A Review of Insulation Stress and Partial Dischargein Medium-Frequency Transformers Under Fast Repetitive Voltage Pulses

The use of wide-bandgap power devices in solid-state transformers enables higher switching frequencies and power densities, but it also exposes medium-frequency transformer insulation to fast repetitive voltage pulses. Short rise times, high dv/dt, high repetition frequencies, and harmonic content can produce nonuniform winding voltages, local field enhancement, and partial discharge. This review examines the resulting insulation stress, discharge mechanisms, detection methods, relevant IEC standards, and design approaches. The reviewed studies show that partial discharge inception and insulation lifetime depend on voltage amplitude, rise time, repetition frequency, polarity, temperature, and residual charge. They also show that power-frequency test results cannot be transferred directly to pulse-width modulation service. Reliable insulation assessment therefore requires complete waveform reporting, interference-resistant sensing, and product-specific test procedures. The resulting evidence should then guide an insulation design that combines electric-field control and thermal management while accounting for parasitic parameters and manufacturing tolerances. Future work should establish common test waveforms and relate discharge activity to long-term material damage under representative operating duty.