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Amid the industrial wave driven by the new power system, renewable energy grid integration, DC microgrids, fast charging for new energy vehicles, power supply and distribution for data centers, and smart grid upgrades, the technical shortcomings of traditional power frequency transformers are becoming increasingly apparent. Issues such as bulky size, heavy weight, lack of power regulation capabilities, inability to adapt to hybrid AC/DC networks, weak harmonic suppression, and high O&M costs no longer align with the current development needs for power equipment—which demand miniaturization, intelligence, high controllability, high efficiency, and bidirectional power flow. Against this backdrop of industry transformation, the Solid-State Transformer (SST) has emerged as a core next-generation power device designed to replace traditional power-frequency transformers and reshape modern power conversion systems. It is hailed by the industry as the “smart power router” and the “next-generation core conversion equipment for the power grid.”
Unlike traditional transformers, which operate passively through electromagnetic induction in an iron core, the SST is a fully controllable, intelligent power conversion system that integrates wide-bandgap semiconductor power devices, high-frequency magnetic materials, high-frequency winding technology, multi-level power electronic conversion, and digital intelligent control. It completely breaks the century-old technical paradigm of 50/60 Hz power frequency electromagnetic conversion. Through high-frequency power electronic conversion technology, it achieves integrated functions such as voltage conversion, electrical isolation, power quality management, bidirectional power flow, and multi-port AC/DC output, making it a revolutionary product in the fields of renewable energy, smart grids, and high-end power electronics.
A solid-state transformer (SST), also known as a power electronics transformer or intelligent solid-state power conversion unit, is a new generation of intelligent power conversion equipment built on high-frequency power electronics conversion technology, high-frequency electromagnetic isolation technology, and digital intelligent control technology. It is entirely distinct from traditional power-frequency electromagnetic induction transformers. In layman’s terms, traditional transformers are “passive electromagnetic conversion devices,” whereas SSTs are “active, intelligent power regulation and conversion systems.”
According to authoritative industry definitions, the SST abandons the operating mode of traditional transformers—which rely on bulky silicon steel core laminations and low-frequency power-frequency windings to achieve voltage conversion. Instead, it uses wide-bandgap semiconductor power devices such as SiC and GaN as core switching units, employs high-frequency magnetic components as isolation media, and utilizes multi-stage AC/DC, DC/DC, and DC/AC power electronic topologies to convert low-frequency utility-frequency electrical energy into high-frequency electrical energy, thereby achieving electrical isolation and voltage-level transformation, before converting it back into standard utility-frequency AC and DC electrical energy. Throughout the entire conversion process, the device can use digital algorithms to regulate voltage, current, phase, power factor, and power flow direction in real time, enabling intelligent control functions that traditional transformers are completely incapable of.
Within the industrial ecosystem of the new power system, the SST holds a highly strategic position and serves as a core, critical component for renewable energy grid integration, DC microgrids, integrated photovoltaic-storage systems, electric vehicle supercharging stations, data center power supply, rail transit traction power supply, and smart distribution grids. While traditional transformers can only perform the two basic functions of “voltage transformation” and “isolation,” the SST integrates more than ten core functions, including voltage step-up/step-down, electrical isolation, bidirectional power transmission, harmonic mitigation, reactive power compensation, voltage stabilization, fault current limiting, AC/DC multi-port output, and intelligent networked dispatch. It is truly an integrated smart power energy router.
From the perspective of product evolution, the power industry has undergone three generations of transformer technology: the first generation consisted of purely mechanical power-frequency transformers; the second generation featured on-load tap-changing transformers with basic voltage regulation capabilities; and the third generation is the SST solid-state smart transformer. Compared to the first two generations, the SST’s greatest innovation lies in its evolution from a passive device to an actively controllable smart device, and from a single-function power-frequency AC converter to a comprehensive, high-frequency, high-efficiency, multi-energy, and controllable power conversion platform—one that perfectly adapts to the core characteristics of renewable energy: randomness, volatility, and bidirectional flow.
