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Advanced Solid State Transformers

Periodic Reporting for period 2 - ASSTRA (Advanced Solid State Transformers)

Reporting period: 2020-01-01 to 2022-06-30

The need for continuous and reliable energy supply and the demand to embrace the discontinuous distributed energy systems are shaping the societal challenge to migrate from the existing concept of AC grids towards the DC, which would enable in a "Smarter" manner to deal with the energy flow and integration of continuously increasing renewable energy resources. Similarly, on the side of the load, the technological development is bringing new challenges with the increasing number of charging stations for electrical vehicles in cities as well as well as on highways, and quickly fluctuating load profiles of data centres, to name only a few. In a DC grid, additional power processing units are necessary to consider making the transformer functional. In this case, the transformer, or together with the power processing unit – solid-state transformer (SST), works at considerably higher frequencies (medium-range) than traditional 50 Hz.
The research on the medium frequency transformers (MFTs) for SSTs (grid applications) aims at reducing the unit mass and geometrical sizes, resulting in a reduced quantity of active materials like copper, ferrites and insulation materials as compared to classical power transformers that are used today in electrical energy transportation and distribution. The research focus is on the MFT design and modelling. The transformer core topology, winding and the insulation system are key aspects which impact the performance of an MFT. The design principles of the insulation system, subject to the DC excitation confronting high levels of stress generated by both electrical and thermal domain, are constrained by a compact design and medium voltage operation. The winding topology should allow high efficiency, throughput and ensure minimal electrical stresses towards the insulation system.
Within this project, the training of two early-stage researchers (ESRs) has been accomplished by means of a mix of industrial (ABB) and academic (TUE) environment. The training network ensured a variety of both scientific and soft skills courses as well as facilitated the participation to workshops which enabled the development of the researchers. As a result, the work carried out within ASSTRA has been disseminated by means of over ten conference visits and journal publications. The major findings and outputs have been disseminated via the video and blog posts on social media and the project website [1].

[1] “ASSTRA,” Asstra Itn. accessed Aug. 09, 2022.
Following the work carried out during the project, the existing design concepts as well as new alternatives have been investigated to identify the design possibilities fitting the specifications determined at the early phase of the project. Main specifications, such as the operating voltage (around 10-20kV), switching frequency (10-20kHz) and power range (MW range) of the solid-state transformer have shaped the detailed constraints and challenges for the final design [2]. One of the topologies that fits the predetermined constraints exploits a novel parallel foil conductor configuration which must have an equal share of the carried current [3], [4]. External passive components resulting from the converter topology (resonant converter) are tailored to minimise the circulating currents. In addition, the design innovation consisting of additional round conductor turns provides the shielding capabilities preventing high electrical stresses of the insulation system [5], [6], these innovative approaches have resulted into a patent [7]. Numerical models have been developed for the above-mentioned designs to quantify their performance and validated experimentally on the manufactured prototypes.

[2] A. Cremasco, D. Rothmund, M. Curti, and E. A. Lomonova, “Voltage Distribution in the Windings of Medium-Frequency Transformers Operated with Wide Bandgap Devices,” IEEE J. Emerg. Sel. Topics Power Electron., pp. 1–1, 2021, doi: 10.1109/JESTPE.2021.3064702.
[3] S. Pourkeivannour, M. Curti, U. Drofenik, A. Cremasco, and E. A. Lomonova, “Mitigation of Circulating Currents in Parallel Foil Windings for Medium Frequency Transformers,” IEEE Transactions on Magnetics, pp. 1–1, 2022, doi: 10.1109/TMAG.2022.3178489.
[4] S. Pourkeivannour, M. Curti, C. Custers, A. Cremasco, U. Drofenik, and E. A. Lomonova, “A Fourier-Based Semi-Analytical Model for Foil-Wound Solid-State Transformers,” IEEE Trans. Magn., vol. 58, no. 2, pp. 1–5, Feb. 2022, doi: 10.1109/TMAG.2021.3094047.
[5] A. Cremasco, E. Logakis, M. Curti, and E. A. Lomonova, “Characterization of Ion Transport Properties in Synthetic Ester Oil by Polarization Current and Dielectric Spectroscopy,” Proceedings of the Nordic Insulation Symposium, vol. 27, no. 1, Art. no. 1, Jul. 2022, doi: 10.5324/nordis.v27i1.4724.
[6] A. Cremasco, M. Curti, J. van Duivenbode, E. A. Lomonova, and D. Rothmund, “Electric Field Models for Liquid-Filled Insulation of Medium-Voltage AC/DC Distribution Technology,” in 2021 IEEE Conference on Electrical Insulation and Dielectric Phenomena (CEIDP), Dec. 2021, pp. 450–453. doi: 10.1109/CEIDP50766.2021.9705406.
[7] A. Cremasco, S. Pourkeivannour, M. Curti, E. Lomonova, D. Rothmund, “A coil and a transformer that have improved electromagnetic shielding,” in patent application WO2022136634A1 published 30 Jun. 2022, url:
Convincingly, the contributions achieved along the project period will enable an enhanced efficiency and compactness of the future SST applications. Foil paralleling comes as a scalable solution to meet a wide range of nominal power ratings. Winding shielding decreases the winding losses and enables a more compact design from the insulation system standpoint. The experimentally supported topology choices have resulted into both solid and liquid insulation designs to withstand real-world and abnormal operating conditions.
Components of the MFT prototype before assembly
CAD drawings for the MFT