Behaviour of Cellulosic Oil Impregnated Insulation Material at High Frequency Stress Used for High Voltage Solid State Transformer Applications

Solid state transformers (SSTs) are a new type of transformers where power electronics and conventional transforming elements are combined into one. The basic principle is that the voltage gets converted into the kHz range before it is transformed to another voltage level. The main advantage of this technology is the size of the transformer - due to the used higher frequency the transformer core and thus the whole apparatus can be built smaller than traditional 50/60 Hz power transformers. Currently there are no oil impregnated cellulosic insulation systems available for high-voltage high-frequency applications. This is partly because the exact breakdown mechanism at high frequency (HF) stress is still unclear and partly because long-term behavior of cellulosic insulation systems under HF-stress is unknown. The main goal of this publication is to provide new experiments and data to contribute on the research of the HF breakdown mechanism in oil impregnated cellulosic insulation systems. For this purpose breakdown experiments on oil impregnated paper samples were conducted at frequencies of 50 Hz, 20 kHz and 40 kHz with unipolar voltage pulses. Tests were performed at 20 degrees C and 90 degrees C in synthetic ester fluid Midel 7131. As voltage source a special self-developed pulsed HF generator was used that provides high-voltage high-frequency voltage up to 6.5 kV between 50 Hz and 40 kHz. As expected, decrease in breakdown voltage with increasing frequency could be shown. But more important, the findings presented in this paper surprisingly also suggest that contrary to most literature not heating due to increased polarization losses is the main responsible breakdown mechanism at high frequency stress.