Dielectric Properties of Polypropylene Containing Nano-Particles

Peer reviewed: NoNRC publication: Ye

Evolution of space charges and conductivity with dc aging of polyethylene-synthetic and natural clay composites

The evolution of the space charge and conductivity with DC poling of two types of polypropylene-(PP-) based nanocomposites (PNCs) was investigated. The PNCs were manufactured with different concentrations of synthetic and natural organoclays. The optimal concentrations of nanofiller that can efficiently mitigate the space charge with DC aging time were 2-wt% for PP-natural-clay and between 2 and 4 wt% for the PP-synthetic-clay. Above these percentages charge transport through overlapping of nanoparticles can occur due to the interaction zone of double layers formed at the nanoparticle/host material interfaces. Under DC field the overlapping increases the conductivity of PNCs and minimizes the benefit of incorporating nanofillers into PP. The total charge stored in unfilled PP increased continuously with time reaching a maximum around 5000 h before decreasing but it also changed slightly in all filled specimens. It was perceived that the smaller the size of nanofiller platelets the more efficient the charge mitigation. The conductivity of specimens containing 6 wt% of natural clay and 8 wt% of synthetic clay reached 486 times the level of the unfilled PP. This observation could be related and due to the crossing of the percolation threshold for these composites. \ua9 2012 Mahmoud Abou-Dakka.Peer reviewed: YesNRC publication: Ye

Evolution of some dielectric properties of polypropylene-nanoclay composites with DC poling

Space charge and DC conductivity evolution with DC poling of two types of polypropylene-based nanocomposites containing different concentrations of nanofiller were investigated. The two nanofillers used were natural and synthetic organoclays. It was observed that the optimal concentration of a nanofiller for mitigating space charge in polypropylene (PP) is 2-wt% and between 2 and 4-wt% for the natural and synthetic clay, respectively. Excessive quantity of nanofiller could lead to overlapping of the interaction zone of double layers formed at the nanoparticle/host material interfaces, which promotes charge transport. Under DC field this overlapping increases conductivity of a nanocomposite and thus could minimize the benefit of incorporating nanofillers into PP. The total charge stored in unfilled PP increased continuously with time but increased only slightly for filled specimens. After only 504 h of DC poling, at 25 kV/mm, the conductivity of specimens containing 6-wt% of natural clay and 8-wt% of synthetic clay reached 48 6 times the level for unfilled PP. This observation could be related to crossing the percolation threshold of these composites. The effect of platelet size on space charge mitigation reported by other authors has been confirmed in this work, i.e. nanofillers with smaller platelets mitigate space charge more efficiently then nanofillers with larger platelets. \ua9 2011 IEEE.Peer reviewed: YesNRC publication: Ye

On-site diagnostic technique for smart maintenance of power cables

Polymeric insulation of power cables has excellent short-term dielectric properties but is subjected to serious degradation problems due to defects, water, ionic contamination, partial discharges, electrical treeing, etc. Since more than 20% of installed underground power cables are older than their design life-time, utilities are faced with important maintenance related questions whether to repair, refurbish or completely replace old cables. In the era of Smart Grid, accurate assessment of the operating state of cable insulation through reliable diagnostic measurements is crucial to the so called "Smart Maintenance", i.e. the replacement of only those cables that may adversely affect the reliability of the network in the near future. At the National Research Council of Canada, an on-site testing technique for estimating the degree of degradation of power cables based on the polarization/depolarization current measurements has been developed and tested for two years on underground power network of a local utility. Using a noiseless, high voltage solid state switch, both the high frequency (HF) and the low frequency components of the depolarization current (I Dep) were measured and analyzed. It was observed that the area under the HF component of the I Dep curve, Q Dep, could be linked to the dielectric loss factor, and the intensity of water treeing in cable insulation. Furthermore, it was observed that the I Dep waveforms of new and aged cables could be fitted to an equation, which contains a parameter that correlates with the degree of insulation degradation. This new insulation degradation indicator and the Q Dep characteristics could be used for monitoring the operating condition of cables in service and would help utilities to develop cable replacement programs based on the actual state of the insulation operating conditions. Long term aging ( 3c 3.5 yrs) of XLPE miniature cables in NaCl solution has shown a large increase of the area under the polarization current curve, Q P with aging time. Comparing the Q P characteristics of miniature cables with those of cables in the field could also help to assess the degree of insulation aging. \ua9 2011 IEEE.Peer reviewed: YesNRC publication: Ye

