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As replacement of mineral oil, the application of ester liquids in large power transformers has become a hot topic in recent years. Understanding esters' dielectric behaviors under ac voltage is of vital importance for the insulation design of power transformers. This paper presents experimental studies on the ac dielectric strengths of synthetic ester and natural ester in quasi-uniform electric field, using mineral oil as a benchmark. The results show that esters have similar breakdown voltages to mineral oil when filtered and dehydrated. However, their breakdown voltages are significantly higher than mineral oil with the presence of particle and water contaminations. Furthermore, the breakdown voltages of esters are reduced more sharply than that of mineral oil with the increase of electrodes' Effective Stressed Area (ESA). Based on the experimental results, an estimation of the withstand voltages of transformer liquids in practical qualities was made when considering the particle effect, water effect and electrode area effect, and it was found that the 1-mm-gap withstand voltages of esters are higher than that of mineral oil.