Interactions studies of HVDC–MMC link embedded in an AC grid

• A HVDC–MMC link connected in parallel with an ac line is studied.
• A small-signal analysis and EMT models are developed and used to study interactions in this system.
• The analysis of eigenvalues and mode shapes was used to assess interactions between different variables of the system.
• For small perturbations, the parallel ac line can improve the dynamic performance of the HVDC link and that the risk of negative interaction between both HVDC stations is negligible.
• The parametric studies conducted in EMTP shows that for large disturbances, negative interactions can occur when an ac line is in parallel with the HVDC link.

HVDC link interconnections under construction or planned in France are part of a highly meshed ac network. This is a relatively new operating condition. The impact of their operation and the risk of abnormal interaction may have an influence on the ac network. The use of voltage source converters (VSCs) with the modular multilevel converter (MMC) topology is becoming more attractive mainly due to their higher performances and lower cost. This paper analyses the operation and interaction of MMC–HVDC links embedded in an ac grid. First a MMC–HVDC link model suitable for small-signal analysis is presented. This small-signal HVDC model is then validated against an EMT-type model for ac systems having different SCR (Short-Circuit Ratio) values. Modal analysis and parametric studies are performed in order to study the impact of an ac line connected in parallel with an HVDC link.