IEEE Transactions on Dielectrics and Electrical Insulation
Contaminated transformer oil has been tested under non uniform electric fields and the effect of ... more Contaminated transformer oil has been tested under non uniform electric fields and the effect of different electrode systems presented in this paper. Three different electric fields were examined i.e. DC, AC and DC biased AC. These experiments revealed that with all the different electrodes arrangements, contaminated particles always formed bridges between electrodes under DC electric field. The bridges were thicker and more particles were attracted with more uniform electric field (spherical electrode) than with a divergent electric field (needle-plane).
Ionic microfield in plasmas depends strongly on electronic subsystem of plasmas. Two models for e... more Ionic microfield in plasmas depends strongly on electronic subsystem of plasmas. Two models for electrons influence are examined in this article: 1) uniform neutralizing background of electrons, 2) so-called low-frequency component of microfield. Simple analytic approximations for ionic microfield distribution in these two models are constructed. These approximations are suitable for plasmas with complex composition in wide range of densities and temperatures. .
ABSTRACT Thermo-chemical degradation of carbon fibre composite (CFC) materials under intensive he... more ABSTRACT Thermo-chemical degradation of carbon fibre composite (CFC) materials under intensive heat fluxes are modelled. The model couples together heat diffusion, polymer pyrolysis with associated gas production and convection through partially decomposed CFCs, and changes in transport properties of the material due to the damage. The model is verified by laser ablation experiments with controlled heat input. The numerical predictions indicate that the thermal gas transport has a minimal effect on the decomposition extent. On the other hand, the model shows that the internal gas pressure is large enough to cause fracture and delamination, and the damage extent may go far beyond the decomposition region as witnessed from experimental verification of the model.
Diffusion in material of inhomogeneous composition can induce phase changes, even at a constant t... more Diffusion in material of inhomogeneous composition can induce phase changes, even at a constant temperature. A transient liquid phase (TLP), in which a liquid layer is formed and subsequently solidifies, is one example of such an isothermal phase change. This phenomenon is exploited industrially in TLP bonding and sintering processes. Successful processing requires an understanding of the behaviour of the
2010 IEEE International Symposium on Electrical Insulation, 2010
A two-dimensional axisymmetric model has been developed for the simulation of a negative surface ... more A two-dimensional axisymmetric model has been developed for the simulation of a negative surface discharge in air. The finite element model is based on a set of continuity equations accounting for the movement, generation and loss of electrons, positive and negative ions and these equations are coupled with Poisson's equation to take into consideration the space charge field. The surface
The analysis of partial discharge (PD) measurement data obtained using radio frequency current tr... more The analysis of partial discharge (PD) measurement data obtained using radio frequency current transducers (RFCT) incorporated into the external earthing connections (i.e bushing tap points and neutral to earth connections) offers the possibility of on-line condition monitoring of large transformers. A method for locating PD sources in windings has been developed, based on analysis of differential equations that model the propagation of PD signals from the source to the measurement point. Analysis reveals that if the discharge at the source is assumed to be impulse-like, then the form of the response at the measurement points can be simulated as a function of source location along the winding. If all parameters of the winding are known then it is possible to model the response, however, in the field this is often not the case. So the method developed here uses information from the PD measurements themselves to estimate the unknown coefficients required to simulate the likely discharge currents. With these estimates it is then possible to compare PD measurement information with simulation data in order to identify the most likely location of a PD source. An experiment based on a section of high voltage winding has been used to produce PD measurement data and to validate the proposed approach.
IEEE Transactions on Dielectrics and Electrical Insulation, 2015
ABSTRACT Analysis of real operating condition revealed that HVDC transformers experience combined... more ABSTRACT Analysis of real operating condition revealed that HVDC transformers experience combined effect of DC biased AC electric field. The dynamics of pressboard particle in contaminated transformer oil under the influence of DC, AC and DC biased AC electric field has been investigated in this paper. Different levels of particle concentrations are tested at different applied voltages. Optical images of the particles accumulation together with conduction current have been recorded during the experiments. A complete bridge between the electrodes of cellulose particles were observed for all the tests carried out under DC and DC biased AC electric field. Opposite to that, for AC experiments, pressboard particles accumulated on surfaces of both electrodes but did not create a full bridge between the electrodes. It is concluded that a combination of DC and AC voltages in a HVDC transformer could lead to a bridge formation within the equipment which could cause failure.
