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Solid-State Transformers


A solid-state transformer (SST) is a power electronics device that converts voltage and manages energy flow. It replaces the passive iron-core transformer, a design that has stayed architecturally unchanged for over a century, with an active, software-controlled system built on wide-bandgap semiconductors (SiC and GaN).

A conventional transformer steps voltage up or down passively through electromagnetic induction. An SST actively converts AC to high-frequency AC, then to DC and back. That allows bidirectional power flow, real-time voltage and frequency regulation, power quality conditioning, and AC and DC ports at different voltages on one device.

Three forces are driving demand for SSTs at the same time:

  • AI-scale data centers that need 500 kW to 1+ MW per rack at 800V DC
  • Renewable generation with variable voltage and frequency that passive transformers cannot manage
  • EV fleet depots, gigafactories and grid-scale BESS (battery energy storage system) sites that need faster interconnection than conventional transformer supply chains can deliver

Lead times for large conventional transformers run two to four years, and SSTs are the main technology being developed to address the shortage. Research institutions and major OEMs have studied and piloted SSTs for two decades. AI-scale data centers and renewable buildouts now create volumes large enough to justify factories instead of prototypes, though execution risk remains significant.


SST vs. Conventional Transformer

Attribute Conventional Transformer Solid-State Transformer
Conversion Principle Passive induction; 50/60 Hz AC only Active power electronics; programmable frequency
Size & Weight Large, heavy, oil-filled Compact, modular; core 100-1000x smaller
Power Flow Unidirectional; fixed ratio Bidirectional; variable ratio; multi-port
Voltage Regulation Fixed turns ratio; slow tap changers Software-defined; millisecond response
Power Quality No active filtering Built-in PFC, harmonic filtering, sag compensation
Fault Isolation Passive; needs external relay and breaker Integrated detection and fast isolation
Grid Services None Frequency, reactive power, voltage, demand response
DC Output None; needs separate inverter/rectifier Native DC port (Heron Link: 600V DC)
Supply Chain & Lead Time 2-4 years for large units; China-dominated Shorter if SiC supply secured
Cost Lower upfront; well-understood lifecycle Higher upfront; falling with SiC costs

Implications

  • Conversion Principle: An SST uses AC-HF AC-DC conversion, so it can output AC or DC at controlled voltage; a conventional transformer cannot output DC or regulate dynamically.
  • Size & Weight: High-frequency operation shrinks the magnetic core by 100-1000x, while conventional units are oil-filled for cooling with a fixed footprint. SSTs fit spaces conventional transformers cannot and allow distributed deployment closer to load.
  • Power Flow: Conventional power flows only primary to secondary. SST simultaneous multi-port management supports V2G, solar export and BESS charge/discharge from a single device.
  • Voltage Regulation: Conventional tap changers give only coarse, slow adjustment. Real-time SST regulation handles renewable generation variability that passive transformers cannot manage without additional equipment.
  • Power Quality: Conventional transformers pass harmonics, sags and surges through to the secondary. Active power factor correction, harmonic filtering and sag compensation in the SST eliminate separate power quality equipment (UPS, filters, capacitor banks) at sensitive loads.
  • Fault Isolation: In a conventional transformer faults propagate across the winding. In an SST a secondary fault does not propagate to the primary, decoupling grid faults from sensitive loads and improving site resilience without a separate UPS.
  • Grid Services: A conventional transformer has no sensing or control capability. SST grid services are all software-configurable, making the SST a grid asset that can generate ancillary service revenue for the site operator.
  • DC Output: The SST DC port aligns with 800V data center and EV charging standards and replaces the transformer + inverter + rectifier skid, eliminating multiple points of failure and equipment cost.
  • Supply Chain & Lead Time: Conventional transformers faced a critical shortage in 2024-2026. SSTs are modular and semiconductor-based; Heron Power targets US domestic production with a 40 GW/year factory. SSTs offer supply chain independence from the transformer bottleneck delaying grid connections globally.
  • Cost: Conventional units cost less upfront for commodity applications. SST system savings from equipment consolidation may exceed the hardware premium; Baglino (Heron) says SSTs can "remove 70% of the gear involved" and system-level savings can be "an order of magnitude" for data centers.

How an SST Works

An SST converts power in three stages:

  • Stage 1: a SiC-based AC/DC converter rectifies medium-voltage AC from the distribution grid (typically 4-35 kV) to high-voltage DC.
  • Stage 2: the HVDC is converted to high-frequency AC (typically 10-100 kHz) through a high-frequency transformer. At these frequencies the magnetic core is orders of magnitude smaller than in a 50/60 Hz design, so SSTs are much smaller than conventional transformers.
  • Stage 3: the high-frequency AC is rectified or inverted to the required output: low-voltage DC for EV charging, data center power or BESS, or low-voltage AC for conventional loads.

