Éric David

Influence of internal mechanical stress and strain on electrical performance of polyethylene electrical treeing resistance

Crosslinked polyethylene (XLPE) and low-density polyethylene (LDPE) insulations used in HV cables are not only subjected to electrical and thermal stresses, but also exposed to mechanical stresses, whether residual internal stresses created during the cooling process of the fabrication, external forces when cables are bent during installation or thermomechanical stresses caused by differential thermal expansion between the conductor and the polymeric material. In order to investigate the possible influence of mechanical stresses on dielectric properties of polyethylene, measurements were conducted on pin-plane XLPE and LDPE samples with various magnitudes of residual mechanical stresses around the embedded electrode. The time to inception, the growing rate and the shape of the electrical trees under different voltages are reported in this paper. Specimens with the highest values of residual stresses were found to have the shortest inception times and the longest trees after one hour of aging under different voltages. When the mechanical stress was allowed to relax, the treeing resistance was measured to be significantly improved.

Low-frequency dielectric response of epoxy-mica insulated generator bars during multi-stress aging

The aging of insulating materials and insulating systems involves physical and/or chemical changes of the insulating material. These changes in turn lead to changes in the dielectric response of the insulating system. Quantitative measurements of the dielectric response function can be performed using either time or frequency domain techniques, both being mathematically related for linear systems. To investigate the evolution of the low-frequency dielectric response of epoxy-mica rotating machine insulation winding system as a function of aging, time domain dielectric measurements were conducted before, during and after a multi-stress aging program, using a full scale accelerated aging test facility. The results obtained in the investigation are reported in this paper. Modeling of the low-frequency dielectric response is also presented in this paper, with the model having been found to be in good agreement with the experimental results

Polymer nanocomposites-major conclusions and achievements reached so far

The dielectric properties of a number of polymeric nanocomposites (PNC) have been investigated and reported, and there are very good reviews available, for example, see [1]-[3]. CIGRE Working group D1.24 has also performed several collaborative investigations on mostly epoxy- and polyethylene-based nanocomposites, which are reported in CIGRE publications [4], [5] as well as in archived papers [6], [7]. Dielectric nanocomposites investigated in the literature include various polyolefins such as polyethylene (PE; and PE blends) and polypropylene, ethylene vinyl acetate, polyamine, epoxy, and elastomers such as silicone rubber, containing various nanofillers such as metallic oxides, silica, alumina, titanium oxide, zinc oxide, and layered silicates (clays). Due to the very high specific surface area of nano-sized fillers, a few percent addition can significantly affect the dielectric properties of a polymeric material. The most common and practical processing methods suitable for thermoplastic nanocomposites are melt compounding, using a mixer, extruder, or both, and mixing in the liquid phase prior to polymerization for thermosetting resins, a process commonly called the in situ polymerization process [8]. Figure 1 gives examples of typical microstructures of polyolefin-based nanocomposites processed by melt compounding. The striking similarity of the microstructure shown in Figures 1(b) and 1(c) should be noted as both were obtained in two different labs from the melt compounding of fumed silica and a thermoplastic resin using a twin screw extruder. Similar microstructures are also reported for isotactic polypropylene/SiO2 nanocomposites melt blended by extrusion [9].

PDC measurements to assess machine insulation

DC testing is probably the most commonly used maintenance and diagnostic tests periodically conducted on machine stator insulation systems. With the commercial availability of more sophisticated equipment it is now possible to continuously monitor both charge and discharge current during a step voltage test, also known as polarization/depolarization current (PDC) test. This test is related to the dielectric response of the insulation system. This paper presents theoretical considerations on the dielectric response of the various types of machine winding insulation systems encountered in the field and the usefulness of using both the charge and discharge currents to assess the condition of the insulation system.

The use of time domain spectroscopy as a diagnostic tool for rotating machine windings

In order to assess stator winding insulation condition, a field instrument measuring the charge and the discharge current flowing through rotating machine stator winding groundwall insulation was developed at Hydro-Quebec. This instrument applies a constant DC-voltage of 1 kV during a certain time, usually 2000 s. Immediately following the charging, the stator winding is short-circuited during another period of time. The current is continuously recorded as a function of time during both steps. It is suggested that the magnitude and the shape of both curves may be indicative of the degree of aging of the insulating material. A number of measurements obtained either from generator windings at Hydro-Quebec, or from laboratory aged specimens were examined; spare bars or coils were also measured and used as reference. Measurements were conducted essentially on asphalt and epoxy bonded insulating material which are the two major stator groundwall insulating material for the 350 or so generators at Hydro-Quebec. The method was found particularly useful to detect delamination or leakage current due to moisture absorption or end-winding contamination.

