It is more complex than a conventional transformer operating at utility frequency, but it can be smaller and more efficient than a conventional transformer because it operates at high frequency.
A solid-state transformer usually contains a transformer, inside the AC-to-AC converter or DC-to-DC converter, which provides electrical isolation and carries the full power.
A solid-state transformer can actively regulate voltage and current.
Variations can input or output DC power to reduce the number of conversions, for greater end-to-end efficiency.
As a complex electronic circuit, it must be designed to withstand lightning and other surges.