2009 IEEE Conference on Electrical Insulation and Dielectric Phenomena, 2009
The formation of Trichel pulses using two dimensional axisymmetric finite element (FE) simulation... more The formation of Trichel pulses using two dimensional axisymmetric finite element (FE) simulations is presented in this paper. The hydrodynamic drift-diffusion approximation is used to model the phenomenon and the simulation is based on solving the coupled system of partial differential equations accounting for the transport of charged particles and Poisson's equation for the electric field. A needle-plane geometry with
2012 Annual Report Conference on Electrical Insulation and Dielectric Phenomena, 2012
ABSTRACT Experiments were carried out on transformer oil contaminated by cellulose fibre. After t... more ABSTRACT Experiments were carried out on transformer oil contaminated by cellulose fibre. After the high voltage was applied to the electrodes, the pressboard particles accumulated in the highest electric field region and eventually formed a bridge. We have developed a mathematical model to simulate the bridging dynamics. The dielectrophoretic and drag forces are considered for simulation of the particles motion and accumulation. The predicted bridge formation rate and the conduction current between the electrodes showed similar trend to the experimental results. The model is a successful means of predicting bridge formation phenomenon compared with the experimental data.
A study has been made of the electrical characteristics and optical emission spectra exhibited wh... more A study has been made of the electrical characteristics and optical emission spectra exhibited when discharge events take place during plasma electrolytic oxidation processing. Both conventional and small area experimental arrangements have been employed, allowing detailed measurement of durations, and temporal distributions, as well as such characteristics as charge transfer, and power. Individual discharges are of short duration, typically tens to hundreds of microseconds, but there is a strong tendency for them to occur in cascades that commonly last between several ms and several tens of ms. The composition, temperature and electron density of the plasma formed during PEO processing are inferred from characteristics of the emission spectra. This confirms that there are two distinct regions of plasma; a lower density peripheral region at~3500 K, and a higher density core at 16,000 ± 3500 K. The implications of these results are considered in terms of the interpretation of different types of experimental measurement, and attention is also briefly given to how such behaviour might relate to the mechanisms of growth.
Broadband approximate expressions for calculating the broadening of the spectral lines of hydroge... more Broadband approximate expressions for calculating the broadening of the spectral lines of hydrogen-like ions in a multicomponent plasma are derived taking into account both the influence of the interaction between plasma particles on the distribution function of the plasma microfield and the effect of the microfield dynamics on the broadening of the central component of the spectral line. With the approximate expressions proposed, the calculation of the shape of a given spectral line of a certain ion in a plasma with a given ion composition requires only a few seconds of computer time. The approximate expressions provide a good computational accuracy not only for the central component of the spectral line but also for the spectral line wings.
Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 2006
A brief analysis is presented of how heat transfer takes place in porous materials of various typ... more A brief analysis is presented of how heat transfer takes place in porous materials of various types. The emphasis is on materials able to withstand extremes of temperature, gas pressure, irradiation, etc. i.e. metals and ceramics, rather than polymers. A primary aim is commonly to maximize either the thermal resistance (i.e. provide insulation) or the rate of thermal equilibration between the material and a fluid passing through it (i.e. to facilitate heat exchange). The main structural characteristics concern porosity (void content), anisotropy, pore connectivity and scale. The effect of scale is complex, since the permeability decreases as the structure is refined, but the interfacial area for fluid-solid heat exchange is, thereby, raised. The durability of the pore structure may also be an issue, with a possible disadvantage of finer scale structures being poor microstructural stability under service conditions. Finally, good mechanical properties may be required, since the development of thermal gradients, high fluid fluxes, etc. can generate substantial levels of stress. There are, thus, some complex interplays between service conditions, pore architecture/scale, fluid permeation characteristics, convective heat flow, thermal conduction and radiative heat transfer. Such interplays are illustrated with reference to three examples: (i) a thermal barrier coating in a gas turbine engine; (ii) a Space Shuttle tile; and (iii) a Stirling engine heat exchanger. Highly porous, permeable materials are often made by bonding fibres together into a network structure and much of the analysis presented here is oriented towards such materials.