Epoxy/BN micro- and submicro-composites: dielectric and thermal properties of enhanced materials for high voltage insulation systems

Hexagonal boron nitride (h-BN) is a very promising material for application in high voltage insulation engineering due to its high thermal conductivity and good electrical insulating properties. In order to study the effect of incorporating BN particles in epoxy resin, composites with different filler sizes and several BN loadings have been fabricated. Two different filler sizes, one micrometric with an average grain size of 9 μm and a submicrometric one with 0.5 μm, have been used to form composites. The amount of either type of BN in the matrix has been varied from 1 to 5 wt%. Dielectric and thermal performances of the test specimens have been assessed by means of Dielectric Spectroscopy, Differential Scanning Calorimetry, surface erosion, AC breakdown tests and thermal conductivity measurements. It has been found that incorporation of BN particles in the epoxy resin resulted in significant improvements of parameters such as resistance to electrical discharge, as well as diminished dielectric losses for the composites at higher temperatures. Furthermore, BN composites with 5 wt% filler loadings have shown a noteworthy enhancement of thermal conductivities, which was more distinct for the submicrometric BN composite.

Nanostructured epoxy/POSS composites: enhanced materials for high voltage insulation applications

In this study, the dielectric and thermal properties of nanostructured epoxy/POSS (Polyhedral Oligomeric Silsesquioxanes) composites were investigated, using a reactive Triglycidylisobutyl-POSS (TGIB-POSS) additive from 1 up to 10 wt%. POSS has been successfully dispersed at a molecular level for low content composites, which show a remarkably improved resistance to corona discharges, with up to 60% less eroded sample volume, along with significantly increased dielectric breakdown strengths and thermal conductivities. Epoxy/POSS composites containing 5 wt% and more of the POSS additive exhibit agglomerations, which have been observed by SEM. Furthermore, dielectric spectroscopy revealed additional interfacial loss peaks for such composites containing 5 wt% POSS and more, in addition to the α- and β-peaks known for epoxy.

Post-heat treatment effect on the dielectric response of epoxy samples

In the present experiment, the dielectric response of various types of epoxies was investigated in the frequency and time domain. The samples consisted of nanostructured epoxy microcomposites and reference microcomposites. Water ingression at the fabrication step and/or at measuring time is known to affect the dielectric response of materials such as epoxy. Prior to measurement, some samples were submitted to 160°C during 48 hours under vacuum. Heating the epoxy sample under vacuum was found to have a substantial effect on dielectric properties specially when the microcomposite was nanostructured. In the nanodielectric case, pre-treatment conditions could produce a drop in the dielectric constant, about 5% at around 10-2 Hz. This observation would be consistent with the removal of the polarisability associated with water molecules.

Enhanced electrical and thermal performances of nanostructured epoxy/POSS composites

Epoxy resins modified with Glycidyl-POSS (Polyhedral Oligomeric Silsesquioxanes) in contents of up to 20 wt% were fabricated and studied in terms of dielectric spectroscopy, differential scanning calorimetry, AC breakdown tests, thermal conductivity measurements and corona resistance. Microstructure analysis have revealed a molecular dispersion of Glycidyl-POSS up to contents of 10 wt%, whereas in the 20 wt% POSS composites crystalline zones due to POSS could be observed by both scanning electron microscopy and transmission electron microscopy. All POSS composites have seen a significant improvement in AC breakdown strength, as well as an enhanced resistance to corona discharges, which in the latter case was getting more pronounced with higher POSS loadings. On the contrary, low POSS contents proofed most effective in improving the thermal conductivity, which should be attributed to the effect of POSS on the nanostructure of the polymeric network.

Dielectric properties of PE/clay nanocomposites

Polyethylene/nanoclay specimens containing from 0 to 5% nanoclays were prepared from a commercially available premixed PE/nanoclay masterbatch containing 50% wt of nanoclay. The masterbatch was diluted to the desired concentration by adding PE along with various amounts of compatibilizer in order to achieve the best possible dispersion of the nanoclay platelets. The dielectric response of the compounded samples was investigated using a combination of time and frequency-domain spectroscopy in order to cover a wide frequency window. Both techniques were in good agreement when the time-domain data was transformed into frequency-domain data. Despite their low concentration, the addition of the dispersed nanoclays led to a significant alteration of the material dielectric response in the form of the appearance of various interfacial relaxation processes and an increase of charge carrier transport within the insulation material. Moreover, an onset of nonlinear charge transport process was observed at moderate fields for specimens containing a relatively low level of nanoclays. The high-field breakdown strength was shown to have been improved by the incorporation of the nanoparticles, particularly when the exfoliation was enhanced by the use of a maleic anhydride grafted polyethylene compatibilizer.