What is a solid-state transformer?

A solid-state transformer (SST) is an AC-AC converter, sometimes called a power electronics transformer (PET), that replaces a conventional magnetic transformer in applications like green energy systems, cloud data centers, EV fast chargers, and smart grid infrastructure. They are smaller and lighter than conventional transformers and can support better power quality, higher energy efficiency, and enhanced functionality.

SSTs operate at higher frequencies than conventional transformers, resulting in smaller and lighter-weight solutions. An SST can be more efficient than a conventional transformer under variable or partial load conditions.

The efficiency difference can be magnified when the SST is used to provide ancillary functions like active voltage regulation, power factor correction, or handling a variable voltage output from renewable energy sources like wind or photovoltaics.

Some SST architectures can support bidirectional power flow for energy storage integration. SSTs can provide reactive power compensation and harmonic filtering. They can be designed to provide a variable interface between AC and DC power in power grids and microgrids.

The development of silicon carbide (SiC) and gallium nitride (GaN) wide bandgap (WBG) power semiconductors is a significant factor in the emergence of SSTs as a viable technology. The use of WBGs enables higher switching frequencies and smaller and less expensive power conversion solutions.

Typical SST architecture

A typical SST uses a classic three-stage power conversion design with an input rectifier stage, a high-frequency dc-dc stage that provides isolation and voltage conversion, and an output inverter stage. The difference with an SST is that the input voltage is typically a grid distribution voltage like 3.6 kV or 7.2 kV. In some designs, even higher voltages are used to support higher power levels like renewables integration (Figure 1).

Multilevel topologies

The use of modular multilevel converter (MMC) topologies can enable the use of relatively low-voltage WBG power devices to handle much higher voltages like those needed in SSTs. An MMC stacks multiple low-voltage power cells in series to handle high-voltage conversion.

The high voltage is distributed across the individual low-voltage cells, preventing any cell from being exposed to excessive voltage stresses. The cells are switched on and off in a specific sequence, creating a high-voltage output with a staircase-like waveform, adding up the contributions of the individual low-voltage cells.

This modular structure also makes it easy to scale up the voltage by adding more modules. It can support fault-tolerant designs and the higher reliability needed for power grid and microgrid applications.

Microgrids for maximum sustainability

SSTs are necessary to support the widespread deployment of microgrids. That can enhance sustainability by enabling bidirectional power flow and seamless integration of renewable energy and storage resources.

That is achieved through advanced power electronics that support voltage and frequency regulation, fault isolation, and efficient energy management, which are not possible with conventional transformers. This makes microgrids more stable, efficient, and capable of integrating diverse, often intermittent, distributed renewable energy resources (DRERs) like solar and wind and distributed energy storage devices (DESDs) (Figure 2).

By combining advanced controls and artificial intelligence (AI) algorithms, SSTs can isolate the microgrid from the main grid during faults or maintenance. The combination of AI and SSTs can extend beyond microgrids.

AI, SSTs, and power grids

According to researchers at the National Renewable Energy Laboratory (NREL), AI can be a transformative technology for improving the efficiency and sustainability of the primary power grid. And SSTs are expected to be a key element in maximizing the benefits promised by AI.

SSTs can incorporate monitoring functions to provide real-time insights into grid operations, including green energy production and energy consumption. AI algorithms can be used to predict electricity supply, price, and trends based on grid demand or renewable energy output. The combination of SSTs and AI will support sustainable and cost-effective electricity supplies.

Summary

SSTs are an emerging technology that promises to revolutionize energy production and distribution. The availability of high-frequency WBG power devices will be important in enabling SSTs, and the integration of AI algorithms will increase the benefits realized from the use of SSTs.

References

AI, Solid State Transformers For The Modern Power Grid, Peak
Generative Artificial Intelligence for the Power Grid, NREL
Recent advancement of energy internet for emerging energy management technologies: Key features, potential applications, methods and open issues, ScienceDirect
Solid State Transformer, Delta Electronics
Solid State Transformer (SST), FREEDM Systems Center
Solid-State Transformer vs Conventional Transformer: Is the Extra Cost Worth It?, Eureka
Solid State Transformers: A Critical Review of Projects with Relevant Prototypes and Demonstrators, MDPI electronics
Solid-State Transformers: A Review—Part II: Modularity and Applications, MDPI technologies
Solid-state transformers: An overview of the concept, topology, and its applications in the smart grid, Wiley
Solid-State Transformers: Fundamentals, Topologies, Applications, and Future Challenges, MDPI sustainability
Voltage Balance Switching Scheme for Series-Connected SiC MOSFET LLC Resonant Converter, MDPI energies

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