Cascaded DC modular solid-state transformer

A solid-state transformer system, comprising:

• a plurality of isolated DC-DC converter modules arranged as upper arm modules and lower arm modules in each phase of a three-phase configuration, wherein each module has a medium voltage port and a low voltage port;
• a medium voltage DC interface formed by connecting positive terminals of uppermost modules and negative terminals of lowermost modules;
• a medium voltage AC interface formed at junctions between the upper arm modules and lower arm modules of each phase;
• a controller configured to generate voltage references for a medium voltage side of the modules, wherein the voltage references comprise a DC common-mode component and an AC differential-mode component, and optionally an AC common-mode component, wherein the DC common-mode component is determined by a desired voltage across the medium voltage DC interface and ensures unipolar voltage operation across the medium voltage port of each module;
• and
• the controller generates AC differential-mode voltage references with opposite polarity for the upper arm modules and the lower arm modules.

A method of controlling a cascaded modular solid-state transformer, the method comprising:

• generating voltage references for medium voltage buses by combining a DC common-mode voltage component, an AC common-mode voltage component, and an AC differential-mode voltage component;
• regulating the DC common-mode voltage component to ensure unidirectional voltage operation across medium voltage ports while connected to a three-phase AC source;
• and
• controlling power flow between a medium voltage AC interface and a low voltage DC interface.