Nathaniel Taylor

Novel Nanocomposites of Poly(lauryl methacrylate)-Grafted Al2O3 Nanoparticles in LDPE.

Aluminum oxide nanoparticles (NPs) were surface-modified by poly(lauryl methacrylate) (PLMA) using surface-initiated atom-transfer radical polymerization (SI-ATRP) of lauryl methacrylate. Nanocomposites were obtained by mixing the grafted NPs in a low-density polyethylene (LDPE) matrix in different ratios. First, the NPs were silanized with different aminosilanes, (3-aminopropyl)triethoxysilane, and 3-aminopropyl(diethoxy)methylsilane (APDMS). Subsequently, α-BiB, an initiator for SI-ATRP, was attached to the amino groups, showing higher immobilization ratios for APDMS and confirming that fewer self-condensation reactions between silanes took place. In a third step SI-ATRP of LMA at different times was performed to render PLMA-grafted NPs (NP-PLMAs), showing good control of the polymerization. Reactions were conducted for 20 to 60 min, obtaining a range of molecular weights between 23 000 and 83 000 g/mol, as confirmed by size-exclusion chromoatography of the cleaved grafts. Nanocomposites of NP-PLMAs at low loadings in LDPE were prepared by extrusion. At low loadings, 0.5 wt % of inorganic content, the second yield point, storage, and loss moduli increased significantly, suggesting an improved interphase as an effect of the PLMA grafts. These observations were also confirmed by an increase in transparency of the nanocomposite films. At higher loadings, 1 wt % of inorganics, the increasing amount of PLMA gave rise to the formation of small aggregates, which may explain the loss of mechanical properties. Finally, dielectric measurements were performed, showing a decrease in tan δ values for LDPE-NP-PLMAs, as compared to the nanocomposites containing unmodified NP, thus indicating an improved interphase between the NPs and LDPE.

Dielectric response measurement of power transformer bushing by utilizing high voltage transients

Insulation has a significant role in most of power components failures. Therefore, proper diagnostics of insulation condition which can be done non-destructively and online in the field is of interest. In this paper, a new insulation diagnostic technique is proposed which utilizes natural transients in power system for online dielectric response measurement. In the proposed technique, lightning and switching impulses with wide range of frequency are used as stimuli for dielectric spectroscopy. Oil impregnated papers and condenser bushing are the objects under investigation and dielectric response of them is derived at those transients. Dielectric responses obtained by this method are verified by the responses measured by insulation diagnostic systems IDAX and LCR meter.

Effect of partial discharges on thermal breakdown of oil impregnated paper

In this paper the effect of partial discharges on the thermal breakdown voltage of oil-impregnated paper is investigated. A disc shaped cavity in a stack of paper sheets formed a PD source. The paper sample was exposed to partial discharges for 18 hours. Dielectric spectroscopy measurements were performed on the samples before and after exposure to PD activity. The dielectric spectroscopy results showed a big change in the real and imaginary part of the permittivity. These results were used to calculate activation energy and heat losses inside the insulation. In order to investigate the possibility of thermal breakdown in the system, the oil-impregnated paper system was simulated by using the FEM software Comsol Multiphysics. The heat produced by PD pulses and dielectric losses was considered in the simulation. The results show that for the oil-impregnated paper insulation, the PD activity can reduce the thermal breakdown voltage by a factor up to 4 times.

Utilisation of voltage and frequency dependence of stress-grading materials in dielectric diagnostics [electrical insulation]

Diagnostics of electrical insulation is increasingly performed with the frequency and amplitude of the applied voltage varied over wide ranges. Cable terminations and high-voltage machine end-windings have stress grading systems whose response alters measured capacitance and loss components even in measurements at power frequency. Non-linear stress grading materials also contribute harmonic currents to the measured response. Excitation at frequencies other than power frequency changes the gradings potential-distribution, affecting the measured capacitance and possibly inciting partial discharges that contribute to measured loss and harmonic currents. Understanding of the effects of such stress grading systems is important for analysis and possible compensation of dielectric measurement data.

Tailoring dielectric properties using designed polymer-grafted ZnO nanoparticles in silicone rubber

Polymer grafts were used to tailor the interphases between ZnO nanoparticles (NPs) and silicone matrices. The final electrical properties of the nanocomposites were tuned by the grafted interphases, by controlling the inter-particle distance and the NP-morphology. The nanocomposites can be used in electrical applications where control of the resistivity is desired. Hansens solubility parameters were used to select a semi-compatible polymer for grafting to obtain anisotropic NP morphologies in silicone, and the grafted NPs self-assembled into various morphologies inside the silicone matrices. The morphologies in the semi-compatible nanocomposites could be tuned by steering the graft length of poly(n-butyl methacrylate) via entropic matrix-graft wetting using surface-initiated atom-transfer radical polymerization. Image analysis models were developed to calculate the radius of primary NPs, the fraction of aggregates, the dispersion, and the face-to-face distance of NPs. The dielectric properties of the nanocomposites were related to the morphology and the face-to-face distance of the NPs. The dielectric losses, above 100 Hz, for nanocomposites with grafted NPs were approximately one decade lower than those of pristine NPs. The isotropic nanocomposites increased the resistivity up to 100 times compared to that of neat silicone rubber, due to the trapping of charge carriers by the interphase of dispersed NPs and nanoclusters. On the other hand, the resistivity of anisotropic nanocomposites decreased 10–100 times when the inter-particle distance in continuous agglomerates was close to the hopping distance of charge carriers. The electrical breakdown strength increased for compatible isotropic nanocomposites, and the temperature dependence of the resistivity and the activation energy were ∼50% lower in the nanocomposites with grafted NPs. These flexible dielectric nanocomposites are promising candidates for low-loss high-voltage transmission cable accessories, mobile electronic devices, wearables and sensors.

