Frequency sensitivity analysis of ac grids connected to MTDC grid

Multiterminal HVDC (MTDC) based on voltage source converters (VSC) has been getting attention in recent years as a potential solution for integrating offshore wind farms, offshore loads and remote asynchronous grids. One important issue raised in developing such a system is the impact of loadfrequency changes on the power flow of different ac grids connected to an MTDC grid. Instantaneous power is balanced by use of frequency droop control inside the ac grids and by dc voltage droop control inside the MTDC. Hence, depending on the converter control implementations, load-frequency changes occurring in one ac grid can be reflected in the other asynchronous ac grids. In this paper these load-frequency sensitivities are investigated and analytical expressions describing these relations are derived. A four terminal dc grid was simulated in PSCAD and results confirmed validity of the derived frequency sensitivity expressions.