Vinu R

Corona activity on the surface of silicone rubber has high impact on variation in hydrophobicity of the material, and on corona inception voltage (CIV) of water droplet sitting on the corona degraded surface. The CIV is high under negative DC as compared to positive DC/AC voltages. Increase in conductivity of water droplet reduces CIV. Surface charge accumulation studies indicate that the charge retention capability of the insulating material gets reduced drastically with silicone rubber damaged due to corona causing degradation of surface. Irrespective of the number of cycles of corona discharge activities carried out, the time constant of charge decay, after the removal of charge injection, remains the same. The water droplet initiated discharges under AC and DC voltages radiate UHF signal in the bandwidth 0.3-1.2 GHz. With single droplet sitting on top of the corona damaged silicone rubber, the amount of charge retained by the water droplet is low when the droplet is placed and charged immediately after corona discharge activity to surface of the insulating material. When the water droplet is placed after a certain time period, the charge accumulation characteristics of silicone rubber are regained. Leakage current measured during water droplet discharge studies through recurrent plot (RP) analysis reveals that, under AC voltage, it is possible to classify corona inception, arcing and bridging of water droplet between high voltage and ground electrode. The RP obtained for the current signals generated due to water droplet discharges under positive and negative DC voltages are different. The amount of damage caused to silicone rubber due to water droplet is high under negative DC voltage as compared to positive/AC voltages. While correlating the level of damage to the insulating material due to water droplet and the RQA parameters of RP, it is observed that higher the recurrence rate along with high determinism can cause severe damage to the insulating material. Structural analysis using ATR-FTIR and pyrolysis-GC/MS studies reveal significant functional group variation on surface of silicone rubber, especially when the number of cycles of corona ageing is more.

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.