Cooling Architectures for Medium-Voltage Solid-State Transformers: Forced Air, Forced Liquid, and Biodegradable Dielectric Fluid Immersion

Thermal management is the largest physical subsystem on every commercial medium-voltage solid-state transformer (SST) product and the deciding factor in determining the form factor, the lifetime, and the failure modes of the device. The MV-SST’s compactness compared to a conventional iron-core transformer is also its thermal-density problem: the same power throughput in a much smaller volume means the residual losses must be removed through a much smaller surface area.

Three cooling architectures dominate commercial MV-SST products. Forced-air cooling, the simplest implementation, uses external fans to move ambient air across radiator fins or heat-sink surfaces. Forced-air cooling is the lowest-cost option and is used on smaller-rated SST products and on traction applications where the cooling air is also the locomotive’s exterior airflow. The trade-off is power density: forced-air SSTs are physically larger and noisier than liquid-cooled alternatives.

Forced-liquid cooling, using deionized water or ethylene-glycol-water mixtures, is the dominant cooling architecture on multi-megawatt MV-SST products. The liquid loop carries heat from the SiC power modules, the medium-frequency transformer (MFT), and the DC-link capacitors to an external heat exchanger. Liquid-cooled SSTs deliver 3 to 5 times higher power density than forced-air alternatives but require a closed-loop pumping system and periodic coolant maintenance. Research from the National Science Foundation has explored hybrid liquid-and-air cooling specifically optimized for 10 kV SiC SSTs at 13 kV to 7.2 kV with 667 kW output at 20 kHz operation.

Dielectric-fluid immersion cooling is the third architecture and the most aggressive. Ester-based biodegradable dielectric fluids serve simultaneously as coolant and as electrical insulator, allowing the SST’s high-voltage stages to be flooded with fluid that conducts heat efficiently while preventing arcing. The immersion architecture inherits decades of operational experience from conventional iron-core transformers, which have used mineral oil and ester-based fluids for the same dual purpose since the early twentieth century. The MV-SST industry is converging on biodegradable ester dielectric fluids over the mineral-oil legacy choice on environmental grounds.

The cooling architecture decision interacts with the SST’s claimed lifetime. The DC-link capacitor failure mode discussed in earlier mgrid coverage is highly sensitive to capacitor case temperature. Lowering case temperature from 70 degrees Celsius to 50 degrees Celsius can double the projected capacitor lifetime. Whether forced-liquid or dielectric-fluid-immersion cooling delivers the 30-to-40-year service life that utility procurement standards demand is the deciding engineering question for MV-SST adoption at distribution-grid scale.

Critical Perspective

Editorial correction: This post is part of MGRID’s Solid-State Transformer industry coverage. As of May 2026, that body of work systematically framed manufacturer announcements, funding rounds, and laboratory demonstrations as commercial deployments. The reality is that field-deployed commercial-class SST in revenue service globally is measured in single digits, and almost every product cited in this series is at “announced” or “funded” stage, not “operational.” Readers should treat the specific claims in this post against the standards documented in our SST Industry Reality Check (the per-claim audit table maps marketing language to verifiable deployment status). The corrective article is the canonical reference for SST industry reality; this post remains published with its original framing so the editorial drift is traceable.

Related Coverage

• FREEDM Systems Center at NCSU: The 2008 NSF Center That Turned Solid-State Transformers Into Real Devices: NSF FREEDM Center’s history in developing SSTs
• NSF FREEDM Center Hits 17-Year Mark As Generation-IV Engineering Research Center Competition Begins: NSF FREEDM Center’s ongoing research and development
• The Federal R&D Investment Ecosystem Under the US Solid-State Transformer Industry: ARPA-E, DOE OE, NSF FREEDM, and Why It Matters: Federal R&D investment in US SST industry, including NSF
• Virginia Tech CPES Is the Second-Most-Important US Academic Solid-State Transformer Research Center After NCSU FREEDM: Virginia Tech CPES as a key US SST research center