Stability Analysis of High-Power DC Grids

During the last several years, increasing effort has been spent to advance offshore wind power. So far, only small test fields have been realized, and the electrification of offshore wind farms still remains a research topic. Currently, offshore wind farms are equipped with three-phase ac grids, some of them will be connected to the transmission grid via a high-voltage direct-current (dc) link. Consequently, a dc system encompassing the whole wind farm might constitute an alternative to a conventional ac grid. In this paper, the transient stability of such a dc grid is analyzed. Based on traveling wave theory, critical frequencies and cable lengths are calculated analytically. Furthermore, a large dc grid for an offshore wind farm is simulated. The goal is to provide guidelines that support stable operation of an offshore dc grid.