High-frequency Power Conversion for Medium Voltage Power Electronics Interfaces


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Virginia Tech


ith the rapid advancements in modern technology and the increasing demand for efficient energy conversion, the field of medium voltage power conversion has experienced significant progress in recent years. This progress is driven by its high efficiency and improved scalability. Medium voltage power conversion finds applications in various areas such as data centers, electric vehicle fast charging, and smart grids. It enables the reduction of power delivery stages and minimizes the required physical space. The scalability and modularity of this technology offer the flexibility to expand the power level as needed. According to the International Energy Agency, data centers and electric vehicle charging are projected to consume over 10% of the world's total electricity consumption by 2040. To power this amount, approximately 800 nuclear power reactors with a capacity of 1 GW each would be required. Therefore, even small savings in power consumption can have a substantial impact. The solid-state transformer (SST) is a promising technique for medium voltage conversion that offers high-frequency operation, resulting in reduced volume and excellent insulation capabilities. Currently, the medium voltage transformer poses a challenge for SST systems due to the requirements for high insulation levels, efficient thermal management, improved efficiency, and higher power density. Unlike conventional line-frequency transformers, the solid-state transformer operates at relatively high frequencies, typically in the range of tens of kilohertz. This higher frequency enables a reduction in the cross-sectional area of the magnetic components, leading to a smaller and lighter design. However, the high-frequency transformer used in the solid-state transformer does face certain limitations. Balancing insulation capability with the goal of achieving high power density presents a dilemma. To ensure medium voltage insulation, a thick insulation layer is required for the transformer. However, the high-frequency Litz wire and compact size of the transformer make it challenging to achieve partial discharge-free operation, unlike traditional line-frequency transformers. To address these challenges and achieve both medium voltage insulation capability and high power density, improvements in the insulation structure have been made. The dissertation firstly proposes the application of a shielding layer and related stress grading layer in the insulation structure. This helps confine the electric field within the primary side winding encapsulation rather than in the air. As a result, there is minimal electric field present in the air, allowing for further reduction in the transformer volume as there is no longer a need for insulation margin. With the enhanced insulation structure, the transformer can operate at even higher frequencies. However, it is important to note that the reduction in size is not directly proportional to the increase in frequency due to the impact of the insulation layer. To address this, a straightforward and comprehensive optimization method is proposed for the first time. This method considers the trade-off between loss and volume, taking into account multiple design objectives and parameters. An optimized 800/400 V, 200 kHz, 15 kW CLLC converter is demonstrated. The peak efficiency of this optimized converter reaches 98.8%, and the power density is 3.7 kW/L. The transformer also exhibits good insulation capability, with a partial discharge-free level reaching 7.7 kV. Additionally, achieving a suitable insulation level for the DC-DC module poses challenges due to thermal limitations. Insulation materials are not efficient thermal conductors, and as insulation levels increase, the thickness of the insulation layer must also increase, resulting in a significant rise in thermal resistance. To address this issue for applications requiring a 13.2 kV grid, an alternative insulation material called FR4 is considered in this dissertation. FR4, which can be implemented as the insulation layer for a PCB winding, offers the advantage of being fabricated together with the winding during the PCB manufacturing process. This process takes place in a vacuum environment, reducing the presence of air cavities that could lead to partial discharge within the insulation structure. Thus, the entire insulation fabrication process can be simplified. To enhance the insulation capability further, the dissertation proposes the incorporation of an arc section within the PCB winding. This design reduces the electric field crowding in the corner area. However, winding losses in the PCB winding remain a concern. To mitigate these losses, an ER core structure is introduced to balance the magnetic flux within the transformer core. This balanced distribution of the magnetic field helps reduce leakage flux into the air, subsequently reducing winding losses. The dissertation also suggests a sandwich winding structure to decrease the magnetomotive force in the winding, in comparison to a completely separate winding structure. Another optimization process for the PCB winding is performed to strike a better balance between size and loss in the transformer. In line with these improvements, another 800/400 V, 200 kHz CLLC transformer is designed utilizing the PCB winding approach. Compared to the Litz wire-based transformer, the efficiency performance is similar, but the power density is doubled due to the low-profile design enabled by the PCB winding. In terms of insulation capability, the FR4 insulation, with its high dielectric strength, allows the transformer to be partial discharge-free even with the same insulation thickness as the epoxy used in the Litz wire transformer for the 13.2 kV applications. Thirdly, considering the power limitation mainly because of the thermal issue in the primary side PCB winding, the PCB Litz wire concept is proposed to further improve the winding loss. To further improve the power level of the PCB winding transformer, the winding should be designed wider to reduce the DC winding resistance. However, the current distributes in a bad manner due to the proximity effect in the winding. That makes winding width increment insignificant to the loss reduce. The Litz wire is widely used in the high-frequency power conversion applications. A similar concept has been proposed in this dissertation in the PCB winding. Using two layers constructing one turns, the interwoven strategy can be implemented in the PCB winding to achieve the flux cancellation effect. That helps to make the current distribute uniformly inside the PCB winding. The PCB Litz construction method and connection method is introduced in this chapter to reduce the design burden with such a complicated winding pattern. Some design considerations are also proposed to optimize the PCB Litz concept. This dissertation solves the challenges in magnetic design in high-frequency DC/DC converters in the solid-state transformer with medium voltage insulation. This includes the Litz wire transformer and the PCB winding based transformer. With the academic contribution in this dissertation, the insulation performance is better for both Litz wire transformer and PCB winding based transformer. The straightforward and comprehensive optimization method is benefit for both academic and industry for transformer design in this application. The proposed PCB winding transformer makes the insulation fabrication much easier compared to the conventional fabrication method. And the PCB Litz concept helps to further reduce the winding loss, which makes it possible to further lift the power level in the PCB winding based transformer.



Solid state transformer, resonant converter, high frequency, transformer design, medium voltage interface, insulation, magnetic