The tree initiation voltage of impregnated crosslinked polyethylene (XLPE)
with a dielectric liquid is drastically improved compared to normal XLPE fo
r both ac and impulse voltage application. It is considered that liquid fil
ls the polymer free volume, and removes space for electrons to be accelerat
ed to high energy. The experimental results support that collision impact o
f electrons accelerated in polymer free volume is the dominant cause of ele
ctrical tree initiation, and also show that liquid impregnation is a promis
ing way to improve performance and prolong the life of polymeric insulating
materials.