Each stage depends on wide-bandgap power semiconductors: mainly SiC MOSFETs, with GaN emerging for lower-voltage stages. SiC supports switching at 10-100 kHz (against 50/60 Hz) and junction temperatures of 200°C+ (against 150°C for silicon). Those two properties make SST miniaturization possible.

Real-time control software runs switching sequences, power factor correction, fault detection, voltage regulation and grid services at the same time. The software makes an SST a programmable energy router instead of a fixed voltage converter.


Primary Applications

Application SST Role Commercial Stage Lead Vendors
Hyperscale Data Centers MV-to-DC conversion, 600-800V DC Pre-production (2027) Heron Power
Utility-Scale Solar PV Replaces inverter skid Development Heron Power, DG Matrix, Resilient Power
Grid-Scale BESS Bidirectional MV-DC interface Pilot Heron Power
EV Fleet Depot / MCS MW-class DC for Megawatt Charging Production-ready (2026) WattEV, Heron Power
Distribution Grid Modernization Solid-state substation; MVDC feeder Pilot Hitachi Energy, ABB, Siemens
Rail Traction Power Onboard transformer replacement Commercial ABB, Siemens
Microgrids & Hybrid AC/DC Sites Multi-port energy router Pilot ABB, Schneider Electric

Application Notes

  • Hyperscale Data Centers: The SST replaces the transformer + UPS + PDU stack and delivers 600-800V DC to AI racks. Nvidia's 800V rack architecture requires DC distribution; an SST delivers MV-to-DC in one device, eliminating multiple conversion stages, and embedded ride-through eliminates a separate UPS. Heron Link targets 2027, with Intersect Power and Crusoe (1.2 GW, Abilene TX) as early customers.
  • Utility-Scale Solar PV: The SST replaces the inverter skid (transformer + inverter + switchgear), the shipping-container assembly that dominates utility solar substations, with MV interconnection in a single unit. Solar variability requires active voltage management. Solar is Heron Power's primary target market.
  • Grid-Scale BESS: BESS needs bidirectional power flow a conventional transformer cannot provide. The SST consolidates the PCS and transformer into a single device with millisecond response for frequency regulation. BESS is about 1/3 of Heron's current business per Baglino.
  • EV Fleet Depot / MCS: MCS sites need 1-3 MW DC at high voltage; the SST delivers it without a separate MV transformer and replaces the MV transformer + DCFC power cabinet. The WattEV SST (1.2-3.8 MW) targets depot MCS and is production-ready in 2026; Heron targets dense EV charging sites.
  • Distribution Grid Modernization: Feeders with high DER penetration (solar, BESS, EV) need active management passive transformers cannot provide. SSTs enable bidirectional feeder flow, DER integration and grid-forming at feeder level. EPRI and national labs run demonstration programs.
  • Rail Traction Power: Weight and volume reduction is critical onboard. Uses include locomotive transformer replacement and trackside substation modernization. ABB (SFT onboard) and Siemens have commercial SST deployments in European rail, the most commercially mature SST application globally.
  • Microgrids & Hybrid AC/DC Sites: Hybrid AC/DC microgrids need a device that can simultaneously interface the AC grid, DC BESS and DC loads at different voltages, managing solar, BESS, grid and DC loads; the SST is the only single-device solution. ABB and Schneider Electric run campus microgrid pilots, with growing interest from FED designers.

Vendors & Development Status

Vendor Country Product / Platform Target Markets Status
Heron Power US Heron Link 5 MW modular SST Data centers, solar, BESS, EV charging Pilot production 2027
Hitachi Energy CH/JP Power electronics transformer platforms Distribution, data centers, renewables R&D and utility pilots
Siemens Energy DE Distribution SST; rail traction SST (SFT) Distribution, rail, campus microgrids Rail commercial; distribution pilot
ABB CH PCS100 SST family Marine, industrial, microgrid, distribution Pilots; marine most advanced
Eaton US/IE SST for EV depots and DER feeders EV depots, commercial buildings, DER Development and pilots
WattEV US MCS depot SST, 1.2-3.8 MW Electric truck depots, MCS Production-ready 2026
DG Matrix US Matrix Power Router Solar, BESS, EV depots Pilots
Resilient Power US SST for solar and data centers Solar, data center power Development

