Hybrid Phasor- and EMT-based Multi-Terminal MMC-HVDC Model with Grid-Forming Control

The objective of this paper is to apply a hybrid phasor-(RMS) and electromagnetic transient-based (EMT) simulation approach to a multi-terminal modular multilevel converter high voltage direct current (MT MMC-HVDC) model. The MMC of the MT-HVDC are implemented with grid-following and grid-forming control. An overview of the MT MMC-HVDC is given and the grid-forming and voltage droop control are described. The AC and DC networks of the MMC are divided into an RMS and an EMT partition with different simulation time steps. The grid-forming and grid-following control with inner AC current control are assigned to the RMS partition. The total energy and inner converter control are assigned to the EMT partition. The model is initialized for a correct transition from power flow to time domain simulation. The approach is implemented in a meshed power system model, where grid-forming MMC provide voltage and frequency for an offshore wind farm and a synchronous grid. The simulations are performed with the AC networks in RMS and the DC networks in EMT. The results show that the interactions between the AC and DC grids can be studied in a hybrid simulation framework. The DC system response is detailed, while the AC system is reduced to machine and controller dynamics.