Solid State Transformers: Concepts, Classification, and Control

Abstract

Increase in global energy demand and constraints from fossil fuels have encouraged a growing share of renewable energy resources in the utility grid. Accordingly, an increased penetration of direct current (DC) power sources and loads (e.g., solar photovoltaics and electric vehicles) as well as the necessity for active power flow control has been witnessed in the power distribution networks. Passive transformers are susceptible to DC offset and possess no controllability when employed in smart grids. Solid state transformers (SSTs) are identified as a potential solution to modernize and harmonize alternating current (AC) and DC electrical networks and as suitable solutions in applications such as traction, electric ships, and aerospace industry. This paper provides a complete overview on SST: concepts, topologies, classification, power converters, material selection, and key aspects for design criteria and control schemes proposed in the literature. It also proposes a simple terminology to identify and homogenize the large number of definitions and structures currently reported in the literature.

Transformer Model Identification by Ārtap

“…The challenging part of the transformer optimization problem highly depends on the application and the applied technology. For instance, in the case of the modern, solid-state transformers, the determination of the minimal losses and optimal value of inductances needs an accurate calculation of the medium, high-frequency harmonics and the caused non-linearities [6][7][8][9][10][11]. Or in the case of large power transformers, the thermal and the electrical properties should be examined together with the mechanical stresses in their windings [3,4,[12][13][14].…”

Decoupled PI Controllers Based on Pulse-Frequency Modulation for Current Sharing in Multi-Phase LLC Resonant Converters

“…The LLC converter can regulate the output current by adjusting: a) the duty cycle through phase-shift modulation (PSM); b) the input dc voltage through a power factor corrector (PFC); or c) the switching frequency through pulse frequency modulation (PFM). The LLC converter is restricted in power due to nominal current and power losses capability in semiconductors and power rate limitations on medium or high frequency transformers (usually available for a couple of kVA) [18], [19]. At high frequency, the transformer core material should be soft magnetic material: ferrite, amorphous alloy or nanocrystalline.…”

“…At high frequency, the transformer core material should be soft magnetic material: ferrite, amorphous alloy or nanocrystalline. The first one has low saturation flux and the others can be too expensive for some commercial applications [19]. Additionally, the parasitic inductances and capacitances acquire more relevance at high frequency, affecting the performance, efficiency, and operation of the transformer.…”

Design Analysis of High Frequency Linear Transformer with Ārtap Framework

“…One of the most common structures of the medium and high frequency solid state transformers is the cascaded H-bridge topology. This can be composed from many stages [2,3], to reach high-current or high-voltage requirements, resolving the power-semiconductor limitations [2,[4][5][6].…”