Modular multilevel DC/DC converter architectures for HVDC taps

DC/DC connections between DC voltage levels in High Voltage Direct Current (HVDC) schemes are useful for interconnection between links and as taps to inject/extract relatively small amounts of power. Taps with high voltage transformation ratios (such as 500 kV to 50 kV) are particularly challenging to achieve with reasonable use of semiconductor ratings. Four architectures of DC/DC converter covering both direct connection and transformer-interfaced are analysed and compared in terms of the ratio of their apparent power rating (rated current times rated current summed across all stacks of the cells) to the throughput power. It is found that the direct conversion architectures have reasonable power capacity factor only for low transformation ratios (of 2:1 or less) but the inclusion of a transformer and intermediate square-wave operation allows much wider transformation ratios with a reasonable power capacity factor. For each design the nature of the cell-balancing currents has been determined since this is a major factor in determining the total current flow in each cell stack.