Development and Application of Solid-State Transformers: Architecture Innovation Based on High-Frequency and Wide-Bandgap Semiconductor Technology

With the advancement of power electronics and semiconductor technology, the application of Solid-State Transformers (SST) has become increasingly mature. Also known as Power Electronic Transformers (PET), SSTs utilize power electronics to perform the functions of traditional transformers. SST is a new type of power transformer that has gradually developed in recent years alongside advancements in high-power power electronics technology. Beyond the basic functions of conventional power transformers—such as voltage level transformation, electrical isolation, and energy transfer—SSTs can achieve many additional functions like power flow control and power quality management. This capability stems from its practical control over electrical parameters, including the magnitude, phase, frequency, number of phases, phase sequence, and waveform of voltage (or current). Structurally, it consists of two fundamental elements: power electronic converters and medium/high-frequency transformers. The power electronic converter primarily includes power semiconductor devices, controllers, filters, and auxiliary equipment, mainly responsible for protection, power conversion, and control. The high-frequency transformer provides isolation and voltage level transformation, typically operating at kilohertz (kHz) frequencies. The primary goals of using high frequency are to significantly reduce the transformer's size and weight, decrease heat dissipation, and improve capacity and efficiency.

Example: An integrated photovoltaic-storage-charging case for a 10kV direct-connection low-voltage distribution network.

I. Typical SST Power Conversion Process (Three Stages)

1.1) AC-DC Rectification Stage: A controllable rectifier (e.g., a Cascaded H-Bridge, CHB multilevel rectifier) converts the utility-frequency AC voltage to DC voltage while achieving power factor correction and input-side power quality control, reducing harmonic pollution to the grid.
1.2) DC to High-Frequency AC Inversion Stage: A high-frequency inverter (e.g., a Dual-Active Bridge, DAB converter) converts the DC voltage into high-frequency AC voltage (typically at kHz levels, e.g., 3kHz, 25kHz, 100kHz), providing the excitation for the high-frequency transformer.
1.3) High-Frequency AC to DC/AC Conversion Stage: After the high-frequency transformer performs voltage level transformation and electrical isolation, a rectifier or inverter converts the high-frequency AC into the DC voltage (e.g., 400V/1kV for EV charging) or utility-frequency AC voltage required by the load, completing the final power delivery.

II. Solid-State Transformer (SST) Architectures

2.1 Cascaded H-Bridge (CHB) and Modular Multilevel Converter (MMC) Architectures
For connecting to medium/high-voltage distribution networks at 10kV, 35kV, or even higher voltage levels, direct high-voltage conversion using single power semiconductor devices is impractical due to their limited voltage withstand capability (mainstream commercial devices are currently 650V-3300V, with a few reaching 6500V). Therefore, modular cascading technology has become the preferred approach for SSTs handling high-voltage inputs.

2.2 Core Isolation Stage: Dual-Active Bridge (DAB) Converter
Regardless of the high-voltage side topology, the core component within an SST for achieving electrical isolation and voltage transformation is typically a high-frequency DC-DC converter. Among these, the Dual-Active Bridge (DAB) is currently the most prominent research and development path.

• Working Principle: The DAB controls the magnitude and direction of power flow by adjusting the phase shift angle between the primary and secondary full-bridge circuits, inherently featuring soft-switching (ZVS/ZCS) characteristics.
  
• Frequency vs. Efficiency Trade-off: To reduce the size of the intermediate high-frequency transformer (size is inversely proportional to frequency), the R&D direction is to continuously increase the switching frequency to 20kHz-100kHz or even higher. However, as frequency increases, switching losses rise sharply. This is precisely where the demand for silicon carbide (SiC) devices is most urgent in domestic SST development.
  
• Device Requirements: The DAB stage requires power devices with extremely low switching losses and excellent reverse recovery characteristics. SiC MOSFET modules (such as the BMF series) from companies like Basic Semiconductor, with their nanosecond switching speeds and turn-off losses tens of times lower than Si IGBTs, have become the key enabler for realizing high-frequency DAB.
  
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Compared to traditional Si devices, adopting wide-bandgap (WBG) devices like SiC and GaN can reduce both switching and conduction losses, further increasing the system's switching frequency and optimizing its performance. A higher switching frequency results in smaller voltage and current ripple, thus allowing for a reduction in the size of passive components like transformers and capacitors, achieving higher power density. Under identical circuit conditions, operating at the resonant frequency, SiC MOSFETs yield lower losses and higher junction temperatures. At this point, the device's switching losses are small. If the switching frequency deviates from the resonant frequency, the proportion of turn-off losses in the power device increases, making the advantages of SiC MOSFETs even more apparent. Therefore, using SiC MOSFETs can further increase the converter's switching frequency, thereby reducing the volume of passive components and enhancing system power density. Simultaneously, for solid-state circuit breakers, higher voltage-rated power devices can significantly reduce the number of cascaded cells in an SST, thus improving power density and reducing control complexity. To decrease the cascade count, it's necessary to increase the voltage level of each cell, for example, raising the rectified DC bus voltage from 800V to 1500V. The increased bus voltage places higher demands on the isolated DC-DC converter. To meet the requirements of a 1500V bus system, approaches include device series-connection, multilevel topologies, or using higher voltage-rated power devices. Considering implementation difficulty and cost-effectiveness, higher voltage-rated power devices represent the most optimal method for achieving high bus voltages and reducing cascade counts.