Frequency Support from Modular Multilevel Converter Based Multi-Terminal HVDC Schemes

Multiterminal HVDC (MTDC) schemes are intended to transfer power from offshore wind farms to land and interconnect the grids of adjacent countries, through Voltage Source Converters (VSC). The modular multilevel converter (MMC) topology of VSCs is a more recent development which has low harmonics, reduced losses and occupies less space. This paper analyses the frequency support characteristics of MMC-based multiterminal HVDC schemes, using the energy transferred from the capacitance of the MMCs, wind turbine rotating mass and other AC systems. A generic formulation of an equivalent synthetic inertia constant is proposed to determine the energy transferred from the MTDC scheme. A 3-terminal MMCHVDC system is utilised to study the influence of cell capacitance of the submodules and the proposed synthetic inertia constant of the MTDC system on the frequency support capability of the MMC capacitors. The time response and energy capability of the different energy sources of the MTDC system are compared.