Electrochemical treeing in PE and XLPE insulated cables - frequency effects and impulse degradation

This paper describes the effect of frequency on the rate of growth of electrochemical trees in PE and XLPE insulation, extruded into thin wall mini-cables over a #14 AWG copper wire (round and square conductors). The cables were subjected to treeing tests at several frequencies in the range of 60 to 8000 Hz. The tree growth rate is increased by increasing the frequency, and there appears to be saturation in this effect at approximately 1000Hz. At all frequencies the tree growth rate in XLPE was considerably less than that in PE. The results of dielectric tests on cables subjected to accelerated treeing tests indicate that the trees reduce both DC and impulse breakdown strength of the mini-cables. When trees penetrate the wall, the impulse strength drops to about 40% of its original value in both PE and XLPE cables. As the streamer trees grow in length, a substantial reduction in DC breakdown strength was observed.