Vendor Notes

  • Heron Power: Heron Link has 34.5 kV input, 600V DC output, hot-swap modules and embedded Li-ion ride-through (~30 sec), and aligns with Nvidia's 800V rack architecture. $140M Series B (Feb 2026; a16z + Breakthrough Energy); 40+ GW customer pipeline; 50 GW orders; pilot production early 2027; 40 GW/year factory planned. Customers include Intersect Power and Crusoe (1.2 GW).
  • Hitachi Energy: Runs MVDC pilot programs for utilities. The largest incumbent power transformer OEM investing in SST, on a longer development timeline than the startups.
  • Siemens Energy: Targets distribution grids and urban energy systems. Its rail traction SST (SFT) is deployed in European rail, the most commercially deployed SST in rail applications globally; distribution SST is in pilot stage.
  • ABB: SST for marine, campus microgrids and industrial power. Marine is the most advanced application, and ABB has the broadest industrial SST portfolio among incumbents.
  • Eaton: Integrates SST with solid-state circuit breakers (SSCB), where it is strong; SSCBs pair naturally with SSTs in solid-state substation designs.
  • WattEV: Designed specifically for Class 8 truck depot MCS; the most focused MCS-application SST commercially available. Depots at Long Beach, San Bernardino and Bakersfield CA.
  • DG Matrix: Modular SST for solar, BESS and EV depot integration; competes with Heron Power and Resilient Power in the US startup SST segment.
  • Resilient Power: Part of the US SST startup cohort alongside Heron Power and DG Matrix.

SiC Supply Chain

SiC (silicon carbide) MOSFETs are the switching devices at the core of every SST design. Without them, SSTs cannot reach the power density, switching speed or thermal performance that justify their cost. As SST volumes move from prototypes to factory production, SSTs become a significant new source of SiC demand alongside EV traction inverters, BESS power conversion systems (PCS), solar inverters and DC fast chargers.

Main SiC device suppliers for SST development:

  • Wolfspeed (US, primary MOSFET supplier)
  • Onsemi (US/CZ)
  • STMicroelectronics (CH/FR)
  • Infineon (DE)
  • ROHM (JP)
  • Chinese OEMs (BYD Semiconductor, CREE/Wolfspeed China JV)

Wolfspeed's financial difficulties in 2025-2026 and its capacity expansion problems are a near-term supply risk for SST programs that have specified Wolfspeed devices.

See: Power Electronics Supply Chain - SiC Cross-Sector Demand | SiC & GaN Universal Power Substrate


Current Technical Limitations

  • Power rating: commercial SSTs are typically 100 kVA to 5 MVA, while conventional distribution transformers scale to hundreds of MVA. High-power sites need SSTs paralleled in modular arrays, which adds complexity.
  • Voltage range: most commercial SSTs handle up to 35 kV on the primary side. HVDC and large substation voltages above 100 kV are still in R&D.
  • DC fault protection: isolating faults on DC buses is harder than on AC because DC arcs are more sustained. Solid-state circuit breakers (SSCB), the paired technology, are also immature.
  • Thermal management: high switching frequencies generate heat in the semiconductor stages. Liquid or phase-change cooling adds cost and complexity compared with oil-cooled conventional transformers.
  • Standards and interoperability: no single IEEE or IEC standard covers SST design, testing or grid interconnection, so each deployment needs custom utility approval.
  • SiC supply: SiC substrates and epitaxy remain constrained, and Wolfspeed's financial instability is a risk for designs specified on its devices.
  • Utility deployment experience: utilities are conservative, and a technology without 20 years of field data faces long sales cycles.

Emerging Grid Architectures Enabled by SSTs

  • Solid-state substation: SSTs and solid-state circuit breakers replace all passive transformers and switchgear. Every power flow parameter is under software control, oil-filled equipment is eliminated and the substation can be reconfigured remotely.
  • MVDC feeder networks: SSTs at MVAC/MVDC interconnect points enable medium-voltage DC distribution loops. DC feeders have lower losses than AC at the same voltage and allow bidirectional flow without reactive power management.
  • DC distribution zones: SSTs power local LVDC networks for buildings, data centers and ports, removing several AC/DC conversion stages at the load. Nvidia's 800V rack architecture is the leading commercial driver.
  • Hybrid AC/DC microgrids: multi-port SSTs manage solar (DC), BESS (DC), grid (AC) and loads (AC or DC) from one device. The Fleet Energy Depot (FED) design is an early commercial example.

See: Grid Infrastructure & Interconnection | Microgrids | BESS | Fleet Energy Depot


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