The dielectric properties of humid cellulose

Christie, J.H.; Sylvander, Steven; Woodhead, Ian; Irie, Kenji

doi:10.1016/j.jnoncrysol.2004.05.014

Cited by 25 publications

(26 citation statements)

References 54 publications

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“…Here C 1 * () is the Dissado-Hill loss peak response function (equation (1)) with its capacitance contribution (C 1 (0)) denoted by C 10 given by  0 (of equation (1) multiplied by a geometrical factor appropriate to its area and thickness). Similarly C 2 * () stands for a q-dc response (equation (5)) [6,11,15], with a capacitive amplitude factor denoted by C 20 given by the relevant  0 in equation (5) multiplied by its appropriate geometrical factor. The equivalent complex capacitance (C tr * ()) of the top row (row 1) of the circuit is:…”

Section: Resultsmentioning

confidence: 99%

“…In our case the characteristic frequencies of the loss peak and the q-dc process initially move to higher values as ageing progresses followed by a levelling off at long ageing times, as is shown in Figures 9 and 10. It has previously been noted that adsorbed moisture can have such an effect upon q-dc transport in various materials (see for example [20,21]) and specifically in oil impregnated pressboard and paper samples [22][23][24]. The experimental procedure that we have adopted here is intended to reduce the initial amount of absorbed moisture to a minimum and inhibit the adsorption of water from the surroundings.…”

Section: Influence Of Ageingmentioning

confidence: 99%

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Charge transport in thermally aged paper impregnated with natural ester oil

Abdelmalik

Dodd

Dissado

et al. 2014

IEEE Trans. Dielect. Electr. Insul.

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This is the accepted version of the paper.This version of the publication may differ from the final published version. Permanent repository link ABSTRACTSamples of a dried composite oil-paper system were thermally aged in the laboratory under N 2 in a sealed container and analysed using their dielectric response. The contributing loss processes were separated using an equivalent circuit model containing constant high frequency capacitances, dc-conductances, a dispersive capacitance, and a low frequency quasi-dc (q-dc) dispersion process as circuit elements. This low frequency dispersion in oil-paper samples is a bulk property of the system that is often mistaken for a dc conduction process when only the imaginary capacitance or acconductance is measured. The Dissado-Hill response function is proposed for the frequency dependence of the dispersive capacitance in the mid-frequency range rather than the Debye function because its cluster concepts correlate with the sample morphology of a matrix of oil-filled cavities. The quasi-dc dispersion is assigned to the transport of mobile charges between charge-clusters associated with locally connected oil filled cavities, which is a process that can be represented on a global scale by the form of hierarchical circuit system that leads to constant phase angle responses, and is here modelled by the Dissado-Hill q-dc function. It was found that the two dispersions moved together to higher frequency with thermal ageing of the composite material. The characteristic frequency common to the dispersions had an activation energy that decreased with thermal ageing, whereas their amplitude increased. This behaviour is discussed in terms of possible changes to the oil-paper composite caused by the ageing process.

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Section: Resultsmentioning

confidence: 99%

Section: Influence Of Ageingmentioning

confidence: 99%

Charge transport in thermally aged paper impregnated with natural ester oil

Abdelmalik

Dodd

Dissado

et al. 2014

IEEE Trans. Dielect. Electr. Insul.

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“…To measure all the above effects, a probe suitable for a range of materials would ideally include measurements at a number of frequencies over the range I M WOODHEAD, I PLATT, J H CHRISTIE, S KRENEK, A BROADBAND SPECTROSCOPIC SENSOR PROBE from below 1 Hz to several GHz. Very low frequencies, typically less than 1Hz, provide information about processes such as curing development in concrete [4] and ion drift [5].…”