At the same time, the SST serves as the core vehicle for the lightweight, low-carbon, and intelligent upgrading of power equipment. Traditional oil-immersed transformers require large amounts of silicon steel, copper, and transformer insulating oil; they are bulky, extremely heavy, and pose a fire hazard due to oil leaks. In contrast, the SST does not require oil-immersed cooling, has no large power-frequency core, features a compact structure, is environmentally friendly, and is simple to operate and maintain—fully aligning with the development trends toward miniaturization, integration, and environmental sustainability in modern power equipment.
Many industry professionals are prone to a common misconception: they believe that SSTs are simply smaller versions of traditional transformers operating at higher frequencies, and that they essentially still rely on electromagnetic transformation. In fact, the operating principle of an SST belongs to an entirely different technological framework than that of a traditional transformer. Traditional transformers rely purely on physical, power-frequency electromagnetic induction, with no conversion of electrical energy forms and no intelligent control; in contrast, the SST employs a composite operating mechanism consisting of “multi-stage electrical energy conversion + high-frequency electromagnetic isolation + precise digital regulation.” The entire operational process is divided into a three-stage energy conversion logic, with each stage building upon the previous one—all of which are indispensable.
The SST’s input is connected to the utility grid’s power frequency AC power (50 Hz/60 Hz). First, the front-end rectification topology converts the AC power into high-voltage DC power. This stage does not use ordinary uncontrolled diode rectification but instead employs a PWM-controlled rectification topology. Leveraging the high-speed switching characteristics of wide-bandgap semiconductor devices, it achieves high-precision, controllable rectification. The core function of this stage is not only to convert the form of electrical energy but also to provide exceptional power quality management capabilities. It actively suppresses grid harmonics, corrects the power factor, and stabilizes the input voltage and current waveforms, transforming fluctuating grid power into smooth and stable high-voltage DC power, thereby providing a clean power foundation for the subsequent high-frequency conversion stage. This is a front-end regulation capability that traditional transformers completely lack.
This is the core of the SST system and the fundamental source of its compact size and high efficiency. The high-voltage DC power obtained through the front-end rectification is converted into high-frequency AC power ranging from several kilohertz to tens of kilohertz via high-frequency topologies such as the dual active bridge (DAB) and CLLC resonant circuits, and then fed into the built-in high-frequency isolation transformer. Relying on the principle of high-frequency electromagnetic induction, this stage achieves electrical isolation between high and low voltages and matches the voltage transformation ratio, before being rectified back into low-voltage DC power.
Key industry principle: Transformer size and weight are inversely proportional to operating frequency. Traditional 50 Hz power-frequency transformers require oversized cores and heavy windings to achieve energy coupling, whereas SST increases the operating frequency by 20 to 400 times. This significantly improves the efficiency of high-frequency magnetic flux changes, drastically reducing the required core cross-sectional area and number of winding turns, and directly reducing the device’s size and weight by 60% to 80%. At the same time, this tier achieves complete electrical isolation, replacing the power-frequency isolation function of traditional transformers, while comprehensively upgrading isolation efficiency, stability, and safety.
The low-voltage DC power, after undergoing high-frequency isolated conversion, passes through the downstream inverter topology to be converted back into standard power-frequency AC according to load requirements, or directly outputs DC, thereby achieving dual AC/DC port output. This stage is equipped with high-precision digital control algorithms that dynamically adjust output voltage, current, and frequency in real time, ensuring stable power output regardless of input grid voltage fluctuations or sudden load changes. It also supports bidirectional power flow; when the load side includes energy storage or power generation equipment, energy can be fed back into the grid, making it perfectly suited for bidirectional renewable energy networking scenarios.
