Real-Time Simulation of Large-Scale AC System with Offshore DC Grid

For the purpose of HVDC or FACTS device studies, the AC power system is often reduced to an equivalent seen by the device at the point of common coupling (PCC). This practice is encouraged by the fact that electromagnetic transient simulation of large AC power systems is computationally expensive and time-consuming and might not respect the realtime constraints (for real-time commissioning studies). However, it is no mean feat to build system equivalents for a device that has multiple PCCs on the same AC system (e.g. a multi-terminal HVDC system). This paper synthesizes the work done at HydroQuébec’s Research Institute on large-system simulation, modular multilevel converter (MMC) and wind power plant (WPP) modeling and gives an example of real-time simulation of a largescale AC system with a complex multi-terminal offshore HVDC grid, based on MMC technology, used to power isolated loads and harvest offshore WPPs. This test system is subjected to disturbances and its behavior is observed and commented. Through those examples, the importance of full-system simulation is shown.