BACKGROUND
The idea of a “solid-state transformer” has been discussed for
some time. Nearly 20 years ago, Navy researchers [2] proposed
a power-electronic transformer that consisted of an ac/ac buck
converter shown in Fig. 1 to reduce the input voltage to a lower
one. This was followed in 1995 by a similar EPRI sponsored
effort [3]. Both of these efforts yielded working prototypes, but
they operated at power and primary voltage levels that were orders
of magnitude below utility distribution levels.
The ac/ac buck converter is perhaps the most direct approach
to single phase ac power conversion. In this arrangement,
switches and are bidirectional. Turning a switch on
permits it to conduct current in either direction. Conversely,
turning it off can block voltage of either polarity. In Fig. 1,
the two devices tied in series indicate an arrangement of some
number of semiconductors that would be needed to achieve the
desired voltage ratings. (The number of devices required would
be a function of voltage level and the type of semiconductor).
The bidirectional switches and are alternately turned
on at a high frequency relative to the fundamental component
of the voltage and current waveforms. Under these conditions,
it can be shown that