A simple summary of the SST’s complete operating chain: fluctuating utility-frequency AC → controlled rectification into pure high-voltage DC → high-frequency inversion to achieve high-frequency isolation transformation → low-voltage DC voltage regulation → controllable AC/DC output. This entire process achieves a multi-stage controllable conversion cycle of “power frequency–DC–high frequency–DC–power frequency,” completely overcoming the passive limitations of traditional power-frequency electromagnetic conversion. It integrates functions such as voltage transformation, isolation, voltage regulation, filtering, reactive power compensation, bidirectional power transmission, and fault protection into a single unit, enabling comprehensive intelligent control of power conversion.
The SST is not a single magnetic component, but rather a highly integrated, modular power electronics system. Its overall structure is divided into four core modules: the input preprocessing module, the front-end AC-DC rectification module, the high-frequency isolated DC-DC conversion module, and the back-end DC-AC/DC output module. These are complemented by digital control, driver, thermal management, and protection modules. The structure features a clear hierarchical organization, a well-defined division of functions, and a modular design that facilitates maintenance and scalability.
Serving as the SST’s entry-point protection unit, this module primarily comprises EMI filtering circuits, surge protection circuits, and current-limiting protection circuits. Its core function is to filter out high-frequency interference from the power grid, suppress lightning surges, and prevent inrush currents during switching operations. It protects downstream semiconductor devices and high-frequency transformers from damage caused by power grid disturbances, ensuring the safety and stability of the system’s input side, and acts as the first line of defense for the SST’s reliable operation.
This module is centered around SiC and GaN wide-bandgap power devices, paired with high-precision sampling circuits and a PWM control unit. Compared to traditional silicon-based devices, wide-bandgap devices feature faster switching speeds, lower losses, and greater high-temperature resistance, making them well-suited for high-frequency, high-precision conversion applications. This module enables unity power factor operation, actively mitigates grid harmonics, suppresses voltage flicker, and converts unstable utility-frequency AC power into a high-precision high-voltage DC bus voltage, providing a stable energy input for the core high-frequency conversion unit.
This is the SST’s core functional module and the key structural element that distinguishes it from traditional transformers. It consists of three parts: a high-frequency inverter bridge, a high-frequency isolation transformer, and a high-frequency rectifier bridge. The high-frequency inverter bridge converts high-voltage DC into high-frequency AC; electrical isolation and voltage matching are achieved via the high-frequency transformer, after which the high-frequency rectifier bridge outputs low-voltage DC. This module handles the core tasks of energy isolation, voltage conversion, and loss control for the entire system. The material of the high-frequency transformer’s windings, its structural design, and loss control directly determine the overall efficiency and temperature rise performance of the SST.
The output module supports dual-mode operation, allowing flexible configuration of either utility-frequency AC output or pure DC output based on the application scenario. This makes it perfectly suited for diverse applications such as AC loads, DC energy storage, photovoltaic systems, and charging stations. Leveraging digital closed-loop control algorithms, it adjusts output parameters in real time and features functions such as voltage stabilization, current stabilization, current limiting, overload protection, and short-circuit protection. Its response time is in the millisecond range, far faster than the mechanical voltage regulation response of traditional transformers.
The SST is equipped with a DSP+FPGA dual-core digital control system, serving as the “brain” of the entire unit. It can collect real-time data on input and output voltage, current, temperature, and power, and dynamically adjust the switching frequency, duty cycle, and power flow direction to achieve intelligent voltage regulation, reactive power control, harmonic suppression, and bidirectional power dispatch. It also features self-diagnostic testing, fault lockout, remote communication, and data upload capabilities. By integrating with smart grid backends, it enables remote monitoring, intelligent operation and maintenance, and networked coordination, making it a truly intelligent power device.
Unlike the oil-immersed cooling used in traditional transformers, the SST employs dry, high-efficiency cooling solutions such as air cooling, water cooling, and phase-change cooling. It contains no insulating oil, eliminating the risk of leaks and fire hazards. The unit features a compact design and high integration. Its cooling system is precisely matched to the temperature rise characteristics of semiconductor devices and high-frequency transformers, ensuring long-term, stable, full-load operation under high-frequency conditions.
