Advanced High-Frequency Magnetic Links for Emerging Grid-Connected Power Converters

Clean and sustainable renewable energy sources (RESs), such as solar, wind, and oceanic wave energy, are increasingly becoming a better replacement for traditional fossil fuel-based electricity generation. Future smart grid infrastructure will incorporate a significant number of magnetic-linked power converters (MLPCs) for integrating RESs as well as electric vehicle charging systems. MLPCs are becoming increasingly important for applications in distribution networks, electric vehicles, or renewable energy applications. Magnetic links (MLs) play a crucial role as an isolating medium between the input and output stages in MLPCs. By increasing the frequency, the size of the MLs can be reduced significantly. However, there are some challenging technical issues at high/ultra-high frequencies, such as increasing parasitic parameter effects, skin effects, proximity effects, early core saturation, higher core loss, and higher temperature, which might affect the overall performance of the power converters. Therefore, there is an urgent need to investigate and characterise the newer magnetic materials for use in such high-frequency MLs (HFMLs) that have higher magnetic flux density, lower core losses, and higher operating temperature.The majority of HFML topologies include non-interleaved windings and single permeability magnetic cores. As a result, the magnetic flux distributes unevenly in the cores, causing the magnetic material that is closer to the conductors to become saturated earlier than the magnetic material further away from the conductors. This inevitably leads to the improper use of magnetic materials in the design of magnetic links used in power conversion systems. Due to the different excitation waveforms in the magnetic link of the dual-active bridge converters, an appropriate design of the magnetic link is required to analyse the magnetic field and voltage distributions in both balanced and unbalanced cases. The state-space model can be applied to analyse the frequency spectrum of magnetic flux and induced voltages. The existing models do not account for non-linear characteristics, such as core saturation, core loss, skin effect, and proximity effects. Therefore, these models are not suitable to provide precise magnetic link transient characteristics under high-frequency conditions. The conventional modular solid-state transformer (SST) topology with multiple active bridge (MAB) converters considers individual multi-winding HFML (MWHFML) in each phase. These systems have the drawbacks of power mismatch between the windings of MWHFML, system complexity for extended modular design, and system cost.Initially, the thesis examines the recent emerging HFML-based power converter topologies for medium- to high-frequency applications, their historical development processes, electromagnetic characteristics, and core loss modelling and measurement techniques for advanced magnetic materials. Additionally, the thesis examines the properties of advanced magnetic materials, including amorphous Metglas alloys 2605SA1 and 2605S3A, as well as nanocrystalline materials F3CC125, 1K101, and 1K107B. Afterwards, a comprehensive characterisation has been carried out using advanced magnetic material-based-HFMLs for insulated gate bipolar transistor (IGBT), Silicon carbide metal-oxide-semiconductor field-effect transistors (SiC MOSFET), and Gallium nitride (GaN)-based dual active bridge (DAB) power converter applications. Later, PLECS and ANSYS/Maxwell software were used to develop a 3D multi-physics model of the HFML, to carry out 3D transient simulations, to characterise the electromagnetic properties, and to optimise the model for SiC and GaN-based power converters. To achieve more accurate electromagnetic performances of the HFML, the experimental characteristics of amorphous and nanocrystalline materials, such as core loss, B-H curves, and thermal behaviour, are used during the simulation process.After that, a new design of MWHFML for a power converter considering a multi-permeability core and an interleaved winding topology has been mathematically modelled and proposed. The multi-permeability core-based HFML (MPCBHFML) has been designed and analysed using nanocrystalline 1K101 and 1K107B, and Metglas 2605SA1 materials. The electromagnetic properties of the proposed MPCBHFML have been investigated and compared with the existing single-permeability core-based HFML (SPCBHFML). The proposed MPCBHFML offered less core saturation, low core loss, and higher power transfer due to its interleaved winding topology. Also, the inductance profile comparison showed more flattening with the proposed design compared to the existing design topology. The electromagnetic properties, such as voltage, current, and B-H curves, are investigated with experimental testing.Afterwards, a non-linear state-space modelling of the three-phase HFMLs (TPHFMLs) has been proposed, considering the non-linear characteristics of the core. The model has been validated with frequency spectrum analysis for TPHFML with three different core geometries, such as 5-limb, 3-limb, and matrix cores. By comparing the magnitude spectrum and pole-zero plots of these three types of TPHFMLs, the results showed that the matrix core-based TPHFML provided better operation in terms of output magnitude for the specific frequency regions. Experimental prototypes have been developed and tested to obtain the B-H curves with 50 V, 10 kHz three-phase excitations.Subsequently, a novel SST topology using a TPHFML has been proposed for medium-voltage (MV) SST applications. Unlike the existing MV SST systems with individual HFML in each phase of a three-phase system, only one TPHFML has been used to simplify the entire SST topology. The proposed system reduced the power mismatch issues in the HFML, simplifies the system configuration, reduces 50% of components, and improves the overall efficiency. The proposed TPHFML-based SST system can offer approximately 11.45% higher efficiency than that of the existing MWHFML-based SST system. Experimental prototyping has been carried out with commercially available amorphous 2605SA1 core material-based U-shaped core (product code: AMCC016B). Considering the maximum current rating and reducing the skin effect and proximity effect losses, laboratory-made Litz wires are used to develop the windings of the TPHFMLs.Finally, the thesis is concluded by summarising the contributions with relevant outcomes and indicating the future research prospects in the characterisation, design, and optimisation of advanced magnetic materials-based HFMLs used in emerging power converter applications.