Primary frequency control of remote grids connected by multi-terminal HVDC

This paper addresses the possibilities to exchange primary reserves within a multi-terminal HVDC (MTDC grid) system as well as between asynchronous ac grids connected through the DC grid. We propose a new method for the challenge of frequency control participation in ac grids connected by a multi-terminal HVDC system. Since HVDC transmission is very often used to exchange a contracted amount of bulk power between different synchronous systems, the primary frequency response characteristic of one ac grid is not by itself transferred to other terminals connected to the dc grid or to other synchronous ac grids. This functionality can, however, be implemented by a special frequency control solution at the converter stations. The control scheme proposed in this paper combines frequency droop control of ac grids with that of dc voltage droop control in dc grids so that all ac grids connected to the dc grid can contribute to frequency support at the point of common coupling. Simulation results from an EMTDC/PSCAD model of a four-terminal HVDC system have illustrated the robust performance of the proposed control method during frequency variations in different terminals.