Investigation on grid-following and grid-forming control schemes of cascaded hybrid converter for wind power integrated with weak grids

Abstract

The conventional line commutated converter based high voltage direct current (LCC-HVDC) system is no longer suitable for transmitting wind power integrated with weak grids. Therefore, a cascaded hybrid converter consisting of LCC and modular multilevel converter (MMC) connected in series is considered as a promising solution in this paper. And a grid-following (GFL) control scheme and a grid-forming (GFM) control scheme are designed for the cascaded hybrid converter. To investigate the stability performances of the proposed two control schemes, this paper develops a small-signal model of the cascaded hybrid HVDC system for wind power integrated with weak grids. Then, based on the eigenvalue analysis method, several issues of concern are investigated for the proposed GFL and GFM control schemes, including their tolerance to a weak grid and a low wind power output, significant participants and sensitivity to parameter detuning. Based on the analysis results, one of the two that is more suitable for weak grids is recommended. The time-domain simulations implemented in PSCAD/EMTDC confirm the validity of the model and analysis results.Index Terms—Cascaded hybrid converter, grid-following, grid-forming, wind power, weak grids.