Leveraging high-frequency power electronics technology and intelligent control systems, SST solid-state transformers have overcome the century-old technical bottlenecks of traditional power-frequency transformers. They offer comprehensive advantages across six key dimensions—equipment performance, energy efficiency, space utilization, intelligence, safe operation and maintenance, and adaptability to various scenarios—making them a core driving force behind the upgrade of next-generation power systems.
Thanks to its high-frequency operating mechanism, the SST completely eliminates the bulky structure of traditional power-frequency transformers—with their large iron cores and windings—achieving a power density far exceeding that of conventional transformers. At equivalent power ratings, the SST occupies only one-third to one-fifth the volume of traditional equipment and weighs over 70% less. It eliminates the need for large distribution rooms or transformer bases, allowing for direct wall mounting, modular integration, and outdoor installation. This significantly conserves land and space resources in substations, distribution rooms, and equipment rooms, making it ideal for space-constrained scenarios such as dense urban power distribution, compact data center layouts, and vehicle- and aircraft-mounted power equipment.
The SST utilizes wide-bandgap semiconductor low-loss devices, high-frequency low-loss magnetic materials, and low-loss Litz wire windings to minimize conversion losses across three dimensions: components, magnetic cores, and wiring. The overall efficiency of the unit exceeds 98.5%, and efficiency remains stable even under light-load conditions, completely resolving the pain points of traditional transformers—high no-load losses and low efficiency under light loads. In power distribution, energy storage, and data center scenarios that operate continuously throughout the year, the SST can save a significant amount of energy annually, aligning with the “Dual Carbon” policy and enterprises’ needs for energy conservation and cost reduction, resulting in exceptionally outstanding long-term economic benefits.
Renewable energy sources such as solar and wind power are characterized by randomness, volatility, and intermittency. When connected to the grid, they can easily cause issues such as excessive grid harmonics, voltage fluctuations, three-phase imbalance, and low power factor—problems that traditional transformers cannot resolve. In contrast, the SST features built-in integrated power quality management capabilities, enabling active filtering, voltage stabilization, reactive power compensation, and three-phase balancing. It eliminates the need for additional filters or reactive power compensation devices; a single unit can perform multiple functions—including transformation, isolation, and power quality management—significantly reducing system integration costs and making it perfectly suited for renewable energy grid-connection scenarios.
The core characteristics of the new power system are bidirectional interaction among generation, grid, load, and storage, as well as hybrid AC/DC networking. The unidirectional power transmission model of traditional transformers cannot accommodate the demands of reverse grid-injection from energy storage, grid-connected distributed generation, and bidirectional load interaction. The SST can precisely control the direction of bidirectional power flow, flexibly enabling intelligent switching between grid power supply, energy storage discharge, and renewable energy grid-connection. It also supports multi-port AC/DC output, allowing direct connection to photovoltaic systems, batteries, DC loads, and AC loads, making it the core hub device for hybrid AC/DC microgrids.
Equipped with a high-speed digital control system, the SST responds to voltage, current, and fault conditions within milliseconds. It can suppress grid disturbances in real time, rapidly isolate faults, limit short-circuit currents, and prevent the propagation of grid faults. Compared to the second-level response time of traditional transformers’ mechanical voltage regulation, the SST’s dynamic control capabilities significantly enhance grid operational stability, effectively reduce the probability of grid failures, and improve power supply reliability.
The SST employs a standardized modular design that allows for flexible parallel expansion to meet power demands without requiring a complete equipment replacement, making it compatible with distributed and modular power networking models. The equipment can be flexibly adapted to various scenarios, including power distribution, energy storage, charging, industrial frequency conversion, and rail transit, offering versatility and scalability far exceeding those of traditional fixed transformers.
