Improved Insulation Design of Medium-Frequency Transformers Using a Semiconductive Coil Former

Abstract:
This paper proposes using semiconductive materials in the manufacturing of the medium voltage (MV) coil former for medium-frequency transformers. By employing these materials, the common-mode (CM) component of the electric field – introduced by the series connection of converter modules on the MV side – can be effectively shielded, enabling a significantly simplified insulation design. The proposed concept is validated through FEM simulation and partial discharge (PD) tests. Experimental results confirm that the semiconductive coil former is PD-free, enabling a design supporting CM voltage levels more than 10% higher than conventional coil formers. The breakdown voltage can be accurately predicted (<5% error) and tailored through design, removing the necessity for large safety margins in insulation design.