The Frequency Regulation Scheme of Interconnected Grids With VSC-HVDC Links

Abstract

Nowadays interconnections of non-synchronous AC grids with VSC-HVDC links become attractive in preventing cascading outages. To share the spinning reserves, the VSC-HVDC link is expected to balance the frequencies of the interconnected grids under tolerable disturbances. Based on the synchronous generator emulation control (SGEC) strategy, this paper presents a communication-free scheme for the frequency regulation of interconnected grids with a VSC-HVDC link. Under a tolerable disturbance, the scheduled powers of the rectifier and the inverter are changed to their actual powers, respectively. As a result, the grid frequencies could be balanced and the adjacent grid will provide frequency support to the disturbed grid. If the disturbance becomes larger, the scheduled powers of the two converters are changed until the maximum or minimum limits and the frequency support to the disturbed grid is limited. Thus, the large disturbance results in a deeper frequency drop in the disturbed grid, which could be arrested by its under-frequency load shedding. As a result, the disturbance propagating to the adjacent grid is suppressed, which does not jeopardize security. The frequency regulation scheme coordinates well with the automatic generation control (AGC) of the grids. Time-domain simulation studies are performed in the PSCAD/EMTDC software environment.