AC Breakdown characteristics of LDPE in the presence of crosslinking byproducts. N. Hussin and G. Chen University of Southampton, Southampton, UK Weibull analysis and AC conductivity AC breakdown test result Introduction Methods Conclusions University of Southampton, Highfield, Southampton, SO17 1BJ, UK Contact details : Figure 2-5 show the breakdown strength characteristic of fresh LDPE, acetophenone soaked LDPE, α-methylstyrene soaked LDPE and cumyl alcohol soaked LDPE respectively. Comparing the fresh sample and the byproducts soaked samples, it does show a small reduction of the α values as the LDPE films were soaked into the byproducts. It suggests that the breakdown strength is reduced by adding the byproducts in the LDPE film. These values are presented in Table 1. Nevertheless, this suggestion is not totally true as the range of breakdown strength of all samples are to be compared. These values fall in the same region which indicate no significant difference can be seen in all samples. Figure 4: The E b plot of α- methylstyrene soaked LDPE LDPE films of 50µm thick were soaked into acetophenone and α- methylstyrene at room temperature meanwhile into cumyl alcohol at 80°C. The samples were used to perform the breakdown strength (E b ) of the LDPE with the byproducts chemical reside in the sample. The AC breakdown measurements were conducted at a ramp rate of 50V/s at room temperature. Ball-bearing electrodes were used. Weibull distribution is used to analyse the AC breakdown result. Apakah kaitan antara permittvity and berakdown strength ? In gas filled void, permittivity is much lower than the surrounding polymer, so field intensification occurs at the end of the walls in the direction of electric field. Figure 2 : The E b plot of clean LDPE The probability of failure for two parameter Weibull distribution can be expressed by: Where β (beta) and α (eta) are respectively the shape and scale parameter, and E is the breakdown field. AC breakdown test has been performed on the clean LDPE and crosslinking byproducts soaked LDPE. From the Weibull analysis it can be concluded that a)Byproducts improved the β values but reduce the α values. b)However the reduction of α values is not significant and by looking at the range of the breakdown field of all samples, it could be concluded that acetophenone, α- methylstyrene and cumyl alcohol do not alter the AC breakdown strength of the LDPE film. The effect of crosslinking byproducts on space charge has been shown in the previous work. The accumulation of space charge in insulation is known to be one of the reason of premature breakdown to occur. Thus it is interesting to see the breakdown strength of LDPE in the presence of the crosslinking byproducts. With the polar nature of the crosslinking byproducts, some dielectric properties of byproduct soaked LDPE is expected to be observed especially during AC measurement. Figure 3: The E b plot of acetophenone soaked LDPE Figure 5: The E b plot of cumyl alcohol soaked LDPE Table 1 : Weibull analysis of breakdown strength test result The small difference of the AC breakdown strength of the clean and soaked samples is similar to the AC conductivity values of the samples which was measured using the Dielectric spectroscopy. This values are calculated from the relation : Where σ’ is conductivity, ω is the frequency, while ε 0 and ε” are respectively the permittivity of air and imaginary part of the permittivity of the material. In Figure 6, the acetophenone and α- methylstyrene soaked LDPE have almost similar conductivity values through out the range of frequency. The deviation of conductivity characteristic of cumyl alcohol soaked LDPE compare to the rest of the samples is still unidentified. These two measurements suggest that crosslinking byproducts has a small effect on the electrical properties that is related to alternating current. Permittivity values of the samples which presented in Figure 7 also show small effect of the byproducts to the electrical property. Figure 6 : AC conductivity of the samples over the frequency Acetophenone α-methylstyrene Cumyl Alcohol Figure 1 : schematic diagram of the breakdown rig. Figure 7 : Permittivity of the samples over the frequency Samples % added crosslinking byproducts (%) Beta, β Eta, α(kV/mm) Range of E b (kV/mm) LDPE LDPE + Acetophenone LDPE + Alpha Methylstyrene LDPE + Cumyl alcohol