Linear oscillatory dynamics of flexoelectric membranes embedded in viscoelastic media with applications to outer hair cells

Abstract not availableM. Abou-Dakka, E.E. Herrera-Valencia, Alejandro D. Re

Space charge evolution in polypropylene containing synthetic and natural organoclays

Fully synthetic tetrasilisic mica from Topy Co., Ltd. and Cloisite\uae 20A powder of Wyoming natural montmorillonite clay from Southern Clay Products were used to manufacture poly-propylene-based nanocomposites with organoclay concentrations of up to 8-wt%. Both types of nanofiller reduce the amount of space charge accumulated under a dc field as compared with the base material without any nanoparticles but their concentrations have to be kept below 3c4-wt% to achieve this effect. For the same particle concentration, nanocomposites containing natural clay, which has significantly smaller aspect ratio of the individual platelets than the synthetic clay, acquire less space charge than the nanocomposites containing fully synthetic clay.Peer reviewed: YesNRC publication: Ye

Effect of additives on the performance of cross-linked polyethylene subjected to long term single and periodically reversed polarity DC voltage

The performance of cross-linked polyethylene with and without tree-retarding additives, aged with single and reversed polarity DC voltage was investigated. Under single polarity field of a 50 kV/mm, cross-linked polyethylene without the tree retardant (XLPE) showed significantly longer time to breakdown than the material with a tree retardant (TR-XLPE). This is attributed to the difference in how the two materials accumulate and retain space charge. In the case of polarity reversals, TR-XLPE showed better performance than XLPE. This seeming contradiction was studied by measuring the dynamics of space charge evolution with time. The Thermal Step Method (TSM) of space charge measurements was used. It showed that after the polarity reversal the TSM current rapidly inversed in the TR-XLPE but not in the XLPE. This implies more rapid charge dissipation in TR-XLPE. The retained charges cause the electric field enhancement in the material when the polarity is reversed. Thus, after each polarity reversal XLPE was subjected to higher local electric stress for a longer time and thus broke down sooner than TR-XLPE. For both materials the space charge decreased faster when the polarity was changed from negative to positive than vice versa. This could be explained by more efficient electron than holes injection. Since the control of the power flow in DC networks requires reversals of voltage polarity, their detrimental effect on XLPE-insulated equipment, such as cables, cannot be avoided. The effect could be mitigated by the application of carefully designed additives. However, as the present study indicates, additives that improve insulation performance under polarity reversals can be harmful under single polarity voltage.Peer reviewed: YesNRC publication: Ye

Space charge development and breakdown in XLPE under DC field

NRC publication: Ye

Dielectric properties of polypropylene containing synthetic and natural organoclays

Fully synthetic tetrasilisic mica from Topy Co., Ltd. and Cloisite\uae powder of Wyoming natural montmorillonite clay from Southern Clay Products were used to manufacture polypropylene-based nanocomposites with organoclay concentrations up to 8%-wt. Both types of nano-filler increase the AC breakdown strength by approx. 10% over the unfilled material. However, the resistance to partial discharges is significantly improved, with both nano-fillers yielding similar results. Nano-fillers increase dielectric losses of PP over broad frequency and temperature ranges. Specimens containing natural clay show higher losses than the specimens containing synthetic clay, especially at higher frequencies and temperatures.Peer reviewed: YesNRC publication: Ye

Phase-resolved pulsed electro-acoustic technique to detect space charge in solid dielectrics subjected to AC voltage

NRC publication: Ye