Interoperability of Modular Multilevel Converters and 2-level Voltage Source Converters in a Laboratory-Scale Multi-Terminal DC Grid

The prospect of future multivendor HVDC systems has led to concerns about interoperability between converter terminals based on different technologies. This paper investigates the interoperability of converter terminals in a small-scale experimental setup, consisting of two 2-level (2L) Voltage Source Converters (VSCs) and two Modular Multilevel Converters (MMCs) with different number of half-bridge sub-modules in each arm. The laboratory setup is utilized to demonstrate interoperability of a 2-level VSCs and an MMCs when operated in parallel on the dc-side as well as in parallel at a common connection point in an ac grid. Experimental results are presented for the investigated 4-terminal configuration with one 2L VSC controlling the dc voltage and all other converters controlling their active power flow. The presented results give an additional empirical confirmation that topological differences between properly designed converter terminals should have limited influence on system level operation and system interoperability of MT HVDC systems.