The effects of crosslinking byproducts on the electrical properties of low density polyethylene

Abstract

Crosslinked polyethylene (XLPE) is widely used for high voltage insulation in power transmission systems. However, it has been found that, after crosslinking with Dicumyl Peroxide (DCP), the crosslinking byproducts such as acetophenone, ?-methylstyrene and cumyl alcohol have a significant influence on electrical properties of XLPE power cables. This thesis distinguished the individual contribution of the crosslinking byproducts on space charge formation, dielectric properties, dc conductivity as well as the ac breakdown strength. Percentage weight increases as well as the Fourier Transform Infrared (FTIR) spectrum were used to monitor the chemical level in the soaked samples. Despite high concentration of byproducts in the LDPE film compared to practical, the measurement results have successfully reveal the contribution of each byproduct on the electrical properties. It should be noted that some consideration should be taken when taking the quantitative value from the result obtained. Space charge accumulation was measured using the pulsed electroacoustic (PEA) technique. Homocharges are observed in acetophenone and ?-methylstyrene soaked LDPE. Meanwhile heterocharge formed in cumyl alcohol soaked LDPE. From the charge decay profile in dc condition, these chemicals are observed to assist the transportation of the charges in the sample bulk due to shallow traps from the byproducts. These shallow traps assist the trapping process into deep traps when ac field is applied to the byproduct soaked LDPE. As a result, more charges trapped in deep traps were found in soaked LDPE compared to clean LDPE. In addition, from the space charge measurement in ac condition, it is proved that the amount of charge trapped in deep traps also depends on the population of shallow traps in the polymer which is contradicted to the literature where the byproducts are normally associated to the deep traps. Permittivity values of acetophenone, ?-methylstyrene soaked LDPE and cumyl alcohol are slightly higher than permittivity value of the clean untreated LDPE. Cumyl alcohol soaked LDPE has higher dielectric loss at lower frequency due to Maxwell-Wagner-Sillars polarisation as well as space charge polarisation effect. In contrast, acetophenone does not change the dielectric loss value and ?-methylstyrene gives very little effect. These byproducts have very high dc conductivity values. It is also proposed that the chemicals provide shallow traps that aid the charge movement and this is consistent with the mobility values that calculated from the conduction current result. The ac breakdown results however show no significant difference from the breakdown strength of clean LDPE. Based on ac space charge results and ac breakdown test results, it is concluded that the byproducts have little effects in ac condition

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