FF approach has been validated by both simulation and experimental results.