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. Analyses were caried out of 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 3-terminal MMCHVDC system was 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 were compared. The data comprises: (influence of equivalent synthetic inertia constant and cell capacitance) active power in per-unit (pu) over time for cases where the synthetic inertia constant is 6.95s, 20.85s and 27.9s; active power in per-unit (pu) over time using a cell-capacitance of 2.5mF, 7.5mF and 10mF; DC voltage in pu over time for cell-capacitance values of 2.5mF, 7.5mF and 10mF; DC Voltage in pu over time for cases where the synthetic inertia constant is 6.95s, 20.85s and 27.9s; (frequency support characteristics of different energy sources due to a sudden 1800 MW generation loss on the main AC grid) active power (pu) over time for the five different cases - no control, DC capacitor only, DC capacitor and wind farms, DC capacitor and other AC system, and coordinated control; DC voltage (pu) over time for the five different cases; frequency (Hz) over for the five different cases; and additional power (pu) over time from equivalent capacitance of MMCs, wind turbine rotating mass and other AC system during the case of coordinated control.