A model of the sintering exhibited by EB-PVD TBCs, based on the principles of free energy minimiz... more A model of the sintering exhibited by EB-PVD TBCs, based on the principles of free energy minimization, was recently published by Hutchinson et al . In the current paper, this approach is applied to the sintering of plasma-sprayed TBCs and comparisons are made with experimental results. Predictions of through-thickness shrinkage and changing pore surface area are compared with experimental data obtained by dilatometry and BET analysis respectively. The sensitivity of the predictions to initial pore architecture and material properties are assessed. The model can be used to predict the evolution of the contact area between overlying splats. This is in turn related to the through-thickness thermal conductivity, using a previously-developed analytical model .
Yttria-stabilized zirconia powders, containing different levels of SiO 2 and Al 2 O 3 , have been... more Yttria-stabilized zirconia powders, containing different levels of SiO 2 and Al 2 O 3 , have been plasma sprayed onto metallic substrates. The coatings were detached from their substrates and a dilatometer was used to monitor the dimensional changes they exhibited during prolonged heat treatments. It was found that specimens containing higher levels of silica and alumina exhibited higher rates of linear contraction, in both in-plane and through-thickness directions. The in-plane stiffness and the through-thickness thermal conductivity were also measured after different heat treatments and these were found to increase at a greater rate for specimens with higher impurity (silica and alumina) levels. Changes in the pore architecture during heat treatments were studied using Mercury Intrusion Porosimetry (MIP). Fine scale porosity (<$50 nm) was found to be sharply reduced even by relatively short heat treatments. This is correlated with improvements in inter-splat bonding and partial healing of intra-splat microcracks, which are responsible for the observed changes in stiffness and conductivity, as well as the dimensional changes.
A review is presented of how heat transfer takes place in plasma-sprayed (zirconia-based) thermal... more A review is presented of how heat transfer takes place in plasma-sprayed (zirconia-based) thermal barrier coatings (TBCs) during operation of gas turbines. These characteristics of TBCs are naturally of central importance to their function. Current state-of-the-art TBCs have relatively high levels of porosity (~15%) and the pore architecture (i.e., its morphology, connectivity, and scale) has a strong influence on the
It has been found that reducing the level of impurity oxides (particularly SiO 2 and Al 2 O 3 ) i... more It has been found that reducing the level of impurity oxides (particularly SiO 2 and Al 2 O 3 ) in 7YSZ, from about 0.2 wt% to below 0.1 wt% raises the sintering resistance and the phase stability of plasma-sprayed coatings. The implications for the usage of these coatings at elevated temperatures are examined. It is concluded that using relatively high-purity powder of this type is likely to confer substantial benefits in terms of the thermomechanical stability of the coatings under service conditions.
Numerical (finite difference) and analytical models have been developed for the simulation of hea... more Numerical (finite difference) and analytical models have been developed for the simulation of heat flow through plasma-sprayed coatings, allowing the effective thermal conductivity to be predicted as a function of microstructural parameters. The structure is assumed to be composed of lamellar material (splats), separated by (thin) pores, within which there are areas of contact (bridges). The analytical model is based on dividing the material into two regimes, within which the heat flow occurs either by unidirectional serial flow through lamellae and pores or by being funneled through the regions of the lamellae above and below the bridges. The validity of this model is demonstrated by a comparison of the predictions obtained from it and those obtained from the numerical model. The effects of pore geometry on conductive and radiative heat transfer within the coating have been investigated over a range of temperatures and gas pressures. It is shown that the main factor controlling the conductivity is the intersplat bridge area. Comparisons are also presented with experimental conductivity data, for cases in which some attempt has been made to characterize the key microstructural features. The study is oriented toward thermal barrier coatings, based on zirconiayttria top coats. It is noted that the effect of microstructural sintering, which tends to occur in these coatings under service conditions, can be predicted using this model.