The high-frequency transformer is the core of the SST, and Litz wire (SST-Litz Wire) is the only suitable winding material for SST high-frequency transformers. It serves as the fundamental material that ensures the SST’s high-frequency, high-efficiency, low-temperature rise, and highly stable operation. The root cause of the vast majority of issues with SST equipment—such as excessive heat generation, substandard efficiency, and excessive high-frequency losses—stems from incorrect wire selection, specifically the misuse of ordinary solid enameled copper wire in place of specialized Litz wire. This section provides an in-depth analysis of the core principles, product advantages, selection criteria, and application value of Litz wire for SST high-frequency operating conditions.
Fatal Flaws of Ordinary Copper Wire in SST High-Frequency Conditions
SST systems typically operate at frequencies ranging from 10 kHz to 100 kHz, which falls within the typical high-frequency power electronics operating range. Under the influence of high-frequency alternating magnetic fields, ordinary solid enameled copper wire exhibits two major, fatal loss effects: the skin effect and the proximity effect.
Skin Effect: High-frequency current no longer flows uniformly through the cross-section of the wire but is concentrated solely on the wire’s surface, leaving the central region unused. This results in a significant reduction in the effective conductive area, a sharp increase in equivalent resistance, and a dramatic rise in eddy current losses.
Proximity Effect: When multiple turns of windings are closely packed, the high-frequency magnetic fields of adjacent conductors interfere with one another, resulting in extremely uneven current distribution—with localized current concentration and localized no-load conditions—further amplifying high-frequency losses.
When these two effects combine, ordinary copper wire in SST high-frequency transformers will experience severe overheating, a drastic drop in efficiency, excessive temperature rise, and accelerated insulation aging. This directly results in the SST failing to meet energy efficiency standards, a shortened service life, and frequent failures, making it completely unable to satisfy the design requirements for SSTs.
Structural Principles of Litz Wire and Logic for Suppressing High-Frequency Losses
Litz wire is a specialized composite electromagnetic wire developed specifically for high-frequency applications. Its structure consists of multiple ultra-fine, individually insulated enameled copper wires formed through precise bundling, transposition, and stranding processes. Each individual wire has its own independent insulation layer and, during the stranding process, uniformly traverses both the inner and outer magnetic field regions of the wire bundle.
Its core operating principle: the ultra-fine single-strand structure mitigates the skin effect, while the overall interleaving structure balances the magnetic field position and current distribution of each individual strand, completely offsetting high-frequency proximity effects. This ensures that the current is uniformly distributed across the entire cross-sectional area of the conductor, maximizing the utilization of the conductive area and reducing high-frequency eddy current losses at the source. In the high-frequency range of 10 kHz to 1 MHz, Litz wire can reduce high-frequency losses by 30% to 60%, making it the optimal wire exclusively designed for SST high-frequency transformers.
As a once-in-a-century revolutionary technology in the power industry, the Solid-State Transformer (SST) has completely transformed the passive operating mode of traditional power-frequency transformers. Through multi-stage power electronic conversion, high-frequency isolation technology, and intelligent digital control, it has achieved a leapfrog upgrade, transforming the transformer from a “passive electromagnetic device” into an “active, intelligent energy router.”
With core advantages such as miniaturization, high efficiency, intelligent controllability, power quality management, bidirectional power flow, and environmental safety, SSTs are perfectly suited for high-end emerging applications such as new power systems, renewable energy grid integration, energy storage microgrids, ultra-fast charging, data center power supply, and rail transit power supply. They represent the core development direction for the future of power equipment—toward intelligence, lightweight design, high efficiency, and low-carbon operation.
At the same time, the industrialization of SST relies heavily on the support of core supporting materials, particularly Litz wire specifically designed for high-frequency transformers. As the sole core wire material capable of addressing the challenges of high-frequency losses and temperature rise in SST, the selection and quality of Litz wire directly determine the energy efficiency, stability, and service life of SST equipment, making it an indispensable key link in the SST industrial chain.
