The Hidden Voltage Stress in Medium-Frequency Transformers

As medium‑voltage (MV) power electronics architectures continue to evolve—especially in solid ‑state transformers (SSTs), grid‑edge converters, and industrial power systems—the role of the medium‑frequency transformer (MFT) has become increasingly critical. One of most common sources of misunderstanding in MV magnetic design is the distinction between terminal‑to‑terminal voltage and winding‑to‑winding voltage.

While these terms may sound similar, they represent very different electrical stresses, and confusing them can lead to underspecified insulation systems, unexpected reliability issues, or costly redesigns later in the development cycle.

For customers developing MV systems, this is not just a technical detail, it is a partner selection issue. The right magnetics partner must be able to identify these internal stresses early, engineer the insulation and winding system around them, and translate complex MV requirements and existing as well as emerging standards into a reliable, manufacturable component. That is where CorePower Magnetics is positioned to help.

Two Voltages, Two Very Different Design Challenges

Terminal‑to‑Terminal Voltage

Terminal‑to‑terminal voltage refers to the voltage difference measured across the external transformer terminals, such as the line‑to‑line voltage on the primary or secondary side. This is typically the voltage level most visible at the system level and is often the first number engineers reference when discussing medium voltage.

This voltage influences:

• External clearances and creepage distances

• Conductor turn-to-turn insulation design

• Connector, bushing, and housing design

• System‑level safety and standards compliance

  
  

In many MV applications, terminal‑to‑terminal voltage can reach several kilovolts, particularly in SSTs or multi‑level converter architectures.

Winding‑to‑Winding Voltage

Winding‑to‑winding voltage, by contrast, represents the internal voltage stress between windings, or between windings and core structures. This stress is often less intuitive, but highly consequential for transformer reliability.

Winding‑to‑winding voltage directly impacts:

• Insulation architecture and material selection

• Partial discharge (PD) performance

• Long‑term dielectric aging and lifetime

• Manufacturability and consistency at scale

  
  

In some converter topologies, including input-series output-parallel (ISOP) architectures, winding-to-winding stress can exceed the terminal-to-terminal stress of an individual winding. Because this stress combines line-frequency voltage with high-frequency switching effects from other series-connected modules, it presents a distinct insulation challenge that must be addressed separately from terminal-to-terminal voltage stress.

Why the Distinction Matters More at Medium Voltage

At low voltage, conservative insulation margins often mask this distinction. At medium voltage, available margins are limited and additional margins to account for uncertainties can become costly in terms of size and performance.

Medium-frequency transformers operate at elevated switching frequencies while simultaneously handling high isolation requirements. This combination introduces localized electric field intensification around winding edges, interlayer transitions, and insulation interfaces. Designs that are “rated” correctly at the terminals may still experience internal overstress if winding-to-winding voltage is not carefully managed.  

This challenge is one of the reasons MFTs frequently become a system design bottleneck in MV power electronics systems. They must satisfy a rigorous set of electrical, thermal, and mechanical constraints simultaneously, with little room for error.

For customers, that complexity is not just a source of uncertainty, it is a design and commercialization risk. The key question is whether the MFT can scale reliably, maintain sufficient insulation margin, and perform predictably under real operating conditions. CorePower helps answer that question by engineering the insulation system, winding structure, thermal behavior, and manufacturability together from the start.