High-voltage high-frequency transformer design for a 7.2kV to 120V/240V 20kVA solid state transformer

Solid state transformer (SST) exhibits good features such as high power density, small volume and weight, controlled power factor, voltage sag ride through, etc. compared with traditional line frequency transformer. The 7.2 kV AC to 120V/240V AC 20 kVA solid state transformer is a key component of the future renewable electric energy delivery and management (FREEDM) systems as the interface between the 7.2 kV distribution grid and the low voltage residential micro-grid. Three cascaded 6.7 kVA high-voltage high-frequency transformers operating at 3 kHz are employed to convert voltage from 3800 V high voltage DC link of each cascaded stage to 400 V low voltage DC link. The transformer is required to withstand at least 15 kV high frequency voltage insulation continuously. Transformer magnetic core materials were reviewed and compared. Winding layout alternatives for leakage, magnetizing inductance and insulation were compared. An insulation strategy based on split core and separate winding structure with inserted insulation layer between the C cores was proposed. One 6.7 kVA high voltage high frequency transformer prototype was built and the test results were reported.