Page 380 - From Smart Grid to Internet of Energy

344 From smart grid to internet of energy

7.2 kV ac input voltage. The first-generation energy router is built with 6.5 kV/
25A Si insulated gate bipolar transistors (IGBT) for ac-dc and dc-dc converters
as seen in the blocks of figure. The input converters which are used to rectify
and regulate dc bus voltages are cascaded to increase rated power of SST. Each
dc-dc converter is followed by a full-bridge inverter that is fed by a single dc
intermediate bus at 400 V. The output converter provides three ports for 120 V/
240 V ac output voltages and a 400 V dc output voltage.
The improved versions of FREEDM architectures are classified and Gen-II
and Gen-III due to accomplished developments in solid state technologies and
device topologies. The Gen-II SST which is also proposed as energy router is
comprised by three-stage architecture similarly to first generation, but it was
differing with single level ac input voltage, dual half-bridge dc converter topol-
ogy and SiC semiconductors operating at 15 kV/10A voltage and current rat-
ings. Moreover, the SiC mosfets of Gen-II SST was capable to interface
distribution bus voltage up to 20 kV with decreased number of active switching
devices. The third-generation SST of FREEDM is improved by replacing dual
active bridges with series resonant converter and by increasing 97% overall effi-
ciency of Gen-II to 98.2% in Gen-III architecture [22].
The SSTs that are proposed as energy routers are essentially based on power
switches. The semiconductors used in SSTs are selected from high switching
frequency devices which are not suitable to be operated at high voltages. There-
fore, serial or cascaded connections of IGBTs and mosfets are used to comprise
energy routers. However, conventional Si semiconductors are high in volume
and weight comparing to novel SiC and GaN semiconductor devices. 15 kV
SiC IGBTs which are used in Gen-III SST seen in Fig. 9.4 are capable to operate
at 10 kHz for reducing the transformer size and volume, and decreasing overall
size of energy router. On the other hand, SiC mosfets provide higher switching
frequencies up to 50 kHz comparing to SiC IGBTs. The Gen-III energy router
SST has three connection terminals at different voltage levels as 7.2 kV input
bus that is connected to 12 kV distribution line, 120 V/240 V ac and 400 V dc
output ports [21, 22].
The control operation of energy router based on SSTs is more sophisticated
comparing to conventional power converters. The control processes are

FIG. 9.4 Gen-III energy router of FREEDM with 15 kV-10A SiC Mosfets.