Selecting frequency and parameters of DC-fault tolerant non-isolated high power MMC DC/DC converter

•Non-isolated MMC DC/DC converter (NIMDC) parameters design is proposed.•The design goals are set to power loss of 1.5%, and voltage ripple of around ±5%.•The NIMDC parameters affect each other in a highly non-linear way.•A DC fault analysis provides minimal NIMDC inductors size as initial design values.•A generalization of the work using pu approach is also proposed. This paper studies MMC-based non-isolated DC/DC converter for DC transmission grids. The key design parameters including operating frequency and size of passive components are evaluated with the aim of ensuring DC fault tolerance and minimizing losses and size. An analytical model is used to perform parametric studies while detailed non-linear model is used for verification. The case study on 600MW, 320 kV/250 kV system reveals the narrow range of optimal cell capacitance and arm inductance while lower-side arms require substantially larger capacitors. With the targeted losses of 1.5%, and voltage ripple of ±5%, it is recommended to use around 150 Hz operating frequency. For the offshore applications, higher frequency enables significantly smaller size with some increase in losses. The line inductor on the low-voltage side should be much larger than the arm inductor and plays a key role in the dc fault responses.