Analysis of deterioration by partial discharge of XLPE using GCMS and FTIR
Deterioration of polymeric dielectrics by partial discharge (PD) might occur at defects in an insulation layer. To elucidate the mechanism of deterioration by PD, measurements were conducted of the gas components produced during deterioration and the substances created on the surface of cross-linked polyethylene (XLPE) samples subjected to PD. Gas component measurements were performed using an experimental apparatus including GC-MS. Results of gas analyses showed that carbon dioxide (CO2) was produced when the XLPE sample is subjected to the PD in a N2+O2 gas atmosphere. The amount of generated CO2 was confirmed to increase concomitantly with the period of voltage application. The FTIR spectroscopic analysis of substances on the surface of deteriorated XLPE revealed carbonyl compounds on the surface of the deteriorated samples. The deterioration was suppressed when a phenolic antioxidant was added: 44-thiobis-3methyl-6tert butyl phenol. The process of deterioration by PD and the effect of the antioxidant on the deterioration by PD were also discussed.
Effect of antioxidants, on deterioration by partial discharge in LDPE and XLPE [insulation materials]
The effect of antioxidants in low-density polyethylene (LDPE) and cross-linked polyethylene (XLPE) on deterioration by partial discharge (PD) was investigated. Experiments were conducted using LDPE and XLPE samples containing phenolic antioxidants. Samples subjected to the PD were analyzed using atomic force microscopy and Fourier transform infrared analysis (AFM and FTIR). AFM images demonstrated that the magnitude of pits created in the sample containing antioxidants was smaller than that in samples containing no antioxidant. FTIR showed that oxidative degradation occurred on LDPE and XLPE surfaces when they were subjected to PD. These results confirmed that the amount of oxidized substances was smaller in samples containing phenolic antioxidants than in those containing no antioxidant. This report elucidates the mechanism of antioxidant action on deterioration by PD.
Degradation of Polyethylene by Partial Discharge
To explore the fundamental concept concerning testing methods of corona resistance of insulating materials, some "roundrobin" tests with polyethylene have been performed by means of parallel-plate electrodes. The specimen films, degraded under the conditions of 6 kV ac impression and of static dry air, are examined with regard to weight, infrared spectrum, and other changes. From these results, information about degradation mechanisms was obtained. From these studies, the corona degradation behavior of polyethylene might be considered to include two kinds of chemical reactions. First, activated oxygen formed by discharge is able to oxidize perfectly the surface of the specimen exposed directly to discharge and to yield H2O, CO2, and other products, although the portion not directly exposed to the discharge is hardly affected owing to the extremely short life of these active species. At the same time, ozone with relatively long life diffuses into polyethylene and is able to form an ozonide by reacting with a terminal double bond, while it makes carboxylic acid and others by combining with the main chain in the polymer. Moreover, nitrogen oxide causes nitrate ester to be formed. Several conditions may control these two competitive reactions and the degree of competition is probably the reason for the fluctuation of results.