Solid-State Transformers

The solid-state transformer (SST) work has two parts. For the ASSIST project, a review covered SST technology: high-voltage semiconductor devices, converter configurations, smart-grid functions, reported prototypes and applications. Later, in collaboration with Cardiff University, a three-stage SST was designed in simulation, with a focus on its isolated dual-active-bridge (DAB) stage.

What a Solid-State Transformer Is

A solid-state transformer replaces a bulky line-frequency transformer with power-electronic conversion stages around a compact medium-frequency isolation transformer. Raising the isolation frequency shrinks the transformer, and because the conversion is actively controlled, an SST can also regulate voltage, control active and reactive power, isolate faults, and provide DC ports for renewable generation, storage and DC microgrids. Its feasibility at medium voltage depends strongly on high-voltage semiconductors: the review followed SiC MOSFETs of 10 kV and above in research, against commercial devices that were then mostly SiC modules up to 1.7 kV and silicon IGBTs at 3.3 to 6.5 kV.

Single-, Two- and Three-Stage SSTs

SSTs fall into three configurations, distinguished by how many DC links they have.

Block diagrams of single-stage, two-stage and three-stage SSTs between the MV and LV AC grids, with no DC link, one DC link, and both HV and LV DC links — The three SST configurations; the three-stage SST, with both HV and LV DC links, is the configuration designed in this work.

Configuration	Structure	Strengths	Limitations
Single-stage	Direct AC–AC conversion through a high-frequency transformer, with no DC link	Fewest components and conversion stages, so potentially the highest efficiency and reliability	No DC link, so no reactive power support or decoupling between the grids; filtering and control are demanding
Two-stage	An isolated AC–DC stage and a DC–AC stage around one DC link, on the high- or the low-voltage side	The DC link enables reactive power compensation, voltage-sag ride-through, fault isolation and DC-link protection	More switching devices, higher losses and ripple current, and harder to build modularly
Three-stage	An AC–DC rectifier, an isolated DC–DC stage and a DC–AC inverter, with DC links on both sides	Independent control of each side, fault isolation and ride-through, reactive power support, and HV and LV DC ports for renewables and storage	The most semiconductor stages, so lower efficiency and more components to keep reliable

The three-stage configuration offers the most complete set of grid functions, including voltage regulation, current limiting, protection and power-factor control, and its two DC links can be brought out as ports for storage, renewables and DC microgrids. This is why most recent research prototypes in the review used it, and why it was chosen for the design. The review also covered the applications that motivate SSTs, from data centres and fast EV charging to DC microgrids, ships and offshore wind.

The Three-Stage SST

A three-stage SST for medium-voltage to low-voltage distribution has three power stages, each with its own control:

MV-side rectifier. A cascaded H-bridge, modular multilevel or three-level neutral-point-clamped (NPC) rectifier connects to the medium-voltage grid. It regulates the HV DC link and controls the active and reactive power drawn from the grid, using grid synchronisation, current control and DC-link voltage control. An NPC stage must also keep its two DC-link capacitors balanced, by adjusting how long it uses redundant switching states.
Isolated DC–DC stage. Dual active bridges provide galvanic isolation and bidirectional power transfer between the DC links, often with their secondaries feeding a common LV DC bus. An outer loop on the LV DC voltage sets the phase shift that controls their power.
LV-side inverter. A three-phase inverter, usually grid-forming, sets the low-voltage magnitude and frequency.

A supervisory layer coordinates the three stages through start-up, shutdown and faults. The design work concentrated on the middle stage.

Designing the DAB Stage

How the DAB Works

A DAB connects two full bridges, each across its own DC link, through a series inductance L_tot (the transformer leakage plus any added inductor) and an n : 1 high-frequency transformer. The transformer provides galvanic isolation and scales the voltage; the inductance is where power is transferred.

Dual active bridge schematic: a primary full bridge across V1 and C1, series inductance Ltot, an n to 1 high-frequency transformer, and a secondary full bridge across C2 and V2 — Dual active bridge: two full bridges linked by the series inductance and the high-frequency transformer.

Each bridge applies a square-wave voltage to its side of the transformer. Under phase-shift modulation, both bridges run at 50% duty at the switching frequency f_sw, and the shift ϕ between them, as a fraction of the switching period, places a voltage across the inductance and so sets the power. The bridge that leads delivers power, so the same circuit transfers power in either direction, and the inductor current naturally discharges the switch capacitances before each turn-on, giving zero-voltage switching over a wide load range.

For a lossless DAB with DC voltages V₁ and V₂ and turns ratio n, the transferred power is

P=\frac{nV_1V_2}{f_{sw}L_{tot}}\,\phi\bigl(1-2|\phi|\bigr),

positive ϕ sending power from the primary to the secondary and negative ϕ reversing it. Power peaks at ϕ = 0.25.

Choosing the Series Inductance

The series inductance is the central design choice: it sets the power capability, the soft-switching range and how finely the controller can adjust power. Three limits bound it. Zero-voltage switching needs enough stored energy to charge and discharge the switches’ output capacitance C_oss in the dead time; the digital controller can change ϕ only in steps of f_swΔT_PWM, and each step must not change the power by more than a tolerable amount at minimum load; and the inductance must still let the converter reach full power:

L_{tot}>\frac{4C_{oss}V_1^2}{I_{pk}^2},\qquad L_{tot,\max}=\frac{nV_1V_2}{8f_{sw}P_{\max}}.

For the 15 kW stage, with operation down to 7.5 kW, the ZVS limit was negligible. The control-resolution limit set the lower bound at about 28 µH, keeping each phase step below about 375 W at 7.5 kW, and full power set the upper bound at about 1.1 mH.

Inductor selection window from 28 microhenries to 1.1 millihenries on a logarithmic axis, with 250 microhenries selected — Feasible inductance window for the 15 kW DAB, and the selected value.

A value of 250 µH was selected, made up of about 10 µH of transformer leakage and 240 µH of external inductance. The design model predicts:

At 15 kW with L_tot = 250 µH	Predicted value
Phase shift	ϕ ≈ 0.030, about 1.5 µs
Power resolution	about 37 W per phase step
RMS current, primary / secondary	9.73 A / 14.60 A
Conduction loss, primary / secondary bridge	15.16 W / 8.95 W, 24.1 W in total
Efficiency, conduction losses only	about 99.84%
Zero-voltage switching	down to about 32 W

These are predictions from the design model in simulation, not measurements; the efficiency counts conduction loss only.

Modulation Options

Phase-shift modulation is not the only way to drive a DAB. Using three-level bridge voltages adds degrees of freedom:

Modulation	How it works	Trade-off
Phase shift	Both bridges at 50% duty; power set by the phase shift alone	Simplest control and a wide operating range; the usual choice for SSTs
Triangular	Three-level voltages timed so the secondary switches at zero current	Lower switching loss and current stress, but less power, and none when V₁ = nV₂
Trapezoidal	Extends triangular modulation to zero-current switching on both bridges	More power than triangular with favourable switching, at the cost of more complex control

In practice, phase-shift modulation combined with a well-chosen inductance and outer-loop control gives stable, efficient operation across a wide range, which is why it was adopted here.

Design and simulation references are collected in the solid-state transformer repository.

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