| With the continuous improvement of voltage levels,the insulation of electrical equipment is facing a more severe operating environment,and the failure of insulation material caused by the electrical treeing is becoming increasingly serious.Epoxy resin(EP)is widely used in the insulation of electrical equipment such as dry-type transformers,current transformers,converter transformer bushing,and electronics packages because of its high-temperature resistance,low curing shrinkage,and stable chemical corrosion resistance.However,the electrical treeing problem occurs frequently in EP in the condition of high frequency and high voltage,which leads to the breakdown and failure of insulation materials.Therefore,it is of guiding significance to study the electrical treeing characteristics of epoxy resin for the safe and stable operation of electrical equipment.Nano-modification is an effective way to improve the electrical properties of dielectric materials.To study the effect of nano-MgO on the electrical treeing characteristics of EP,nano-MgO/EP composites with different mass fractions(0～1wt%)were prepared.The initiation and growth of the electrical trees were observed under power frequency voltage.The results show that the electrical treeing resistance of composites increases with the increase of the doping ratio when it is relatively low.When the mass fraction of nano-MgO is 1wt%,the electrical treeing initiation probability and electrical tree length of composites reduce by 45%and about 300%,respectively,and the growth rate is the slowest.The corresponding AC breakdown strength increases by 14.1%and the composites have the best resistance to electrical treeing.According to the analysis of trap characteristics,the trapping energy level increases from 0.9 eV of pure EP to 1.11 eV at a lwt%doping ratio.With the addition of nano-MgO,the trapping energy level of the composites increases,and the mobility and concentration of carries decrease,improving the electrical treeing resistance of nano-MgO/EP composite.The electrical treeing properties of pure EP and EP composite doped with 1wt%MgO at different frequencies were studied.The results show that the electrical treeing initiation voltage of pure EP decreases with the increase of frequency,dropping by 54%at 130 kHz compared to that at 50 Hz.With the increase of frequency,the electrical treeing initiation speed increases and peaks at 400 Hz before decreasing to the lowest at 130 kHz.With the increasing frequency,the growth rate of the electrical tree rises and becomes highest at 130 kHz.Meanwhile,the expansion coefficient becomes larger and the color of the electrical tree becomes deeper.At all frequencies,the initiation voltage and expansion coefficient of nano-MgO/EP is higher than that of pure EP.The initiation speed and growth speed of electrical treeing of MgO/EP are lower than that of pure EP,and the color of the electrical tree is slightly lighter than that of pure EP.Therefore,nano-MgO can improve the electrical treeing resistance of EP in a wide range of frequencies. |
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