What other "states" are there besides the "solid‑state" of transformers?
Many readers may share the same confusion as Toutiao's editorial team: Is there really no "liquid‑state" transformer? What does the industry mean by the term "mechanical‑state"? And what exactly is a solid‑state transformer?
To answer these questions, we need to start with the operating principle of conventional power‑frequency transformers.
In 1831, physicist Michael Faraday discovered a remarkable phenomenon: a changing magnetic field can "induce" an electric current in a closed coil. This gave birth to the principle of electromagnetic induction.
Common conventional power‑frequency transformers today achieve voltage transformation based on this very principle of electromagnetic induction.
Operating at 50 Hz or 60 Hz, conventional power‑frequency transformers perform voltage conversion via silicon‑steel sheet iron cores and copper windings. When alternating current flows into the primary winding, an alternating magnetic field is generated inside the transformer. The magnetic flux passes through the iron core and induces voltages of different magnitudes in the secondary winding, realizing voltage transformation.
Conventional power‑frequency transformers contain no power electronic devices. They consist merely of copper windings, silicon‑steel sheet iron cores, oil tanks and other components, operating purely according to physical laws.
Now let us look at the Solid‑State Transformer (SST), also known in the industry as the Power Electronic Transformer (PET). Unlike conventional power‑frequency transformers, it accomplishes electric‑energy conversion and power regulation using semiconductor solid‑state switching devices.
Solid‑state transformers adopt an indirect yet highly flexible voltage‑conversion path: AC → DC → high‑frequency AC → DC/AC.
Alternating current entering a solid‑state transformer is first rectified into direct current by semiconductor devices. The resulting direct current is then inverted into high‑frequency alternating current and fed into an internal compact high‑frequency isolation transformer for electrical isolation and voltage transformation. Finally, the current undergoes another round of rectification or inversion to deliver electricity at the target voltage level.
A solid‑state transformer does not employ the bulky, heavy coils and iron cores found in conventional power‑frequency transformers. Its system is controlled by power‑electronic components, namely semiconductor solid‑state switching devices.
From the above analysis, "solid‑state" does not refer to the state of matter as in a solid. Instead, it can be interpreted as a "stationary state". While conventional power‑frequency transformers rely on the motion of mechanical structures to generate magnetic fields for voltage transformation, solid‑state transformers are fully controlled by solid‑state semiconductor chips with no mechanical movement - hence the name "solid‑state".
In fact, solid‑state devices form a broad product family. Besides solid‑state transformers, the well‑known solid‑state drives operate on analogous principles.
Therefore, this "solid‑state" is not that "solid‑state". There are no so‑called "liquid‑state" or "gaseous‑state" versions as one might imagine.
Conventional transformers have served us for over a century. Why develop solid‑state alternatives?
Since the discovery of electromagnetic induction, transformer technology has evolved over more than a hundred years, delivering proven stability and durability.
Why then have solid‑state transformers emerged at this juncture?
Simply put, modern new‑type power systems face richer and more diverse demand scenarios.
New applications such as AI computing‑intensive data centers, new‑energy grid integration and smart microgrids are continuously emerging. These bring about AC‑DC hybrid power supply, dynamically fluctuating loads, and demands for intensive use of space resources. Conventional power‑frequency transformers, with their limited functions, long energy‑conversion chains, large footprint and relatively low efficiency, can no longer adequately meet these requirements.
To draw an analogy: a conventional power‑frequency transformer is comparable to a water tower that works solely by water‑pressure differentials. A solid‑state transformer, by contrast, is an integrated water‑supply unit equipped with variable‑frequency water pumps, intelligent electronically‑controlled valves and sealed pressure‑bearing water tanks. It is more compact and far smarter.
Its core strengths can be summed up in five attributes: compact, flexible, fast, versatile, intelligent.
**Compact · Reduced Footprint**
Solid‑state transformers feature higher power density. At equal power ratings, their volume is roughly 70 % smaller than that of conventional power‑frequency transformers. In space‑constrained environments such as data centers, solid‑state transformers can save substantial floor space.
**Flexible · Independent Controllability**
Unlike conventional transformers, solid‑state transformers are actively controllable. They can independently regulate output voltage, active power and reactive power, integrating functions including reactive‑power compensation, harmonic mitigation and voltage support.