IEEE Transactions on Dielectrics and Electrical Insulation
Contaminated transformer oil has been tested under non uniform electric fields and the effect of ... more Contaminated transformer oil has been tested under non uniform electric fields and the effect of different electrode systems presented in this paper. Three different electric fields were examined i.e. DC, AC and DC biased AC. These experiments revealed that with all the different electrodes arrangements, contaminated particles always formed bridges between electrodes under DC electric field. The bridges were thicker and more particles were attracted with more uniform electric field (spherical electrode) than with a divergent electric field (needle-plane).
Ionic microfield in plasmas depends strongly on electronic subsystem of plasmas. Two models for e... more Ionic microfield in plasmas depends strongly on electronic subsystem of plasmas. Two models for electrons influence are examined in this article: 1) uniform neutralizing background of electrons, 2) so-called low-frequency component of microfield. Simple analytic approximations for ionic microfield distribution in these two models are constructed. These approximations are suitable for plasmas with complex composition in wide range of densities and temperatures. .
ABSTRACT Thermo-chemical degradation of carbon fibre composite (CFC) materials under intensive he... more ABSTRACT Thermo-chemical degradation of carbon fibre composite (CFC) materials under intensive heat fluxes are modelled. The model couples together heat diffusion, polymer pyrolysis with associated gas production and convection through partially decomposed CFCs, and changes in transport properties of the material due to the damage. The model is verified by laser ablation experiments with controlled heat input. The numerical predictions indicate that the thermal gas transport has a minimal effect on the decomposition extent. On the other hand, the model shows that the internal gas pressure is large enough to cause fracture and delamination, and the damage extent may go far beyond the decomposition region as witnessed from experimental verification of the model.
Diffusion in material of inhomogeneous composition can induce phase changes, even at a constant t... more Diffusion in material of inhomogeneous composition can induce phase changes, even at a constant temperature. A transient liquid phase (TLP), in which a liquid layer is formed and subsequently solidifies, is one example of such an isothermal phase change. This phenomenon is exploited industrially in TLP bonding and sintering processes. Successful processing requires an understanding of the behaviour of the
2010 IEEE International Symposium on Electrical Insulation, 2010
A two-dimensional axisymmetric model has been developed for the simulation of a negative surface ... more A two-dimensional axisymmetric model has been developed for the simulation of a negative surface discharge in air. The finite element model is based on a set of continuity equations accounting for the movement, generation and loss of electrons, positive and negative ions and these equations are coupled with Poisson's equation to take into consideration the space charge field. The surface
The analysis of partial discharge (PD) measurement data obtained using radio frequency current tr... more The analysis of partial discharge (PD) measurement data obtained using radio frequency current transducers (RFCT) incorporated into the external earthing connections (i.e bushing tap points and neutral to earth connections) offers the possibility of on-line condition monitoring of large transformers. A method for locating PD sources in windings has been developed, based on analysis of differential equations that model the propagation of PD signals from the source to the measurement point. Analysis reveals that if the discharge at the source is assumed to be impulse-like, then the form of the response at the measurement points can be simulated as a function of source location along the winding. If all parameters of the winding are known then it is possible to model the response, however, in the field this is often not the case. So the method developed here uses information from the PD measurements themselves to estimate the unknown coefficients required to simulate the likely discharge currents. With these estimates it is then possible to compare PD measurement information with simulation data in order to identify the most likely location of a PD source. An experiment based on a section of high voltage winding has been used to produce PD measurement data and to validate the proposed approach.
IEEE Transactions on Dielectrics and Electrical Insulation, 2015
ABSTRACT Analysis of real operating condition revealed that HVDC transformers experience combined... more ABSTRACT Analysis of real operating condition revealed that HVDC transformers experience combined effect of DC biased AC electric field. The dynamics of pressboard particle in contaminated transformer oil under the influence of DC, AC and DC biased AC electric field has been investigated in this paper. Different levels of particle concentrations are tested at different applied voltages. Optical images of the particles accumulation together with conduction current have been recorded during the experiments. A complete bridge between the electrodes of cellulose particles were observed for all the tests carried out under DC and DC biased AC electric field. Opposite to that, for AC experiments, pressboard particles accumulated on surfaces of both electrodes but did not create a full bridge between the electrodes. It is concluded that a combination of DC and AC voltages in a HVDC transformer could lead to a bridge formation within the equipment which could cause failure.
