Anh T. Hoang

Charge Transport in LDPE Nanocomposites Part I—Experimental Approach

This work presents results of bulk conductivity and surface potential decay measurements on low-density polyethylene and its nanocomposites filled with uncoated MgO and Al2O3, with the aim to highlight the effect of the nanofillers on charge transport processes. Material samples at various filler contents, up to 9 wt %, were prepared in the form of thin films. The performed measurements show a significant impact of the nanofillers on reduction of material’s direct current (dc) conductivity. The investigations thus focused on the nanocomposites having the lowest dc conductivity. Various mechanisms of charge generation and transport in solids, including space charge limited current, Poole-Frenkel effect and Schottky injection, were utilized for examining the experimental results. The mobilities of charge carriers were deduced from the measured surface potential decay characteristics and were found to be at least two times lower for the nanocomposites. The temperature dependencies of the mobilities were compared for different materials.

Electrical characterization of a new enamel insulation

Investigations of dielectric properties of a newly developed enamel wire insulation created by adding chromium (III) oxide (Cr2O3) filler to polyamide-imide enamel base are presented. Results of measurements of electrical conductivity and complex permittivity at various temperatures as well as surface potential decay are discussed and compared with corresponding properties of standard enamel insulation. Contributions of different polarization relaxation processes in both enamels are examined based on the obtained master curves of dielectric response. In addition, the properties of chromium oxide filler are characterized separately and utilized further for analyzing its impact on the performance of enamel wire insulation by means of computer simulations. The experimental and simulated results demonstrate that the introduction of chromium oxide yields changes in the electrical properties that allow for mitigating the voltage stress in a wound insulation system. Furthermore, a correlation between the obtained results and the earlier described improved resistance to partial discharge activity of the chromium oxide filled enamel is discussed.

Charge Transport in LDPE Nanocomposites Part II—Computational Approach

A bipolar charge transport model is employed to investigate the remarkable reduction in dc conductivity of low-density polyethylene (LDPE) based material filled with uncoated nanofillers (reported in the first part of this work). The effect of temperature on charge transport is considered and the model outcomes are compared with measured conduction currents. The simulations reveal that the contribution of charge carrier recombination to the total transport process becomes more significant at elevated temperatures. Among the effects caused by the presence of nanoparticles, a reduced charge injection at electrodes has been found as the most essential one. Possible mechanisms for charge injection at different temperatures are therefore discussed.

Influence of nanoparticle surface coating on electrical conductivity of LDPE/Al 2 O 3 nanocomposites for HVDC cable insulations

LDPE/metal oxide nanocomposites are promising materials for future high-voltage DC cable insulation. This paper presents data on the influence of the structure of the nanoparticle coating on the electrical conductivity of LDPE/Al2O3 nanocomposites. Al2O3 nanoparticles, 50 nm in size, were coated with a series of silanes with terminal alkyl groups of different lengths (methyl, w-octyl and n-octadecyl groups). The density of the coatings in vacuum was between 200 and 515 kg m−3, indicating substantial porosity in the coating. The dispersion of the nanoparticles in the LDPE matrix was assessed based on statistics for the nearest-neighbor particle distance. The electrical conductivity of the nanocomposites was determined at both 40 and 60 °C. The results show that an appropriate surface coating on the nanoparticles allowed uniform particle dispersion up to a filler loading of 10 wt.%, with a maximum reduction in the electrical conductivity by a factor of 35. The composites based on the most porous octyl-coated nanoparticles showed the greatest reduction in electrical conductivity and the lowest temperature coefficient of electrical conductivity of the composites studied.

Charge decay on enamel wire surface

Partial discharge resistant enamel insulation filled with chromium oxide (Cr2O3) has been developed recently. In the paper, dynamics of surface charges on samples of the material is analyzed and is compared with that on conventional enamel. The experiments were conducted on test objects prepared as windings of enameled wires. Surface charging was implemented using dc corona from a needle electrode which was placed against the grounded winding. After charging, the potential induced by deposited charges was measured by non-contact technique. The surface charge density pattern was reconstructed utilizing Φ-matrix method. The decay of charges accumulated on insulation surfaces was recorded. It was found that the decay process was considerably faster for the filled enamel than for the conventional one, which may be one of the factors contributing to the improved partial discharge resistance of the former.

Partial discharge behavior of a newly developed enamel insulation at various voltage rise times

Investigations of partial discharge (PD) characteristics in twisted pairs are presented. The test objects were made of conventional and chromium oxide filled enameled wires and were exposed to pulse width modulated (PWM) voltages of different rise times. To compare the performance of each insulation system, measurements of both PD inception and extinction voltages were conducted. Additionally, the number of PDs per cycle and their amplitude at different voltage levels were determined. Experimental results showed that the total PD exposure was considerably reduced for the chromium oxide filled enamel insulation as compared to that of the conventional material at the shortest voltage rise time. However, for filtered PWM waveforms with relatively longer rise times this difference became less prominent. The analysis of the results indicated that the observed effects are determined by the presence of chromium oxide filler and its electrical properties.

Open-circuit Thermally Stimulated Currents in LDPE/Al2O3 Nanocomposite

Polyethylene doped with nanoparticles of inorganic oxides has been studied extensively during the last decade, aiming at its possible use as suitable material in insulation of high voltage direct current cables. To elucidate electrical properties of such material, thermally stimulated discharge (TSD) currents are analyzed in samples of low-density polyethylene (LDPE) filled with Al2O3 nanoparticles and compared with the response of pure LDPE. The latter exhibits a presence of two peaks in the TSD spectrum, which yield levels of activation energies close to earlier published data. For LDPE nanocomposite, a single broad peak is found instead, which when decomposed into a number of single peaks yields higher and broadly spreading activation energies. This indicates a presence of deep traps in the nanocomposite and explains significant reduction of its DC conductivity and strong suppression of space charge accumulation, as compared with unfilled LDPE.