The large-scale integration of renewable energy sources in the power system, combined with the need for an increased transmission capacity has led to a growing interest in multi-terminal high voltage dc (MTDC) grids. In the future, these grids will be integrated with different existing asynchronous ac grids, eventually resulting in hybrid AC/DC power systems. This paper investigates interactions between asynchronous ac grids in a hybrid AC/DC power system. In the study, a symmetrical monopolar ±400kV four-terminal VSC-based MTDC grid connected to three different multi-machine ac systems is modelled in DIgSILENT PowerFactory. One of the ac grids has four generators while the others have two generators each. Governor and automatic voltage controllers are included for each generator so as to capture the complete generator dynamics. DC cables are modelled as PI models with lumped parameters. All dc grid terminal converters are operating in dc droop and reactive power control modes. A small signal analysis is carried out in the test system to investigate interactions between asynchronous ac grids. From the modal analysis of the poorly damped eigenvalues, it is shown that speed state variables of all generators in the study system are observable in these modes; indicating dynamic interactions between generators located in asynchronous ac grids. The change in the level of these dynamic interactions is studied for different time responses of the MTDC terminal converter controllers. It is found that faster and slower converter control response times lead to lower and higher interactions between the asynchronous ac grids, respectively. Results from a time domain simulation of the study system for a fault in one of the ac grids support the findings of the small signal analysis. The study results show that dynamic coupling exists between ac grids across dc grids and that the level of interaction is influenced by the converter controller settings.
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