Medium-Voltage Solid-State Transformers: Topologies, Control and the Engineering Gap

A distribution transformer is a purely passive device: it changes voltage and does nothing else. A solid-state transformer (SST) replaces the line-frequency transformer with a medium-frequency transformer (MFT) and power-electronic converter cells on both sides. Because transformer core size scales inversely with operating frequency, moving the isolation stage from 50/60 Hz into the 1–100 kHz range shrinks the magnetic circuit and adds functions — voltage regulation, power factor correction, DC interfaces, active control — that a passive transformer cannot provide on its own [1].

The harder question starts at medium voltage. Once the input is a 10 kV or 35 kV class distribution feeder, the designer is no longer choosing components inside one architecture — the architecture itself is the decision. This article summarises what two 2025 peer-reviewed reviews conclude about stage count, rectifier choice, isolation and the system-level gaps that still separate laboratory demonstrations from grid assets [1][2].

1. Stage count is the first and most expensive decision

SST literature classifies converters primarily by the number of conversion stages around the MFT [1]:

The first review is explicit that no configuration dominates. Selection balances component count, modularity, whether DC stages are needed, controllability and size. Its conclusion: single-stage SSTs are attractive where economy and power density dominate, while multi-stage SSTs remain viable because they are easier to control and more flexible to design [1].

2. The front-end rectifier decides your power quality

For grid-tied medium-voltage systems, input-stage power factor correction is close to mandatory, and the rectifier choice sets the achievable current quality [1]:

• Diode bridge — cheapest and simplest, but unidirectional and poor at current shaping. Suitable only when power quality is not a contractual requirement.
• VIENNA rectifier — a unidirectional three-level integrated boost rectifier. Its three-level structure gives lower current ripple and higher power density than classical two-level boost rectifiers, and it outperforms other boost PFCs when bidirectional power transfer is not needed.
• SWISS rectifier — an integrated active filter rectifier with an integrated buck stage. Better efficiency and simpler control than a standard buck-type PFC, at the cost of additional switching elements.
• Full-bridge / six-switch boost PFC — the most prevalent SST rectifier structure when bidirectional power flow is required, at the cost of a more complex and expensive structure.

3. Single-stage designs live or die by the matrix converter

Where no DC link is wanted, AC/AC matrix converters are the dominant family: direct matrix converters (DMC), indirect matrix converters (IMC) and sparse variants [1]. Three engineering properties matter more than the topology names:

• Inherent step-down. A matrix converter's output voltage is at most 0.871 of the input, which constrains the voltage window available to the isolation stage.
• No natural freewheeling path. Inductive loads — including the MFT itself — require an external clamp circuit, which is a reliability-relevant addition.
• Device count versus conduction loss. DMC offers more even switch loading and lower conduction losses but more switches; IMC behaves like a DC-link converter without storage and increases currents at low output frequencies; sparse topologies trade active switches for passive ones and, in the ultra-sparse case, give up reactive power control capability.

The three-to-two DMC is currently the most widely used matrix structure in SST implementations [1]. Monolithic bidirectional switches would substantially reduce device count, but the review notes they are not yet commercially mature — a reminder that single-stage elegance is often paid for in control complexity and component stress.

4. The isolation stage sets the device class — and the cost curve

Isolated stages in modern SST designs use dual-active-bridge (DAB), resonant DAB or LLC-type converters around the MFT [1]. Because 10 kV and above cannot be switched by a single commercial device, cascaded topologies series-connect low- and medium-voltage SiC modules — so the available SiC voltage class (1.2 kV, 1.7 kV, 2.3 kV, 3.3 kV and research devices at 15 kV) directly determines module count, insulation coordination, complexity and cost.

Published MV prototypes illustrate the range: a 10 kV / 400 V, 500 kVA electronic power transformer; a 7.2 kV single-stage AC/AC SST built on 15 kV SiC MOSFETs and a current-source series-resonant converter; and a 15 kV class intelligent universal transformer for utility applications [1].

One counter-intuitive point deserves attention: an SST is not automatically more efficient than a plain low-frequency transformer. Conduction and switching losses make it less efficient than a single LFT — but it can be more efficient than the LFT plus the cascaded converter stages it replaces. Efficiency must therefore be compared at system level, over the real load profile, not at component level [1].

5. The real gap is protection and coordination, not topology

The second review focuses on what happens after the converter works [2]. Its central finding is a disparity between component-level maturity and system-level readiness:

• Fault current. Unlike a passive LFT, an SST contributes limited fault current because semiconductors switch within microseconds. Conventional overcurrent-based protection assumes the transformer's fault contribution — so protection philosophy, sensor placement and coordination settings must be redesigned around the SST, not inherited from the transformer it replaces.
• Communication. Coordinated operation with utility systems requires low-latency communication, which raises protocol standardisation and cybersecurity questions that component datasheets do not answer.
• System readiness. The review names fault isolation, thermal resilience and coordinated control as the areas where the gap is widest between laboratory results and deployable equipment. Multi-criteria evaluation — efficiency, cost, protection compatibility, fault ride-through — should replace single-metric comparisons.

6. What this means if you are specifying an MV SST

1. Fix the stage count before the topology. Decide whether you need MV or LV DC links; that single answer eliminates most of the design space.
2. Specify power quality as a requirement, not a hope. Name the harmonic and power-factor limits, then choose VIENNA, SWISS or bidirectional boost PFC accordingly.
3. Check device supply against voltage class. Confirm which SiC classes are in volume production for your module count and insulation scheme.
4. Demand a protection concept, not a protection device. Ask how fault current, fault isolation and coordination with upstream protection are handled.
5. Compare efficiency over the load profile. A single peak number hides the light-load behaviour that determines real energy cost.
6. Insist on thermal and insulation evidence. Partial discharge, thermal cycling and damp-heat results are the difference between a prototype and an asset.

Related: SST Technology Introduction · MV SST vs Traditional Transformer · SiC Technology in Medium Voltage Applications

Sources

1. Predescu, D.-M.; Roșu, Ș.-G. "Solid State Transformers: A Review—Part I: Stages of Conversion and Topologies." Technologies 2025, 13(2), 74. Open access, CC BY. DOI: 10.3390/technologies13020074
2. Chandran, D.R.; Kumar, S.; Sanath, D. "Solid State Transformers: A Comprehensive Review of Technology, Topologies, Applications, Research Gaps, and Future Directions." Journal of Power and Energy Engineering 2025, 13(6). Open access. DOI: 10.4236/jpee.2025.136003

Disclaimer: This article is a technical summary compiled from the publicly available peer-reviewed sources listed above for industry information purposes only. Design decisions on a specific project require project-specific engineering analysis.