Accelerated ageing of Oil-Paper under Medium-voltage Pulsed Stresses

MSc thesis of Philip Mathew

Solid-state transformers (SST) are expected to replace conventional transformers in the electrical grid by offering larger power densities and flexible power flow control. The medium-frequency transformer (MFT) is a critical part of the SST, the insulation of which will experience mixed-frequency stresses characterised by fast-rising pulsed waveforms of tens of kilovolts repeating at frequencies up to hundred kilohertz. To evaluate the performance of oil-impregnated paper under such stresses, a variable frequency pulse modulator was developed to apply pulse waveforms across capacitive test samples. The samples were then tested at 20 kHz and 50 kHz. The results show a clear reduction in breakdown strength and lifetime at higher frequencies, which is attributed to increased dielectric heating leading to failure.”

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