D. M. Tu

High-field induced electrical aging in polypropylene films

According to Kao’s model of electrical discharge and breakdown in condensed insulating materials, charge carrier injection from electrical contacts and subsequent dissociative trapping, and recombination play a decisive role in the breaking of polymer chains and the creation of free radicals or low-weight molecules, and hence traps. We believe that electrical aging is due to this gradual degradation process. The increase in structural degradation and trap concentration reflects the degree of electrical aging and hence the lifetime of the electrically stressed polymers. On the basis of this concept, we have derived a theoretical formula for the prediction of the lifetime of insulating polymers. We have also carried out experiments on polypropylene films at high electric fields. It is found that destructive breakdown occurs when the accumulation of field induced traps reaches a certain critical value. Experimental results are in good agreement with our theoretical model.

Dielectric Properties and Crystalline Morphology of Low Density Polyethylene Blended with Metallocene Catalyzed Polyethylene

Dielectric properties of low-density polyethylene (LDPE) blended with metallocene catalyzed polyethylene (MPE) to 1, 3, 5 wt% of the latter are reported. It was found that the 1 wt% MPE blend had the lowest volume resistivity, the highest direct current (DC) breakdown strength and the least accumulated space charge. The crystalline morphology of the blends was studied through differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD) and small-angle light scattering (SALS) measurements. It was found that blending increased the percentage crystallinity, decreased the spherulite size and caused the formation of imperfect spherulites, i.e. spherulites containing defects and impurities in their crystalline phases, and thus fewer impurities and defects on their boundaries. The improvement in dielectric properties of the blends, especially the 1 wt% MPE blend, is attributed to the increase in crystallinity and the formation of imperfect spherulites.

Effect of space charge in nanocomposite of LDPE/TiO/sub 2/

Nanocomposite of low density polyethylene (LDPE)/TiO/sub 2/ was prepared via solution blending method. The disperse performance of TiO/sub 2/ in LDPE was observed with Scanning Electron Microscope. It is shown that TiO/sub 2/ could be dispersed uniformly in LDPE through controlling temperature and content of the solution of LDPE, and the largest conglomeration diameter of TiO/sub 2/ in LDPE is less than 80 nm. At the same time, space charge distribution of samples with and without nano TiO/sub 2/ was measured with Pulsed Electroacoustic Method. Hetero-polar space charge near electrodes is much less in sample of nanocomposite of LDPE/TiO/sub 2/ than that in pure LDPE under lower direct current (DC) stress (no more than 40 kV/mm), and space charge in the central of sample of LDPE/TiO/sub 2/ is much more uniform than that in pure LDPE. Thus electrical stress concentration is improved under DC stress in LDPE/TiO/sub 2/. After being pre-stressed under stress of 50 kV/mm for 1h, samples with and without nano TiO/sub 2/ were short-circuited, and space charge in these samples was measured. It is shown that decaying rate of the remnant of space charge in LDPE sample containing TiO/sub 2/ increases with increasing of the TiO/sub 2/.

Effect of voltage reversal on space charge and transient field in LDPE films under temperature gradient

Voltage reversal will strongly affect the stability of cable in service, especially under temperature gradient due to the temperature rise of the cable. In this paper, the results of space charge profiles and field distortions in low density polyethylene (LDPE) films after dc pre-stress of 50 kV/mm for 2 h under temperature gradient and then during different voltage reverse polarities or during different voltage reverse periods were presented. The test results show that the charge accumulation and field distortion are different under different voltage polarities due to the dependence of charge injection on electrode materials. In this work, under the Al / SC (Semiconductive) electrode system with a temperature difference of 40°C, the maximal transient field occurs near the low temperature Al electrode when polarity varies from negative to positive while near the high temperature SC electrode when polarity varies from positive to negative. In addition, the faster the voltage reversal at room temperature the higher the maximal transient field, but the faster the voltage reversal the lower the maximal transient field under temperature gradient. Moreover, the maximal field appears during voltage inversion at room temperature while appears at pre-stress under temperature gradient.

Effect of Free Radical Scavenger on Space Charge Distribution in Polyethylene during Electrical Aging

Electrical aging of polyethylene was inhibited effectively by incorporation of a free radical scavenger (FRS). Samples of polyethylene with and without the FRS were electrically aged for various times and then the space charge distributions of these samples under dc stress and short-circuited were measured, respectively. It is shown that during electrical aging, the space charge distribution of all samples changed in a complex manner, and splitting or superposing of positive or negative charge peaks occurred at the same time. In addition, the total amount of space charge linearly increased with aging time, which corresponds to the remnant breakdown strength of the samples. Considering to the degradation of polyethylene under other types of stresses, the effects of the FRS of reducing the production of space charge and inhibiting electrical aging in polyethylene were discussed. Finally, the feasibility of space charge as a parameter in quantifying electrical aging of polyethylene was discussed.