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Design of a High-Frequency Transformer and 1.7 kV Switching-Cells for an Integrated Power Electronics Building Block (iPEBB)

iPEBB Topology
Fig 1. iPEBB di-directional topology with the transformer highlighted in green.
  This work presents the design of a 250 kW integrated power electronics building block (iPEBB) for future electric ship application. This work focuses on the motivation, design choices, and initial prototyping of a 500 kHz transformer which forms the core of the converter. The transformer is designed for 13.8 kVAC and features a unique structure that meets insulation requirements while maintaining a compact and lightweight design. The prototype was designed with target ratings of 250 kVA, 1000 Vpri / 1000 Vsec, 500 kHz with insulation suitable for operating in 20 kVDC or 13.8 kVAC systems. The size of the prototype is approximately 30 cm by 25 cm by 10 cm and weighs less than 5 kg. To reduce the size of the transformer, the target switching frequency for the inner bridges of the iPEBB is 500 kHz. The bi-directional topology of the iPEBB is shown in Fig. 1. To achieve this high switching frequency, soft switching is employed for the inner bridges (blue). The outer bridges (grey) will be hard switching (10-20 kHz) and interface to the ac or dc line. The inner bridges interface with the transformer (green) .

  In conventional medium-voltage transformers, the insulation can become a sizeable fraction of the transformer weight and present a thermal barrier preventing effective cooling. To address this, the proposed transformer design uses an insulating sheet to separate the primary- and secondary-sides (Fig 2 (a)). The sheet is coated with conductive and semiconductive layers to reduce the electric field strength. Mica is used as the insulating sheet and has dimensions to meet the primary-to-secondary voltage insulation and creepage requirements. The insulating sheet is covered with a layer of conductive paint on the two surfaces where the primary and secondary coils and core sections are located. The conductive layers are referenced to the potentials of the two respective sides, thereby eliminating electric field stress in the air gaps around the windings (Fig 2 (b)). N49 ferrite was used for the core, and 6600/46 AWG litz wire was used for the windings. Based on initial test results, the transformer is expected to have core losses of 300 W and winding losses of 850 W at 250 kW, resulting in an efficiency of 99.5%. Future work will involve testing the transformer with the rest of the iPEBB converter.
Transformer Picture
Fig 2. Top view of the transformer prototype (a) before the conductive and semiconductive paints is applied, (b) with the top core block removed to show the Litz wire coils and conductive (black) and semiconductive paint (white) paints.

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