频率对高密度聚乙烯电树老化特性的影响

为了解频率对高密度聚乙烯电树老化特性的影响,在50Hz~90kHz较宽频率范围的交流电压作用下,研究了冰水淬火高密度聚乙烯(HDPE)薄膜的电树老化特性。结果表明,频率对电树起始形态具有重要的影响,随着电压频率的升高,树枝型电树的起始几率逐渐降低,丛状型电树的起始几率逐渐升高,电树逐渐由树枝型起始为主向丛状型起始为主转变,树干型和直击型为高频下所特有的电树起始形态。随着电树的生长,电树形态存在转换的可能,低频下,起始占主导的树枝型电树向丛状和树干型转变;高频下,起始占主导的丛状型电树则极易转变为树干和击穿型,导致绝缘的破坏。电树的发展可分为起始、滞长、生长和击穿期4个阶段。频率的提高加快了电树的发展速度且减少了电树的发展阶段,使发生击穿的几率大为增加。

Effect of Frequency on Electrical Treeing Characteristicsof High-density Polyethylene

The characteristics of electrical treeing in ice-water quenched high-density polyethylene (HDPE) film under AC voltage of different frequency ranging from 50 Hz to 90 kHz was studied. The electrical treeing experiments are carried out at room-temperature. The processes of electrical treeing during the experiment are observed via a digital micro-observing system. It was found that under different frequency of applied voltage the initiation shape of electrical tree was different, it means the different mechanisms of electrical tree initiation. There are 4 kinds of trees in inception period: branch tree, trunk tree, bush tree and breakdown tree. Trunk tree and breakdown tree with high initiation speeds are special kinds appearing in high frequency (HF). The branch tree is the primary shape in the range of low frequency, while the bush tree is the primary shape in the range of HF. With the increase of frequency, the occurrence ratio of branch tree decreases, while the occurrence ratio of bush tree increases. It was also shown that as the propagation of electrical tree, the conversion of tree shape is strongly affected by the frequency. The electrical tree usually has a simple shape and rarely changes under low frequency in one hour or more. However, in the case of a higher frequency, the electrical tree shape changes more frequently and has a large probability to breakdown. The result also showed that there were 4 steps in the electrical tree propagating process: initiation step, stagnation step, growth step and breakdown step. Under the same voltage, high frequency could accelerate the electrical tree propagation. Higher frequency could speed up the tree growing and increase the probability of breakdown caused by electrical tree. The influence of frequency on material polarization process, fatigue and thermo effect were considered as the main causes of the experiment results. The more serious destruction caused by high frequency electrical treeing degradation can well explain the reason of insulation premature inactivation in mid-frequency or high-frequency electrical equipment.