Fig. 4

Different effects of scheduled HVDC ramps on the Rate of Change of Frequency (RoCoF). We show the modeled interdependencies between the ramps of scheduled HVDC flow to Continental Europe and the hourly RoCoF in the GB (left) and Nordic (right) synchronous areas. The dependencies have opposite directions, which is confirmed by the Kendall correlation coefficient \(\tau\) of the scattered data. We explain these effects with different relative ramp speeds s, which we calculate based on the ramp rates r of the HVDC links (ENTSO-E 2019b). On the CE-GB border, a cable with 1000 MW capacity is allowed to ramp 100 MW/min, i.e., the ramp rate is \(r=0.1 / \mathrm {min}\) (share of full load per minute). On the CE-Nordic border, a cable such as Kontek has 600 MW capacity and Nordic ramp restrictions allow 600 MW per hour, i.e., a ramp rate of \(r=0.017 / \mathrm {min}\). Using the mean \(\Delta P\) of the absolute hourly flow changes on the links, we estimate the typical Rate of Change of Power as RoCoP\(=\Delta P \cdot r\), which results in different relative ramp speeds s (see main text). These different ramping speeds most probably explain the different effects on the hourly RoCoF