Coordination of Fast Frequency Support from Multi-Terminal HVDC Grids

A high-voltage direct-current (HVDC) system should provide frequency support to all ac grids connected to it. Presently, only a reduced number of ac grids receive this ancillary service. In this paper, an HVDC grid is modeled to analyze and compare the effectiveness of three different fast frequency support algorithms. These supplementary schemes allow the exchange of additional power among the ac systems interconnected by the HVDC grid. However, undesired power flows and reduced power transfers may result if all converters within the dc grid are fitted with the frequency support loop. This may be attributed to simultaneous frequency oscillations on multiple ac systems during the support operation. To prevent this adverse effect, an auxiliary dead-band controller (ADC) is proposed, which uses measurements of rate of change of frequency and frequency deviation to modify the dead-band set-point of the fast frequency controllers. Results show that the proposed ADC scheme improves the performance of fast frequency control algorithms fitted to multi-terminal HVDC grids. Moreover, the dc grid is capable of facilitating support to all connected ac grids as needed while restoring a stable operation and improving power transfer capability.