814 / 2022-04-01 02:18:36

Aging of epoxy resin under sinusoidal waveform at different frequencies

insulation aging,high voltage testing,high frequency,epoxy resin,lifetime curve

Purpose/Aim

In this paper immediate time to breakdown and lifetime curves of epoxy resin is measured under AC sinusoidal waveform at different frequencies. The aim is to obtain lifetime curves of different epoxy resin in combination with different type of fillers. Based on this, most suitable epoxy resin and filler percentage for insulation system of high voltage high frequency transformers can be selected.

Experimental/Modeling methods

A summary of existing literature related to the impact of frequency on breakdown strength and lifetime of insulating material is provided first. A reliable method to produce thin sheets (~100  μm) of epoxy resin is introduced afterward. The experimental setup consist of a medium frequency medium voltage transformer to produce electric stress at different frequencies, An H-bridge to drive the transformer, a cylinder-plane electrodes system immersed under oil to perform the breakdown and lifetime tests, and a protection device to switch off the system as soon as a breakdown occurs.

Results/discussion

Immediate breakdown voltage is measured using a ramp sinusoidal voltage with a slope of 1 kV_p/s, at different frequencies. Impact of sample thickness on immediate breakdown strength is presented. Lifetime curves are measured at accelerated electric stress using step method and are obtained at different frequencies. Slopes of the lifetime curves are calculated and compared for different frequencies, and conclusion regarding impact of frequency on lifetime is presented. Experimental challenges that might arise while doing this type of measurement are discussed and key points are summarized.

Conclusions

In this paper a test setup for testing insulation material under high frequency sinusoidal waveform is developed.  A step-up transformer with ferrite core is used in a resonance circuit to convert pulse waveform of an H-Bridge into a high voltage sinusoidal waveform. It is shown that both immediate breakdown voltage and the lifetime of epoxy decreases with increase of voltage frequency. It is found that due to variation of sample thickness, edge partial discharges may occur at different voltage levels for one sample compared to another sample. This introduce a significant error on the data points in a lifetime curve. To obtain a valid lifetime curve special attention must be paid to this phenomena and it must be avoided carefully.