Frequency changes in AC systems connected to DC grids: Impact of AC vs. DC side events

Each converter station within a DC grid can be equipped with a power frequency droop control loop in addition to the power voltage droop control normally used for autonomous power sharing. Presence of the power frequency droop control allows exchange of frequency support among the AC systems connected to the DC grid. Following an event (or disturbance), there are two distinctly different patterns of dynamic variation of the frequencies of the AC systems connected to the DC grid. In this paper, an analytical basis is provided to support the different trends. Simulation results using a 4-terminal DC grid model set up in DIgSILENT PowerFactory are used to validate the analysis. The different patterns of dynamic variation of the frequencies for AC and DC side disturbances could have significant implications for extraction of inertial/frequency support from offshore wind farms connected through MTDC grids.