Influence of frequency-droop supplementary control on disturbance propagation through VSC HVDC links

The construction of the European Super Grid is a valuable solution to deal with the growth of electricity demand and the increasing penetration of renewable energies. Key feature is the introduction of large capacity High Voltage Direct Current (HVDC) links between countries bordering the North Sea. This paper investigates the level of coupling of two AC grids connected via an HVDC link. Two reduced dynamic equivalent transmission systems resembling Great Britain and Scandinavia are developed in DIgSILENT PowerFactory. A balanced monopolar HVDC link equipped with two-level Voltage Source Converter (VSC) is used to connect the two grids. A frequency droop control that modulates the power transfer along the DC link has been designed for the converters to provide frequency support between the grids. A variety of disturbances were applied to the AC grids to test the extent to which faults in one grid propagate to the other. It was found that the presence of frequency droop control increases significantly the degree of coupling between the two grids.