Magnetic Core Selection Guide for High-Frequency Transformers and Inductors in 2MW Solid-State Transformers

1. Introduction: Demanding Requirements for Magnetic Components in SST

Solid-state transformers are becoming a key technology for medium-voltage grid integration, EV fast-charging hubs, and data center power delivery. In a 2MW three-phase SST with 20kV input, 800V output, operating at 70kHz using a Dual Active Bridge topology — under such demanding conditions, the core selection of high-frequency transformers and inductors directly determines power density, thermal performance, and overall system reliability.

For magnetic component suppliers, understanding the specific requirements that SST systems place on transformers, and providing targeted core selection and design support, is the foundation for serving high-end customers. This article provides technical reference for magnetic core selection of high-frequency transformers and inductors used in SST systems.

2. Core Material Comparison at 70kHz

Core selection involves balancing saturation flux density, high-frequency losses, and mechanical processability. The table below evaluates mainstream materials for high-frequency transformers at 70kHz.

Material	Saturation B (T)	Loss at 70kHz	Suitable for 2MW SST Transformer	Key Limitation
Nanocrystalline	~1.2	Low to Moderate	Recommended	Higher cost, brittle nature
High-performance Ferrite	0.3-0.5	Very Low	Acceptable (large volume)	Low Bsat → large core size
Amorphous	~1.5	High	Not recommended	High loss at 70kHz
Silicon Steel	~1.8	Extremely High	Not applicable	Excessive eddy current loss

Conclusion: For transformer designs in 2MW SSTs that prioritize power density, nanocrystalline cores are currently the most viable option. Their high saturation flux density (1.2T) significantly reduces core volume compared to ferrite, directly helping customers achieve higher system power density.

3. Key Design Parameters for SST Transformers

3.1 Operating Flux Density (Bmax) Derating

A common mistake is attempting to operate the core near its saturation point. Although nanocrystalline has a Bsat of 1.2T, at 70kHz, core loss increases exponentially with flux swing (Pv ∝ f^α × Bm^β).

Practical recommendation: For 2MW SST transformer designs, to effectively control temperature rise, limit Bmax of nanocrystalline cores to 0.3T – 0.4T. This provides sufficient saturation margin and keeps thermal dissipation manageable, ensuring long-term transformer reliability.

3.2 Core Shape and Insulation Design

Core shape: C-core or stacked toroidal core are recommended. These structures facilitate high-current winding installation, provide better leakage inductance control, and simplify high-voltage isolation for the 20kV input side.
Insulation design: High-frequency operation increases voltage stress. For transformers requiring 20kV isolation, reinforced insulation measures are mandatory:
PEEK bobbins or triple-insulated wire
Vacuum epoxy potting ensuring voids < 50µm
Partial discharge testing to verify insulation integrity

3.3 Air Gap Treatment

For DAB topologies that intentionally use leakage inductance as part of the resonant tank, distributed gap or gapless design is recommended. A discrete air gap can create localized eddy current hot spots and increase EMI.

4. Recommended Transformer/Inductor Parameters for SST Applications

Parameter	Recommended Choice	Rationale
Core material	Nanocrystalline (e.g., VITROPERM 500 F)	High Bsat, acceptable loss at 70kHz
Bmax setting	0.3 – 0.4 T	Thermal margin, avoid saturation
Core shape	C-core or Toroidal	Winding ease, leakage control
Insulation rating	Reinforced (20kV withstand)	Meet MV side safety requirements
Cooling method	Liquid-cooled cold plate compatible	2MW generates significant heat
Loss calculation	iGSE (Improved Generalized Steinmetz Equation)	Standard Steinmetz is inaccurate for square-wave excitation

5. Engineering Recommendations for Magnetic Components

Based on recent experience developing multi-MW SST transformer prototypes:

Do not push Bmax beyond 0.5T at 70kHz. While nanocrystalline can theoretically operate at 1.2T, the exponential rise in core loss at 70kHz will lead to thermal runaway in a 2MW system. Lower flux density is key to transformer reliability.
Consider temperature derating. Ferrite materials experience a sharp drop in permeability above 100°C–120°C (Curie temperature effect). Ensure hot-spot temperatures stay well below this threshold under worst-case conditions.
Use iGSE for loss estimation. The traditional Steinmetz equation assumes sinusoidal excitation and is inaccurate for square-wave or trapezoidal waveforms in DAB converters. iGSE provides much more accurate core loss predictions to help customers optimize system design.

6. Conclusion

Designing high-frequency transformers and inductors for a 2MW, 70kHz, 20kV-to-800V solid-state transformer places extreme demands on magnetic components. Nanocrystalline cores offer the best balance of high saturation flux density and acceptable high-frequency losses, provided that the operating flux density is conservatively derated to 0.3T–0.4T. A C-core structure with reinforced insulation and liquid cooling is recommended.

As a professional manufacturer of electronic transformers and inductors, we support SST applications with:

Core material selection guidance
Custom design and prototyping of high-frequency transformers and inductors
Contact us to get a custom high-frequency transformer solution tailored to your project.