The influence of survival mechanical stress and voltage frequency on electrical tree in XLPE

The initiation and growth of electrical trees under high electric stress have been studied in XLPE to explore the effects of the internal mechanical stress. Treeing tests were carried out under an applied voltage of 7 kV rms and both tree initiation and growth were monitored. The needle with a tip radius around 5 μm was inserted into an XLPE block at elevated temperature that was cut from a XLPE power cable. Two types of samples have been prepared i.e. one with a significant internal mechanical stress around the tip of the needle and the other without. It was found that in the presence of the mechanical stress around the tip the tree initiation time is significantly shorter and growth rate is faster compared with those without. The combining effects of the internal mechanical stress and the frequency of the applied voltage on the structure of electrical tree during the growth stage have also been investigated. For example, in the presence of the internal mechanical stress, the results showed that the tree has a mixture of branch-like and bush-like structures at low frequency. In the absence of the mechanical stress, the results indicated that the tree at the early stage has many thicker branches at high frequency and thinner branches at low frequency.