IEEE Electrification - June 2021 - 5

TECHNOLOGY LEADERS
Multiwinding-Transformer-Based
dc-dc Converter Solutions for
Charging Stations
By Marco Liserre, Felix Hoffmann, and Thiago Pereira
T
HE GROWING DEMAND
for sustainable mobility
and, consequently, also for
a wider use of electrical energy in the
transportation sector is posing serious
challenges not only to onboard
vehicle systems but also to the electrical
network that allows the integration
of more charging systems.
These systems are often requested to
provide power in a short time (fastcharging),
and this request is, unfortunately,
mostly in the morning
hours when the electrical network is
already loaded, sometimes even very
heavily. Consequently, future fastcharging
stations are requested from
one side to integrate energy generation
systems (like photovoltaic) and
storage systems to realize, when possible,
a net-zero energy system and
from the other side to provide
advanced supporting functionality
for the electric network, such as voltage
control or load management to
counteract the effects of the peak
power requests when generation and
storage are not present and/or not
enough. In this sense, a smart transformer
could present an optimal
solution offering low-voltage (LV) and
medium-voltage (MV) dc distributions
to supply several charging
points with high efficiency and to
integrate storage in a charging-park
Digital Object Identifier 10.1109/MELE.2021.3070933
Date of current version: 8 June 2021
and coordinate charging, energy generation,
and storage following a dispatchment
plan.
Thus, to offer the optimal tradeoff
between fast-charging and minimal
effects for the electrical network
through the coming years, intensive
research into charging technologies
has been performed to provide integration
of more systems into the
charging stations (generation and/or
storage), higher efficiency and low
infrastructure with low
maintenance need
for an higher number
of flexible charging
points capable of
charging different ve -
hicles with different
voltages and powers,
and new functionalities
for reducing the
effects on the electrical
grid, sometimes
leading to requests
for power bidirectionality to implement
concepts like power-to-grid.
Isolated DC-DC Converters for
EV Applications and Their
Requirements
In this context, the dc-dc converters
arise as a key component in both
onboard and off-board systems for
adapting the voltage and/or control
the power transferred between the
charging station and electric vehicle
(EV). Additionally, since the galvanic
2325-5897/21©2021IEEE
A smart transformer
could present an
optimal solution
offering low-voltage
and medium-voltage dc
distributions to supply
several charging points
with high efficiency.
isolation between the ac (grid) and
the dc side must be assured according
to the IEC61851-23 standard, the
question is where this isolation will
take place. Therefore, dc-dc converters
offer an optimal option to guarantee
isolation with reduced volume
and high efficiency due to the advantages
of a medium-high frequency
transformer that is not yet a standard
component and might impose
some design challenges, for example,
the maximum number
of windings into
the off-shelf magnetic
cores.
In the last three
decades, several isolated
dc-dc converter
topologies have been
widely investigated
in the literature for
integrating distributed
dc power sources,
storage units, and dc
loads, enhancing the power transfer
capability and increasing the conversion
efficiency. However, the isolated
multiport dc-dc converters,
in particular those based on the
multiwinding transformer (MWT),
have drawn more attention due to
their advantages in applications
where multiple dc sources and/or
loads should be integrated while
also ensuring galvanic isolation and
adopting different voltage levels
among their ports.
IEEE Electrification Magazine / JUNE 2021
5
IEEE Electrification - June 2021

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