Lightning Impulse Breakdown and Acceleration Voltages of a Synthetic Ester Liquid at Large Gaps

In addition to mineral oils, ester liquids including both synthetic and natural ester have become an option of insulating liquids for high voltage large power transformers. Breakdown characteristics of insulating liquids under lightning impulse have been widely investigated which contributes to transformer insulation design. This paper presents the experimental studies on the breakdown and acceleration voltages of a synthetic ester liquid under standard lightning impulse at large gaps. Tests were conducted with needle-plane electrodes at various gaps from 25 mm to 200 mm. The results indicate that both the breakdown and acceleration voltages increase with gap distances but with a reducing increasing rate. As expected, negative polarity results in higher breakdown and acceleration voltages than positive polarity. In addition, the 50% breakdown voltages become the same as the acceleration voltages when the gap distance increases to 75 mm for both polarities. At 200 mm gap distance, the measured 50% breakdown voltages (same as acceleration voltages) are 187.6 kV and 413.72 kV for positive and negative polarities, respectively.