2009 IEEE Conference on Electrical Insulation and Dielectric Phenomena, 2009
The formation of Trichel pulses using two dimensional axisymmetric finite element (FE) simulation... more The formation of Trichel pulses using two dimensional axisymmetric finite element (FE) simulations is presented in this paper. The hydrodynamic drift-diffusion approximation is used to model the phenomenon and the simulation is based on solving the coupled system of partial differential equations accounting for the transport of charged particles and Poisson's equation for the electric field. A needle-plane geometry with
2012 Annual Report Conference on Electrical Insulation and Dielectric Phenomena, 2012
ABSTRACT Experiments were carried out on transformer oil contaminated by cellulose fibre. After t... more ABSTRACT Experiments were carried out on transformer oil contaminated by cellulose fibre. After the high voltage was applied to the electrodes, the pressboard particles accumulated in the highest electric field region and eventually formed a bridge. We have developed a mathematical model to simulate the bridging dynamics. The dielectrophoretic and drag forces are considered for simulation of the particles motion and accumulation. The predicted bridge formation rate and the conduction current between the electrodes showed similar trend to the experimental results. The model is a successful means of predicting bridge formation phenomenon compared with the experimental data.
A study has been made of the electrical characteristics and optical emission spectra exhibited wh... more A study has been made of the electrical characteristics and optical emission spectra exhibited when discharge events take place during plasma electrolytic oxidation processing. Both conventional and small area experimental arrangements have been employed, allowing detailed measurement of durations, and temporal distributions, as well as such characteristics as charge transfer, and power. Individual discharges are of short duration, typically tens to hundreds of microseconds, but there is a strong tendency for them to occur in cascades that commonly last between several ms and several tens of ms. The composition, temperature and electron density of the plasma formed during PEO processing are inferred from characteristics of the emission spectra. This confirms that there are two distinct regions of plasma; a lower density peripheral region at~3500 K, and a higher density core at 16,000 ± 3500 K. The implications of these results are considered in terms of the interpretation of different types of experimental measurement, and attention is also briefly given to how such behaviour might relate to the mechanisms of growth.
Broadband approximate expressions for calculating the broadening of the spectral lines of hydroge... more Broadband approximate expressions for calculating the broadening of the spectral lines of hydrogen-like ions in a multicomponent plasma are derived taking into account both the influence of the interaction between plasma particles on the distribution function of the plasma microfield and the effect of the microfield dynamics on the broadening of the central component of the spectral line. With the approximate expressions proposed, the calculation of the shape of a given spectral line of a certain ion in a plasma with a given ion composition requires only a few seconds of computer time. The approximate expressions provide a good computational accuracy not only for the central component of the spectral line but also for the spectral line wings.
Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 2006
A brief analysis is presented of how heat transfer takes place in porous materials of various typ... more A brief analysis is presented of how heat transfer takes place in porous materials of various types. The emphasis is on materials able to withstand extremes of temperature, gas pressure, irradiation, etc. i.e. metals and ceramics, rather than polymers. A primary aim is commonly to maximize either the thermal resistance (i.e. provide insulation) or the rate of thermal equilibration between the material and a fluid passing through it (i.e. to facilitate heat exchange). The main structural characteristics concern porosity (void content), anisotropy, pore connectivity and scale. The effect of scale is complex, since the permeability decreases as the structure is refined, but the interfacial area for fluid-solid heat exchange is, thereby, raised. The durability of the pore structure may also be an issue, with a possible disadvantage of finer scale structures being poor microstructural stability under service conditions. Finally, good mechanical properties may be required, since the development of thermal gradients, high fluid fluxes, etc. can generate substantial levels of stress. There are, thus, some complex interplays between service conditions, pore architecture/scale, fluid permeation characteristics, convective heat flow, thermal conduction and radiative heat transfer. Such interplays are illustrated with reference to three examples: (i) a thermal barrier coating in a gas turbine engine; (ii) a Space Shuttle tile; and (iii) a Stirling engine heat exchanger. Highly porous, permeable materials are often made by bonding fibres together into a network structure and much of the analysis presented here is oriented towards such materials.
