pwr_january-2025 - 25

TRANSFORMERS
What Is a Solid-State Transformer?
Unlike conventional transformers that
rely on heavy iron cores and low-frequency
operation, solid-state transformers
(SSTs) use a multi-stage architecture
and high-frequency transformers (HFTs)
to achieve significant improvements in
size, efficiency, and functionality.
Essentially, SST functionality relies
on three core stages (Figure 1).
1. Input Stage (AC-DC Conversion).
This stage converts low-frequency
alternating-current (AC) into direct-current
(DC), forming the foundation for
high-efficiency power management.
Wide-bandgap semiconductors like
silicon carbide (SiC) and gallium nitride
(GaN) are central to the process, offering
reduced switching losses, enhanced
thermal stability, and the ability
to operate at higher frequencies. These
innovations allow SSTs to achieve compact
designs with enhanced power
density. The input stage also provides
reactive power compensation, which
promises seamless integration
with
the grid and stabilizing power delivery
under dynamic conditions.
2. Isolation Stage (High-Frequency
DC-DC Conversion). In this stage, an
HFT isolates and adjusts voltage levels
between the high- and low-voltage
sides. By leveraging advanced magnetic
materials (such as ferrites and
amorphous alloys), HFTs minimize core
losses while maintaining high thermal
stability and power density. Operating
at frequencies ranging from tens of kilohertz
to several megahertz, the HFT
significantly reduces size and weight
compared to traditional transformers,
making it ideal for space-constrained
environments like urban substations or
offshore wind platforms.
3. Output Stage (DC-AC Conversion).
This stage reconverts DC back
into AC or retains it as DC, depending
on the application. Supporting bidirectional
power
flow, the output stage
enables seamless integration of distributed
energy resources (DERs), energy
storage systems, and renewable energy
sources. Precise voltage and current
regulation at this stage bolster grid
stability and efficiency, positioning SSTs
as intelligent nodes in modern grids.
Compared to conventional transformers,
SST designs notably include
advanced features, including modular
configurations, such as multilevel converters
and dual-active bridges, which
enhance scalability and operational flexibility.
At the same time, integrated sensors
and intelligent control algorithms
can enable real-time monitoring, voltage
regulation, harmonic filtering, and fault
January 2025 | POWER
1. Overview of the solid-state transformer
(SST) infrastructure, showcasing modular
design, high-frequency transformers, and
advanced power electronics for compact, efficient,
and bidirectional power flow. Note:
AC = alternating current; DC = direct current;
HV = high voltage; and LV = low voltage.
Source: Agarwala et al., 2024.
isolation. SSTs also support grid communication
protocols, ensuring seamless
integration with renewable energy
systems and DERs. Notably, however,
the functionality and applicability of
SSTs depend significantly on their configuration,
which determines their performance
in various scenarios.
One-Stage Configuration. The
design involves direct AC-to-AC conversion
without a DC link. While costeffective,
lightweight, and suitable for
basic voltage transformation, it lacks
the advanced capabilities needed for
reactive power compensation and renewable
energy integration. The configuration
may be ideal for applications
in rural and industrial settings where
simple step-down voltage conversion
is necessary.
Two-Stage Configuration. Incorporates
a DC link on either the primary or
secondary side on either the high-voltage
(primary) or low-voltage (secondary)
side of the transformer, enabling more
advanced functions. These include, for
example, reactive power compensation,
improved voltage regulation, and integration
with DERs and energy storage.
The configuration is better suited for
electric vehicle (EV) fast-charging stations,
where DC conversion is critical,
and for renewable energy microgrids
that need reliable voltage regulation and
storage integration.
Three-Stage Configuration. Features
dual DC links on both the highand
low-voltage sides. While complex
and costly, this configuration provides
the highest level of operational flexibility,
bidirectional power flow, robust
reactive power management, and
seamless DER connection. Applications
generally envisioned include
urban substations, offshore wind platforms,
and data centers, which generally
need compact, efficient, and highly
controllable power management.
www.powermag.com
2. Solid-state transformers (SSTs) can enable
efficient bidirectional power flow, renewable
energy integration, and enhanced voltage
regulation in modern distribution networks.
Note: MV = medium voltage. Source: Shadfar
et al., 2021, International Transactions on Electrical
Energy Systems
cases that can produce different kind of
results, " Pascualy said. " So, I think that
that is just a matter of maturing. "
The first crucial step could arrive with
more widespread deployment of hybrid
SSTs, which integrate the functionalities
of traditional transformers and advanced
SSTs to achieve modular and efficient
designs. Hybrids, envisioned to feature
multiple stages of voltage conversion,
such as AC-DC and DC-AC, and can include
both high-voltage and low-voltage
DC links, are largely unavailable commercially.
The University of Texas at Austin,
supported by a DOE grant under the
Transformer Resilience and Advanced
Components (TRAC) program, has so far
developed and demonstrated a 500-kVA
hybrid solid-state transformer (HSST)
that combines dual-active-bridge-based
SST technology with a conventional drytype
transformer. The project explored
advanced capabilities such as voltage
regulation, fault detection, and dynamic
state estimation for modern grid applications.
More research is ongoing at North
Carolina
State
University's FREEDM
Systems Center focusing on innovations
such as SSTs to enhance renewable energy
integration and grid efficiency.
While challenges remain in fully commercializing
SST technology, Pascualy is
optimistic about its potential to revolutionize
grid modernization. " The grid is
receiving a lot of attention right now,
but we're adding so much more to it, "
he said. Collaborative efforts between
technology companies, utilities, and research
institutions will be crucial in driving
the widespread adoption of SSTs,
he predicted. ■
-Sonal Patel is a POWER senior editor.
25
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