P. Rain

Dielectric properties of LDPE-SiO 2 nanocomposites

The dielectric behaviour of nano-SiO2 filled low density polyethylene is investigated over a frequency range of 10 mHz–10 MHz and for different temperatures from 250 K to 350 K. It is shown that the presence of nanoparticles change significantly the dielectric behaviour of the polymer system. The frequency variations of the permittivity and of the tan delta emphasize a α-relaxation process, for each of the nanocomposite samples. The relaxation is more important and occurs at higher frequencies with the increase of the filler content. The increase of the temperature leads to a shift of the relaxation frequency to higher values. Results from chemiluminescence measurements and from X-ray diffraction analysis are discussed in connection with the dielectric behaviour.

Characterization of electro-thermal aging of a filled epoxy resin

A DGEBA-type epoxy resin filled with silica flour has been submitted to long-term electro-thermal, or purely thermal, aging. Following this aging dielectric spectroscopy and current measurements were made. Changes in low frequency permittivity and loss tangent have been observed. The relative contributions to aging of the electric field and the temperature are discussed.

Mechanical and Dielectric Properties of Glass-Mica-Epoxy Composites along Accelerated Thermo-Oxidative Aging

The stator windings of large air-cooled rotating machines are insulated with mica-glass fabric impregnated with epoxy. These composites are submitted to thermal, mechanical and electrical stresses. Long-term electrical insulation may be affected by the occurrence of partial discharges. A specific aging arrangement has been set-up allowing accelerated thermo-oxidative aging thanks to the application of a pressure up to 10 bars on samples submitted to controlled atmospheres, especially dry O2, at temperatures from ambient to 200°C. Mechanical and dielectric properties of samples have been followed along aging, using dynamic mechanical analysis (DMA) and dielectric analysis (DEA). The effects of the oxidation on polymer network, i.e. crosslinking and elimination reactions, were observed thanks to the evolution of the glass transition temperature (Tg). The effect of 50-180°C temperature cycles has been compared with thermal oxidation at constant temperature. Correlations with weight losses are also underlined.

Influence of T/sub g/ on space charge development in epoxy resin

Space charge development has been studied in a filler free epoxy resin at temperature below (328 K) and above (353 K) T/sub g/. Space charge generation was observed at low DC electric field (2 kV/mm) at each temperature. The nature of the space charges is different at these two temperatures. The role of the water content on the space charge generation is shown and discussed.

Space charge measurements in epoxy resin under DC voltage

Various regimes of homocharge and heterocharge generation have been observed in the range 5-40 kV/mm dc. Three temperature domains have been studied depending on the glass transition temperature of the epoxy resin (Tg/spl sim/65/spl deg/C): 20-40/spl deg/C for glassy state properties, 55/spl deg/C (just below T/sub g/), and 80/spl deg/C (above T/sub g/).

Temperature dependence of space charge behavior in silicone

The generation of space charges in a silicone elastomer has been characterized under DC voltage as a function of the temperature using the LIPP method. Both heterocharges and homocharges have been observed depending on the field magnitude, polarization duration and temperature. Heterocharges appear in less stressed conditions. Then, homocharges take place of these heterocharges. The phenomena depend on the electrode polarity.

Detection of Artificial Insulation Defects in a Medium-Voltage Motor by Dielectric Spectroscopy Analysis

Artificial defects have been introduced during the manufacturing process inside the composite insulation of stator models of medium-voltage motors dedicated to the application of traction. The dielectric analysis at low frequency (10-3-103 Hz) has been conducted at 30°C for each of the seven different models. The sensitivity of the dielectric properties to humidity is illustrated. This is the reason why the measurements were made with the models in a dried state. The dielectric analysis allowed the detection of most of the seven defects sought. These defects have also been identified. The value of the capacitance at 1 kHz appeared in particular as a reliable marker for identification. The observed differences between models with defect and the reference model are qualitatively justified by dielectric considerations.

Study of the generation of the pressure pulse in the laser induced pressure pulse method: Optimization of the process

The laser induced pressure pulse method -LIPP- is commonly used for measurement of distribution of space charge in insulating materials. The aim of this paper is to study the mechanisms giving rise to the generation of the pressure pulse and to optimize the generating process to obtain the better sensitivity of the technique. For transparent liquids at the laser wavelength, the two mechanisms which may generate the pressure pulse, i.e. thermal expansion and thermal vaporization at the liquid/target interface, are clearly identified depending on the laser energy. The surface roughness of the target also played an important role in this generation. For opaque liquids, thermal dilatation and evaporation at the liquid/air interface are involved. With the use of Indian ink, the energetic conversion revealed better for the smaller investigated liquid thickness, i.e. 20 µm. A concentration of 20% of Indian ink in water provides the best sensitivity at 20°C, respectively 25% at 60°C.

Hygrothermal ageing of a filled epoxy resin: Measurements of the insulation properties and qualitative modelling

Samples of epoxy filled with silica have been subjected to hygrothermal aging at 80 °C and 80% relative humidity for several months. These samples were characterized by the measurements of resistances, partial discharges and breakdown voltages. Results for both silanized and non-silanized fillers were obtained. After ageing, large differences were observed in both breakdown voltage and resistance. The breakdown voltage was reduced by a factor of 10 after 74 days of ageing without silanization whereas the decrease was a factor of 2 with silanized fillers. The resistance was reduced by more than 4 decades in the former case and one decade in the second case. The breakdown was preceded by the occurrence of partial discharges. Partial Discharges Inception Voltage (PDIV) dropped by a factor of 10 between 32 days and 74 days of ageing. The PDIV was sensitive to the sample temperature: it was lower at larger temperatures. Partial Discharges (PD) patterns suggest that the discharges occurred in gaseous cavities. Based upon these measurements, a model of breakdown phenomenon based upon cavities filled of water vapour is proposed and discussed.

Influence of the electrode metal on the injection of space charges in PET and epoxy resin

Using the LIPP method, negative homocharges have been observed in PET sheets after a few hours poling under 6kV/mm DC at 60 °C. This observation was made with various electrode metals: gold (Au), aluminium (Al) and silver (Ag). With gold electrodes, the injection is much slower than with the other two metals. Furthermore, the gold metallization is the only one leading to the injection of positive space charges in the same conditions. Similarly, at 55 °C under 7kV/mm, positive homocharges have been observed in epoxy sheets only with gold-deposited electrodes and not with aluminium electrodes.