Principal Investigator: Dr. Yue Zhao
Over the past decade, the medium voltage (MV) solid state transformers (SST) have been
extensively studied and demonstrated as an enabling technology to replace the bulky line
frequency transformer and provide advanced grid supporting functionalities. To connect to the
MV AC grid, most of the proposed solutions utilize series connected modular converters on the
AC side. Several MV SSTs, both Silicon based and Silicon Carbide (SiC) based systems, have been
developed and reported in the public domain. However, the major issues of these system are the
low efficiency, e.g., less than 98%, and high volume due to the use of multi-stage architecture
and the bulky DC link capacitors. Therefore, as stated in the Needs Document D1.1 developing
MV SSTs that are highly efficient, low volume, is still desirable. Architecture studies of how these
applications apply and impact the grid are still needed.
Through a program funded by the Department of Energy (DOE), a highly efficient highdensity
modular SST concept has been developed, from the base unit, i.e., Power Cell, which is
an air-cooled isolated DC/AC conversion unit rated at 75 kVA, to the Power Stack, which consists
of 3× Power Cells. Then with several Power Stacks, a MV Tower can be developed. Two all SiC
full-scale Power Stack prototypes have been developed and tested to reach full power 450 kVA
with up to 4160V AC output. Due to the use of SiC, the power density and efficiency of the Power
Cell are much higher than that of the state-of-the-art. However, there are two challenges
remaining to be solved, i.e., (1) how to develop a modular controller hardware architecture to
scale up in terms of the power and/or voltage; and (2) what is the most practical control software
architecture to enable the flexible inter-connection among Power Cells and/or Stacks. To address
these challenges, which also aligns with the Need D1.1 and M1.3, in the Year 1 of this project,
innovative work has been done in the architecture studies, especially from the controller
hardware and software perspectives.
With a fully functioning SST system, the goal of Year 2 work is to develop and demonstrate
the advanced features that this SST can bring to the distribution system. As outlined in the RFP,
due to the higher order harmonics generated by the large IBRs, DERs, EV Chargers, and other
switching converters, the heating effects on the distribution transformers can be substantial. To
address this issue, as a use case, we will connect the SST with the secondary side winding of the
transformer to enable full controllability, especially eliminate the harmonics generated by the
non-linear loads. In addition, other features, such that voltage regulation, power factor
correction and etc., which are not able to provide by the traditional transformer, will be
demonstrated.
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