IEEE Power Electronics Magazine - March 2022 - 26

voltage-controlled grid forming mode, thus supplying highquality
ac power to residential systems and appliances.
Moreover, it provides an additional opportunity of ac integration
and interaction of DER and ES at the building side.
On the other hand, this approach requires all energy provided
to the building to be processed by the ER, which can
lead to increased power losses.
The approach presented in [18] proposes the addition
of a static transfer switch (STS) for bypassing the backto-back
converter in several operation scenarios, thus
increasing the energy efficiency. Moreover, this concept
enables high reconfigurability of the power electronic
stage, which brings the ER an extra level of redundancy
and control flexibility. For example, in the normal ongrid
operation, the STS is closed, and the load is supplied
directly from the grid. The ER operates in parallel
to the grid, thus performing the function of a power
conditioner. The power exchange between the dc and ac
buses could be controlled by each of the inverters or by
their parallel configuration. Such redundant approach
increases the fault-tolerance of the ER substantially and
offers excellent fault ride-through capability because of
the ability to easily isolate grid- and building-side faults
from each other.
Moreover, the dc bus of a back-to-back converter (Figure
4(c)) could be used for powering dc loads, such as
LED lighting or USB-C PD compliant consumer electronics,
by employing dedicated interface dc-dc converters7. It
is expected that the power delivery capacity of the USB-C
PD will be extended to 240 W soon, which will foster the
development of residential dc nanogrid technology. This
all drives the innovation towards dc buildings, where
power delivery is realized using the dc bus with the nominal
voltage ranging between 350 and 400 VDC [14], [19].
The ER in that case operates as a bidirectional rectifier
(Figure 4(d)), which in several cases is also referred to
as an active front end (AFE) converter. Due to the safety
requirements imposed by several countries, the AFE may
also include a galvanic isolation stage, which is typically
realized using series resonant dc-dc converters operating
in the dc transformer mode. In contrast to classical ac,
the emerging dc power delivery approach avoids any frequency
stability and reactive power issues and reduces
the system cost [20]. Many modern A-class home appliances
and HVAC systems today can be adopted for operation
in dc nanogrids as they are naturally " dc-ready " , i.e.,
have the two-stage power conversion with an intermediate
dc-link. As a result, the reduced number of energy conversions
and increased power delivery capacity of the dc
nanogrid can increase the energy efficiency of a building
by up to 18.5% [21]. Furthermore, the natural congestion
management and more simple interfacing and balancing
of multiple DER and ES make dc nanogrids an enabling
technology for the last mile power delivery.
7 https://www.dc.systems/products
26 IEEE POWER ELECTRONICS MAGAZINE z March 2022
TTN and Smart Transformer
TE requires also smart hub at the distribution level, which
can aggregate subsystems in one smart electrical system
(Figure 5(a), where TTN and TEN nodes can be implemented.
Those systems require a superior power converter
and controller called a Smart Transformer (ST)
[22], which can control/minimize power flow and grid distortions,
allows easy connection of Energy Storage (ES)
in the most efficient way, increases the reliability of the
network and reduces the appearance of blackout. ST is
also capable of exchanging information about market
prices of electricity, actual energy demand and other
information. Moreover, ST not only replaces a traditional
power transformer but also offers many other functions,
such as smart demand side management of distributed
systems response, integration of RES and ES, bidirectional
communication, improved power-quality, control at
islanded and grid mode of operation, similar to the ER at
the end-user level. Therefore, the research and application
of power electronics converters as distribution transformers
have become a wide area of interest [23]−[31].
ST is designed to replace a conventional distribution
transformer converting medium-voltage (MV) ac to lowvoltage
(LV) ac at power levels from several dozen kVA up
to several MVA. The most common configuration of ST is
presented in Figure 5(b), where power conversion is based
on the multistage process and the modular structure
based on the power electronics building blocks (PEBB)
concept includes:
■ ac-MV/dc-MV conversion where the power electronics
converter based on the cascade connected topology
makes the operation at the MV level possible;
■ dc-MV/dc-LV conversion reducing the dc voltage level
from MV to LV based on the Input-Series Output-Parallel
(ISOP) structure and incorporation of high-frequency
transformer (HFT) that provides galvanic isolation
demanded by the grid code due to safety reasons;
■ dc-LV/ac-LV conversion where the converters are connected
in parallel, which is demanded by the high current
on the LV side.
Modular structure of ST using PEBB gives high power
and voltage scalability and access to MV and LV dc terminals,
which is a great advantage for the coming era
of electromobility and energy storage systems. It can be
directly connected to the dc grid without any additional
energy conversion stages. Additionally, the modular system
brings lower voltage and current ratings of power
electronics components, which both lowers the costs
and reduces stress by lower dv/dt and di/dt that minimizes
EMI emission. Moreover, the described system
has a unique feature of increased total system reliability
due to module power routing (preventing uneven power
semiconductor aging [25], [26]) and post-fault operation
by system redundancy that provides system maintenance
during normal operation of the system. However, the
modular structure of ST also brings some challenges due
https://www.dc.systems/products
IEEE Power Electronics Magazine - March 2022

Table of Contents for the Digital Edition of IEEE Power Electronics Magazine - March 2022