Section: Introductionmentioning

confidence: 99%

A Broadband Spectroscopic Sensor Probe

Woodhead

Platt²,

Christie³

et al. 2008

International Journal on Smart Sensing and Intelligent Systems

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Abstract-The electrical properties of many materials are closely related to their composition and to their moisture content in particular. For sensor development, characterising the response of a particular volume of material over a broad frequency range is desirable, since separate measurements could generate errors due to spatial variability. A coaxial probe has been designed for measurement of the permittivity of smooth and flat, solid or liquid samples over the frequency range from 1 Hz to 6 GHz. Although the probe is capable of a very wide frequency range, separate instruments are generally required, and here we focus on measurements above 1MHz. We demonstrate measurements in the frequency domain using a vector network analyser, and in the time domain using a broadband oscilloscope. For switching, we employed a coaxial switch and demonstrate how that is included within the instrument calibration. Calibration of the probe used three references: an open circuit, short circuit (indium foil) and a reference liquid, ideally chosen for a permittivity similar to that of the sample. The sample complex permittivity was calculated by a numerical model which used as inputs, the measured reflection coefficient and physical measurements of the probe geometry.Index terms: coaxial probe, spectroscopy, calibration, broadband, network analyser. I INTRODUCTIONThe dairy, timber, wool, and many other primary processing industries demand rapid on-line measurement of product properties, particularly moisture. The chief advantage of the dielectric measuring technique is its relatively low cost compared with neutron backscatter, X-ray analysis, and magnetic resonance imaging. Dielectric techniques are also non-ionising, and offer clean, rapid, on-line measurements of bulk material, suited to many diverse products. For a particular product, there will be one or more frequency ranges where the dielectric constant has the highest sensitivity to changes in moisture content (or other The chosen measurement frequency for determining moisture content or composition may lie within a very broad range. Reasons for choosing a particular frequency will include sensitivity of the reading to changes in moisture content and the dependence of the reading on other parameters such as density and dielectric loss. For a particular polar molecular species, as the frequency is increased, the dielectric loss reaches a maximum and is accompanied by a reduction in the permittivity. The frequency of maximum loss is the relaxation frequency, and for free water begins to significantly affect the dielectric properties above approximately 3 GHz. In contrast, the relaxation frequency for bound water is typically 100 MHz [3]. Electrical conductivity is directly useful at low frequencies, particularly for materials with low moisture content where it provides a very useful measure of moisture content, and at high frequencies where it influences the dielectric loss. Apart from the Maxwell-Wagner effect which is caused by conducting inclusions in a dielectric, the p...

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“…FDS presents the response of the material to sinusoidal excitation of varying frequency. A notable fact is that dielectric response of a wide range of solid materials including oil impregnated cellulose [116,[120][121][122][123][124][125], poly-oxymethylene, and porous glass [126] are principally governed by the absorbed moisture. Therefore, dielectric response measurements in time and frequency domains have been significantly increased in use during the past two decades to estimate the moisture content in cellulose based insulation of transformers.…”

Section: Theory Of Dielectric Polarisationmentioning

confidence: 99%

“…On the other hand, reduction of modelled conductivity at low frequencies of oil samples; M8, NA4 and NA7 is lower than the reduction of the measured conductivity. One could claim that charge carrier loss can occur at low frequencies as a consequence of a long measuring time [120]. For example, oxidation results in decreasing charge carriers in the system.…”

Section: Equivalent Circuit For Explaining Low Frequency Behaviourmentioning

confidence: 99%

Condition monitoring of biodegradable oil filled transformer

Mudiyanselage¹,

Bandara²

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Natural and synthetic ester insulating oils have higher fire points and excellent biodegradable characteristics. Therefore, in order to reduce the adverse environmental impact and to improve the fire safety of transformers, there is an increasing demand for natural and synthetic ester insulating liquids as a transformer insulating oil. However, present understanding on ageing behaviour of ester oil-paper composite insulation system and knowledge on application of existing condition monitoring tools for ester based insulation systems are inadequate. This impedes the cost effective and reliable field applications of ester insulating oils, particularly application of natural esters. To reduce this knowledge gap, series of controlled ageing experiments are performed in this research project to provide a better and comprehensive understanding on ageing behaviour of ester oil-paper insulation systems. Furthermore, applicability of existing chemical and electrical based condition monitoring techniques for ester oil-paper insulation systems is systematically investigated in this research project.In this thesis, ageing behaviour of dry pressboard insulation in mineral and three different ester insulating oils under simulated transformer operating environment is investigated. Moreover, the ageing behaviour of natural ester-pressboard composite insulation in moisture rich environment is also compared with that of mineral oil-pressboard system. Degree of polymerisation of pressboard samples measured at different ageing interval is used in this research to determine the ageing condition of pressboard. Moreover, applicability of oil related diagnostic parameters such as concentration of dissolved furanic compounds, acidity value, Dielectric Dissipation Factor (DDF), viscosity and colour to assess the degree of degradation of both ester and mineral insulating oils is thoroughly investigated in this research project. The potential of FTIR (Fourier Transform Infrared spectroscopy) techniques for characterising the degree of degradation of paper insulation is also discussed in this thesis. In addition, this thesis concentrates on characterisation of the charge dynamic in insulating oil through modelling of their dielectric responses.The comparison of gassing behaviour of ester and mineral insulating oils under two different low temperature faults and low energy electric discharge condition is also presented in this thesis.Dissolved Gas Analysis (DGA) results presented in this thesis depicts that faults gases detected in ester and mineral oil samples subjected to a similar fault are akin in type but quantitatively different. The quantity of fault gases produced in two different natural ester oils is also dissimilar.For example, soy-based natural ester produces a large quantity of ethane (C2H6) under low temperature overheating condition than sunflower oil based natural ester. Therefore, this research project investigates the applicability of well-established DGA interpretation schemes namely Duval II | P a g e triangle, IE...

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The dielectric properties of humid cellulose

Cited by 25 publications

References 54 publications

Charge transport in thermally aged paper impregnated with natural ester oil

Charge transport in thermally aged paper impregnated with natural ester oil

A Broadband Spectroscopic Sensor Probe

Condition monitoring of biodegradable oil filled transformer

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