**Fast · Millisecond‑Scale Fault Isolation**
Solid‑state transformers achieve fault isolation within milliseconds. They can block bidirectional fault propagation and effectively enhance the security of power‑grid supply.
**Versatile · Adaptable to Diverse Scenarios**
Conventional power‑frequency transformers only perform AC‑to‑AC conversion and cannot directly output direct current, making them unsuitable for DC distribution networks and new‑type loads such as AI computing‑intensive centers. Solid‑state transformers support multi‑port AC and DC operation, flexibly outputting either alternating or direct current to suit DC‑load scenarios including energy‑storage systems and computing‑intensive centers. In addition, they enable bidirectional power flow, fitting applications such as wind‑solar‑storage systems and source‑grid‑load bidirectional interaction in flexible distribution networks.
**Intelligent · An Electronic "Brain"**
Conventional power‑frequency transformers have fixed voltage transformation ratios. Dynamic voltage regulation depends on external on‑load tap‑changers with limited adjustment ranges. Solid‑state transformers feature high‑level digitization and support remote coordinated dispatching.
Will solid‑state transformers displace conventional power‑frequency transformers?
The term "Solid‑State Transformer" first appeared in power‑electronics‑related patents and literature from the 1960s through the 1980s.
In the 21st century, several United‑States‑based institutions launched systematic research on solid‑state transformers for medium‑voltage distribution‑network applications, and the technology became a research hotspot for smart distribution grids and the Energy Internet.
Statistics show that the global solid‑state transformer market is expanding rapidly at a compound annual growth rate of nearly 60 %. The total market size is projected to exceed one hundred billion yuan by 2030.
China's research on solid‑state transformers ranks among the world's top tier. The country possesses full‑fledged capabilities in system integration and engineering implementation, with leading real‑world deployments worldwide. Driven by industries including computing‑infrastructure construction and new‑type power‑system development, China has become the world's largest single regional market for solid‑state transformers. Its global market share is expected to reach 75 % by 2030.
In terms of application‑sector composition and market distribution, AI computing‑intensive data centers, driven by rigid requirements for high‑density computing power supply and high energy efficiency, are poised to become the primary large‑scale commercial application scenario for solid‑state transformer technology.
So will solid‑state transformers take the place of conventional power‑frequency transformers?
The answer is no in the short‑to‑medium term.
Restrained by complex system architectures, high upfront investment, and the need to accumulate more long‑term on‑grid operational data, solid‑state transformers will not replace conventional power‑frequency transformers on a large scale in the near future. Instead, they will serve predominantly as flexible power‑conversion equipment for supplementary deployment in characteristic scenarios of new‑type power systems. As wide‑bandgap semiconductor device costs decline and relevant standard systems mature, conditions for the industrial‑scale roll‑out of solid‑state transformers will gradually improve.
Since 2013, solid‑state transformer products developed by State Grid Electric Power Research Institute (NARI Group Corporation) have been put into batch operation across Jiangsu, Zhejiang, Hebei and other provinces. In 2018, the ±10 kV / 2 MW fault‑ride‑through DC solid‑state transformer (NES‑5660) developed by the same institute became the industry's first DC solid‑state transformer complying with the latest bipolar short‑circuit specifications and meeting top‑tier fault‑ride‑through requirements; it was deployed in a demonstration project in Wujiang, Suzhou.
Today, State Grid Electric Power Research Institute (NARI Group Corporation) boasts complete industrial‑collaboration capabilities for solid‑state transformers and leading R&D and test platforms. It has built a full‑value‑chain layout covering key components such as power devices and high‑frequency transformers; core technologies including high‑voltage cascading, flexible control and fault ride‑through; as well as complete‑machine integration and factory acceptance testing of solid‑state transformers. Core devices and technologies are domestically self‑controllable.
The newly launched second‑generation all‑scenario "Rui‑series" solid‑state transformers comprise four product lines: Ruipan, Ruiting, Ruirong and Ruishu, tailored respectively for data centers, ultra‑fast charging stations, new‑energy grid integration and smart‑grid applications. Compared with the prior generation, the second‑generation models deliver roughly doubled power density and 50 % lower power losses. The under‑development third‑generation products adopt higher‑capacity silicon‑carbide devices to achieve substantial increases in unit capacity and overall power density.