Dielectric and physico-chemical properties of epoxy-mica insulation during thermoelectric aging

Three stator bars were subjected to 1142 hours of thermoelectric aging. At intervals during this time, the aging of the insulation was investigated by capacitance (C) and dissipation factor (DF) measurements and by the physicochemical techniques of infrared spectroscopy (FTIR) and Differential Scanning Calorimetry (DSC). Results indicated different degradation paths under discharge conditions, where the volume of voids within the insulation played a major role. Postcuring reactions were predominant during the first aging cycles. Such reactions were detected as reductions in capacitance and dissipation factor parameters, as a depletion of the chemical group epoxide at the absorption band 909 cm-1 as measured by FTIR, and as an increase of the glass transition temperature (Tg) of the epoxy resin. Since the postcuring reactions occur globally in the insulation, they were detected by both dielectric and physicochemical techniques. With the advance of the aging program the epoxy resin underwent structural changes, seen as modifications of functional groups CH2, CH3 and C=O. The air atmosphere where partial discharges occurred also promoted the formation of reactive species. When the air volume was high enough, the findings of FTIR measurements suggested a local degradation mechanism of mica involving cation exchange reactions between partial discharge byproducts and potassium layers in the mica.

Measured and modeled capacitance, loss and harmonics in stator insulation with nonlinear stress control

Diagnostic measurements on stator insulation suffer disturbance from nonlinear resistive stress-control in the end corona protection (ECP). The extent of this effect is studied here, by measurements and modeling. Frequency-domain dielectric response measurements on bars with epoxy-mica insulation in new condition are presented as capacitance, loss and current harmonics, for the guarded insulation in the slot region, the end regions alone, and the combination. The voltages are from 0.3 up to 14.4 kV, at frequencies from 100 Hz down to 0.1 mHz, to cover the range relevant to common diagnostic methods. The ECP approximately doubles the change of capacitance with frequency, and has a much stronger relative effect on voltage-dependence and on current harmonics in an unaged insulation system. The ECP currents are modeled numerically, using material properties from further measurements on samples of the silicon-carbide based ECP tape. A nonlinear 1-dimensional model gives a fair fit to the measurements except at the highest frequencies. Reduction of the ECP disturbance in measurements by subtraction of modeled values is discussed. The main practical limitation of the model is likely to be uncertainty of the input parameters describing the geometry and material, rather than a need of more detail in the model.

A study on dielectric response of bitumen in the low-frequency range

From the current state of literature, the dielectric property of bitumen has not been understood extensively, nor its relation with other properties such as polarity and rheology. In this study, dielectric spectroscopy measurement in a low-frequency range (10−2–106 Hz) was performed on both pure bitumen in different grades and wax-modified bitumen (WMB). From the performed tests we found the following: (i) the dielectric response of base bitumen is strongly temperature and frequency dependent, which is also highly linked to the rheology of the system. (ii) No remarkable differences in the dielectric constant among different grades of bitumen from the same crude oil source can be seen. (iii) Regular changes of dielectric loss tangent (tan δ) among the different grades of bitumen can be observed, which can be a good indicator for the linkage between the dielectric and rheological responses. In addition, it can also be perceived that the dielectric spectroscopy may have the potential to become a new approach for the multi-scale characterisation of road infrastructure materials.

Partial Discharge analysis in a metal-dielectric air gap on machine insulation at arbitrary testing voltage

Partial Discharge (PD) tests were performed in a metal-dielectric air gap between a spherical electrode and the surface of machine insulation, which consists of mica, epoxy resin and glass-fiber. The PD activities were investigated at three types of applied voltages: triangular, square and trapezoidal waveform. In the trapezoidal waveform, the duration of the linearly rising part of the applied voltage was varied. The effect of the time derivative of the applied voltage dU/dt on the PD activities was studied. The PD activities were studied at a power frequency (50 Hz) and at a lower frequency (10 Hz) of the applied voltage. The results show that the time derivative of the applied voltage has a significant effect on the PD behavior. In each waveform, most of the PD pulses occur during the linearly rising period, but never occur during the linearly falling period. At lower frequency, compared with the positive pulses, the number of negative ones decreases dramatically with the increasing rising time, and PD activities even disappear at the triangular waveform.

Effect of superimposed impulses on AC partial discharge characteristics of oil-impregnated paper

The effect of HV impulses on surface and cavity partial discharges (PD) of oil-impregnated paper (OIP) is investigated in this work, by subjecting specimens to impulse voltages superimposed on a continuous AC voltage. The superposition of transient and AC voltage is relevant to insulation systems in power-system equipment, which is exposed to transients such as lightning and switching impulses during its operation. For several different PD-generating defects in oil-impregnated paper, the PD behaviors before and after the impulse are compared. The influence of moisture content is studied by using OIP with moisture content <0.5%, 3.0% and 5.5%. The results show a distinctive behavior of PD due to the impulse for each defect and insulation condition, either by change in the PD rate and magnitude or by causing PD inception or extinction. Recordings of the voltage signals and associated PD measurements during and after the incidence of a high voltage transient are of potential use for condition assessment of the insulation of power transformers and their bushings, as a form of online diagnostics.