Solid State Transformer (SST)

Key components inside a solid state transformer

Solid State Transformer Structure: Inside a Packaged Medium-Voltage SST

A packaged, field-grade solid state transformer looks less like a circuit board and more like a substation cabinet. The main sections of a medium-voltage SST enclosure are the following.

Fan and pump section (cooling)

Because an SST concentrates a lot of power into a small volume, thermal management is central. A cooling section houses fans, pumps, a coolant loop, and heat exchangers that carry heat away from the semiconductors and the medium-frequency transformer.

MV dead-front section and door

The medium-voltage compartment uses a dead-front design, meaning there are no exposed energized parts when the door is opened.It holds the MV terminations and bushings and is protected by interlocks so the equipment cannot be accessed while live.

LV dead-front section and door

The low-voltage compartment terminates the LV AC and/or DC output, again dead-front for operator safety, and houses the output-side protection and disconnects.

The tank

Analogous to a conventional transformer tank, this sealed section contains the medium-frequency magnetics and, in many designs, a dielectric coolant, providing insulation, heat transfer, and environmental protection for the most sensitive components.

Customer interface

The interface brings it together: input and output terminals, communications ports, monitoring, and an HMI/SCADA connection so operators can see and control the unit remotely.

Key Features of an SST Solid State Transformer

The features of an SST solid state transformer describe what the device can do that a passive unit cannot:

• Bidirectional power flow for storage, vehicle-to-grid, and DER export
• Active voltage regulation, holding output steady as input or load swings
• Reactive power support
• Harmonic filtering and power factor correction
• Fault current limiting instead of a large uncontrolled fault contribution
• Native DC ports (for example, an 800 V DC bus)
• Built-in monitoring and communications
• A compact, largely oil-free design compared with a conventional distribution transformer

Power Semiconductors for Solid State Transformers: MOSFETs, IGBTs, Diodes, Thyristors, and Gate Drivers

The switching devices are the heart of every solid state transformer. Medium-voltage silicon carbide (SiC) MOSFETs are the workhorse on the MV side and in the high-frequency stages, because they switch fast with low loss and block high voltage, which is exactly what keeps the magnetics small. Insulated-gate bipolar transistors (IGBTs) still appear in some designs, particularly at higher currents or where mature silicon is preferred. SiC Schottky and freewheeling diodes handle rectification and current recirculation. Thyristors can serve in protection, bypass, or crowbar roles, providing a rugged path to divert fault energy.

Tying them together are isolated gate drivers. In an SST they face brutal conditions — very high dv/dt, reinforced insulation requirements across medium voltage, and the need for fast short-circuit protection— so the gate driver is as much a reliability component as the switch it commands. In series-connected device strings, active gate drivers also help balance the voltage each device sees.

SST Control, Sensing, and Auxiliary Power Functions

A solid state transformer runs on a hierarchical control system: a central controller sets the overall targets while per-cell controllers manage each module. Microcontrollers and FPGAs execute the fast control loops, taking in isolated current and voltage measurements and issuing switching commands with tight timing. Fiber-optic links carry signals across the medium-voltage insulation boundary without creating a conductive path, and isolated auxiliary power supplies feed the gate drivers and control electronics safely. A communications layer exposes monitoring, setpoints, and protection status to the outside world — which is also why cybersecurity is now part of the control conversation.

Solid State Transformer vs Conventional Transformer

A solid state transformer and a conventional line-frequency transformer do the same basic job — change voltage and isolate — but almost everything else differs. The table below compares them across twelve attributes.

An SST is not a drop-in replacement for every distribution transformer. It earns its place where control, DC output, or footprint matter more than the lowest AC-to-AC loss and the lowest first cost.

Types of Solid State Transformer Architecture: Single-Stage to Three-Stage

Solid state transformers are grouped by how many conversion stages they use and where their DC links sit.

Newer research proposes broader, multi-dimensional taxonomies of SST design, but the stage-and-DC-link view above covers the practical field.

Modular (multi-cell) vs single-cell designs

Modular approaches — input-series output-parallel cascaded cells and modular multilevel converters — stack many lower-voltage devices to reach medium voltage. This is scalable and fault-tolerant, but it means many parts and constant cell-voltage balancing. Single-cell designs lean on high-voltage SiC devices, using fewer parts but facing harder insulation and device limits.

Hybrid solid state transformer

A hybrid solid state transformer pairs a conventional transformer with a partially rated converter that processes only about 10–20% of the total power, reaching roughly 98.6% efficiency — close to a conventional unit — while the bulk of the load flows through the passive transformer. It is cheaper than a full SST and bulkier than one, and it is a sensible near-term fit for AC grids that mainly need voltage regulation and reactive support rather than a DC bus.

SST Transformer and the Power Grid: Smart Distribution, Microgrids, and DER Integration

On the power grid, an SST transformer is more than a voltage changer — it is a controllable node. In smart distribution it provides fast voltage regulation and reactive power support, filters harmonics, and precisely controls power flow.Its bidirectional DC and AC ports make it a natural interface for distributed energy resources — solar, wind, and storage — improving a feeder’s hosting capacity for variable generation. In microgrids and hybrid AC/DC grids it can act as an energy router between subsystems,coordinating islanded and grid-connected operation. And because monitoring is built in, it doubles as a grid-edge sensor, giving operators visibility they never had from passive iron. These are the same dynamics we model in our work on power grid stability and grid-forming inverters.

Solid State Transformer Advantages and Limitations

Advantages

Solid state transformers pack a lot of capability into a small package. Their high-frequency operation makes the magnetics far smaller and lighter. They offer native DC ports, active voltage regulation, and power-quality services such as harmonic filtering, power factor correction, and reactive support. They support bidirectional flow for storage and vehicle-to-grid, isolate faults and limit fault current, and produce negligible inrush. Monitoring and communications are built in, and they use far less insulating oil than a conventional distribution transformer — a maintenance and environmental benefit.

Limitations and engineering challenges

The limitations are real and mostly economic and reliability-related. First cost is high, driven by SiC devices, medium-voltage packaging, and certification. There is an efficiency penalty in pure AC-to-AC service, where a conventional transformer’s roughly 99% is hard to beat. Field reliability under surges, heat, and years of medium-voltage stress is still being proven. Insulation and partial discharge in the medium-frequency transformer are demanding, as is managing dv/dt stress on devices and insulation. Modular designs require constant cell-voltage balancing. Because an SST limits rather than supplies large fault current, downstream protection coordination must be rethought — an SST simply cannot deliver the large, sustained fault current that conventional protective devices are set to detect. Cybersecurity becomes a design requirement once a transformer has firmware and communications. SiC device supply and immature standards round out the list.

Hype vs reality in 2026 The demand signals are genuine: DC-native data center architectures, acute transformer lead-time pressure, the first 22.9 kV-class designs announced at roughly 1 MVA, and medium-voltage products now scaling into the multi-megawatt range. But utility-wide replacement of distribution transformers is still years away. The near-term wins are DC-heavy sites — data centers and charging depots — not wholesale swaps of the passive fleet. Treat this section as time-sensitive and refresh it quarterly.

Standards and Certification for Solid State Transformers

There is no single published SST product standard yet. An IEEE recommended practice for the design and integration of solid state transformers in the electric grid, project IEEE P3105, is in development,and until it publishes, the ecosystem relies on a patchwork of existing standards that each touch part of an SST.