Modelling for AC/DC System Interaction Studies in EMT-type Software

With the growing integration of multi-terminal HVDC (MT-HVDC) systems into transmission networks, modern power systems are evolving into hybrid AC/DC grids that challenge traditional planning and stability assessment methods. Current stability and protection investigations generally rely on simplified representation of either the AC or DC side, limiting the ability to capture dynamic interactions and fault behaviour in hybrid grids. To address this, a meshed AC grid combined with expandable DC network model is proposed. This setup enables realistic simulations of fault propagation, component stress, and system interactions. A case study shows that equivalent AC network representations substantially underestimate fault severity, resulting in overly optimistic FRT compliance. In contrast, detailed EMT models reveal earlier FRT breaches and higher component stresses. Three factors govern overall system stability evaluation: AC grid representation, DC system design and converter control. Additionally, to support future HVDC planning, a comprehensive evaluation strategy is proposed. This strategy combines sensitivity-based assessments with hard and soft key performance indicatorss (KPIs).