1、 Moisture and other impurities

　　水在变压器油中有两种状态：高度分散、且分布非常均匀，可视为溶解状态；呈水珠状一滴一滴悬浮在油中，为悬浮状态。悬浮状水滴在油中是十分有害的，因他们在电场作用下将被极化而沿电场方向伸长，会畸变油中的电场分布，并可能在电极间连成小桥。

　　There are two states of water in transformer oil: highly dispersed and evenly distributed, which can be regarded as dissolved state; water droplets suspended in the oil drop by drop. Suspended water droplets are very harmful in oil, because they will be polarized and elongated along the direction of electric field under the action of electric field, which will distort the distribution of electric field in oil and may form small bridges between electrodes.

　　当油中还含有其他固体杂质时，击穿电压的下降程度随杂质的种类和数量而异。纤维的含量即使很少，也对击穿电压有很大影响。因为纤维是极性介质并且易吸潮，很易沿电场方向极化定向而排列成小桥，从油中分解出来的碳粒却对油的击穿电压影响不大。

　　When there are other solid impurities in the oil, the decrease of breakdown voltage varies with the type and quantity of impurities. Even if the fiber content is small, it has a great influence on the breakdown voltage. Because the fiber is a polar medium and easy to absorb moisture, it is easy to form a small bridge along the direction of electric field polarization. However, the carbon particles decomposed from oil have little effect on the breakdown voltage of oil.

　　二、油温

　　2、 Oil temperature

　　变压器油的击穿电压与温度的关系比较复杂，随电场的均匀程度、油的品质以及电压类型的不同而异。

　　The relationship between breakdown voltage and temperature of transformer oil is complex, which varies with the uniformity of electric field, oil quality and voltage type.

　　均匀电场油间隙的工频击穿电压与温度的关系如上图所示。曲线2为潮湿的油，当油温由0°C开始上升时，一部分水分从悬浮状态转化为危害较小的溶解状态，使击穿电压上升；但在温度超过80°C时，水开始汽化，产生气泡，引起击穿电压下降，从而在60-80°C的范围内出现最大值；在0-5°C时，全部水分转为乳浊状态，导电小桥最易形成，出现击穿电压的最小值；再降低温度，水滴冻结成冰粒，油也将逐渐凝固，使击穿电压提高。曲线1是干燥的油，这时随着温度升温，击穿电压略有下降。

　　The relationship between power frequency breakdown voltage and temperature of oil gap in uniform electric field is shown in the figure above. Curve 2 is wet oil. When the oil temperature starts to rise from 0 ° C, part of water changes from suspended state to dissolved state with less harm, which makes breakdown voltage rise; however, when the temperature exceeds 80 ° C, water begins to vaporize, resulting in bubbles, which leads to the drop of breakdown voltage, resulting in the maximum value in the range of 60-80 ° C; when the oil temperature is 0-5 ° C, all moisture changes to turbid state and conducts small bridge When the temperature is lowered, the water droplets will freeze into ice particles, and the oil will gradually solidify, which will increase the breakdown voltage. Curve 1 is dry oil, and the breakdown voltage decreases slightly with the increase of temperature.
                

　　在极有均匀电场中，随着油温的上升工频击穿电压稍有下降，电压的下降可用电子碰撞电离理论来说明，水滴等杂质不影响极不均匀电场中的工频击穿电压。

　　In the extremely uniform electric field, with the increase of oil temperature, the power frequency breakdown voltage slightly decreases. The decrease of voltage can be explained by the theory of electron impact ionization. The impurities such as water droplets do not affect the power frequency breakdown voltage in extremely uneven electric field.

　　不论在均匀电场中，还是在不均匀电场中，随着温度的上升，冲击击穿电压均单调地稍有下降。这也可借助电子碰撞电离理论加以解释，而水滴等杂质的影响很小，因为在冲击电压作用下来不及形成杂质小桥。

　　Both in the uniform electric field and in the inhomogeneous electric field, the impulse breakdown voltage decreases monotonously with the increase of temperature. This can also be explained by the theory of electron impact ionization, and the influence of impurities such as water drops is very small, because the impurity bridge is not formed under the action of impulse voltage.

　　三、电场均匀度

　　3、 Uniformity of electric field

　　保持油温不变，而改善电场的均匀度，能使优质油的工频击穿电压显著增大，也能大大提高其冲击击穿电压。品质差的油含杂质较多，故改善电场对于提高其工频击穿电压的效果也较差。在冲击电压下，改善电场也是能显著提高油隙的冲击击穿电压，由于冲击电压下作用时间短，杂质来不及形成小桥，所以冲击击穿电压与油的品质好坏几乎无关。

　　Keeping the oil temperature constant and improving the uniformity of electric field can increase the power frequency breakdown voltage and impulse breakdown voltage of high-quality oil. The oil with poor quality contains more impurities, so the effect of improving the electric field on improving the power frequency breakdown voltage is also poor. Under the impulse voltage, improving the electric field can also significantly increase the impulse breakdown voltage of the oil gap. Due to the short action time under the impulse voltage, the impurities have no time to form a small bridge, so the impulse breakdown voltage has little to do with the quality of the oil.

　　四、电压作用时间

　　4、 Voltage action time

　　油隙的击穿电压会随电压作用时间的增加而下降，加电压时间还会影响油的击穿性质。在电压作用短至几个微妙时击穿电压很高，击穿有时延特性，属电压击穿；电压作用时间为数十到数百微秒时，杂质的影响还不能显示出来，仍为电击穿，这是影响油隙击穿电压的主要因素是电场的均匀程度；电压作用时间更长时，杂质开始聚集，油隙的击穿开始出现热过程，于是击穿电压再度下降，为热击穿。

　　The breakdown voltage of the oil gap will decrease with the increase of the voltage acting time, and the breakdown property of the oil will be affected by the voltage increasing time. The breakdown voltage is very high when the voltage action is as short as a few microseconds, the breakdown voltage is very high, the breakdown time delay characteristic, belongs to the voltage breakdown; when the voltage action time is tens to hundreds of microseconds, the influence of impurities can not be displayed, it is still electric breakdown, which is the main factor affecting the breakdown voltage of oil gap, which is the uniformity of electric field; when the voltage is applied for a longer time, the impurities begin to accumulate and the breakdown of oil gap begins Thermal process occurs, so the breakdown voltage decreases again, which is thermal breakdown.

　　五、油压的影响

　　5、 Influence of oil pressure

　　不论电场均匀度如何，工程变压器油的工频击穿电压总是随油压的增加而增大，这是因为油中气泡的电离电压增高和气体在油中的溶解度增大的缘故。但是经过脱气处理的油，其工频击穿电压就几乎与油压无关。

　　Regardless of the uniformity of electric field, the power frequency breakdown voltage of Engineering transformer oil always increases with the increase of oil pressure, which is due to the increase of ionization voltage of bubbles in oil and the increase of solubility of gas in oil. However, the breakdown voltage of degassed oil is almost independent of oil pressure.