Understanding electrical treeing activity in electron beam irradiated XLPE cable insulation

Tree-like-trees are observed with electron beam irradiated XLPE cable insulation under AC voltages. Charge accumulation studies indicate that the level of charge deposited by +/- DC corona charging is almost same. However, the amount of accumulated charge increases with increase in level of irradiation of electron beam. The decay time constants are marginally high with positive charge deposition. Moreover, the decay time constant of accumulated charges increases with increase in level of irradiation of the material. The rise time of injected current signal formed during tree growth in positive and negative half cycle is about 900 and 700 ps, respectively. The UHF signal generated during tree growth in virgin and with irradiated specimen has its frequency content in the range 0.5-1.5 GHz. Weibull distribution studies indicate that the life time of electron beam irradiated specimen drastically gets reduced due to treeing. Sequential measurement of UHF signal indicates that tree growth involves high and low intensity discharges. Phase resolved partial discharge studies using spectrum analyzer indicates that discharges occur at the raising portion or near zero crossing of the applied AC voltage. FTIR spectroscopy and analytical pyrolysis-GC/MS analyses of the electron beam irradiated samples reveal the formation of alkoxy and alkyl peroxy radicals, and carbonyl and hydroxyl groups via scission of the crosslinks. The weak crosslinking sites are proposed to act as nodal points from where tree growth occurs.