I. INTRODUCTION
The idea of “solid-state transformer” has been
introduced by Navy researchers in 1980 [1], and later
developed with isolation transformer included [2, 3]. As an
alternative to the bulky distribution transformer, it greatly
improved the power density of the power distribution
system. In a typical solid-state transformer system, rather
than connecting the transformer to line frequency buses
directly, semiconductor devices are used to increases the
operating frequency of the transformer, as shown in Fig. 1,
which largely reduces the volume of the transformer [4].