DC-DC converters are expected to play a significant role in future DC transmission grids and in providing connection points to HVDC lines. They will facilitate power exchange between two DC systems of different or similar voltage levels, and they may also provide other functions like DC fault isolation and power flow control.
CIGRE has recently established working group B4.76 which is focused on DC-DC converters and this submission provides initial B4.76 recommendation for non-isolated DC-DC converter topology.
This article presents the topology for non-isolated MMC-based DC-DC converter. The initial design study illustrates that such DC/DC converter will have overall semiconductor count comparable to a MMC AC-DC (used with HVDC transmission) converter of similar rating. A full controller schematic is presented and operating principles are discussed.
A 600MW, 320kV/250kV test system with parameters is presented in detail. A model for this converter is developed in PSCAD and a range of responses are presented. The simulation results confirm good responses for full power reversal, for severe DC faults on each DC bus, and for a step change on DC terminal voltage.
It is concluded that this converter is a good candidate for applications in future DC grids, and the test system is suitable for power flow, stability and protection studies of DC grids.

D. Jovcic, P. DWORAKOWSKI, G. KISH, A.
JAMSHIDIFAR, A. NAMI, A. DARBANDI, X. GUILLAUD