A model of the sintering exhibited by EB-PVD TBCs, based on the principles of free energy minimiz... more A model of the sintering exhibited by EB-PVD TBCs, based on the principles of free energy minimization, was recently published by Hutchinson et al . In the current paper, this approach is applied to the sintering of plasma-sprayed TBCs and comparisons are made with experimental results. Predictions of through-thickness shrinkage and changing pore surface area are compared with experimental data obtained by dilatometry and BET analysis respectively. The sensitivity of the predictions to initial pore architecture and material properties are assessed. The model can be used to predict the evolution of the contact area between overlying splats. This is in turn related to the through-thickness thermal conductivity, using a previously-developed analytical model .
Yttria-stabilized zirconia powders, containing different levels of SiO 2 and Al 2 O 3 , have been... more Yttria-stabilized zirconia powders, containing different levels of SiO 2 and Al 2 O 3 , have been plasma sprayed onto metallic substrates. The coatings were detached from their substrates and a dilatometer was used to monitor the dimensional changes they exhibited during prolonged heat treatments. It was found that specimens containing higher levels of silica and alumina exhibited higher rates of linear contraction, in both in-plane and through-thickness directions. The in-plane stiffness and the through-thickness thermal conductivity were also measured after different heat treatments and these were found to increase at a greater rate for specimens with higher impurity (silica and alumina) levels. Changes in the pore architecture during heat treatments were studied using Mercury Intrusion Porosimetry (MIP). Fine scale porosity (<$50 nm) was found to be sharply reduced even by relatively short heat treatments. This is correlated with improvements in inter-splat bonding and partial healing of intra-splat microcracks, which are responsible for the observed changes in stiffness and conductivity, as well as the dimensional changes.
A review is presented of how heat transfer takes place in plasma-sprayed (zirconia-based) thermal... more A review is presented of how heat transfer takes place in plasma-sprayed (zirconia-based) thermal barrier coatings (TBCs) during operation of gas turbines. These characteristics of TBCs are naturally of central importance to their function. Current state-of-the-art TBCs have relatively high levels of porosity (~15%) and the pore architecture (i.e., its morphology, connectivity, and scale) has a strong influence on the
It has been found that reducing the level of impurity oxides (particularly SiO 2 and Al 2 O 3 ) i... more It has been found that reducing the level of impurity oxides (particularly SiO 2 and Al 2 O 3 ) in 7YSZ, from about 0.2 wt% to below 0.1 wt% raises the sintering resistance and the phase stability of plasma-sprayed coatings. The implications for the usage of these coatings at elevated temperatures are examined. It is concluded that using relatively high-purity powder of this type is likely to confer substantial benefits in terms of the thermomechanical stability of the coatings under service conditions.
Numerical (finite difference) and analytical models have been developed for the simulation of hea... more Numerical (finite difference) and analytical models have been developed for the simulation of heat flow through plasma-sprayed coatings, allowing the effective thermal conductivity to be predicted as a function of microstructural parameters. The structure is assumed to be composed of lamellar material (splats), separated by (thin) pores, within which there are areas of contact (bridges). The analytical model is based on dividing the material into two regimes, within which the heat flow occurs either by unidirectional serial flow through lamellae and pores or by being funneled through the regions of the lamellae above and below the bridges. The validity of this model is demonstrated by a comparison of the predictions obtained from it and those obtained from the numerical model. The effects of pore geometry on conductive and radiative heat transfer within the coating have been investigated over a range of temperatures and gas pressures. It is shown that the main factor controlling the conductivity is the intersplat bridge area. Comparisons are also presented with experimental conductivity data, for cases in which some attempt has been made to characterize the key microstructural features. The study is oriented toward thermal barrier coatings, based on zirconiayttria top coats. It is noted that the effect of microstructural sintering, which tends to occur in these coatings under service conditions, can be predicted using this model.
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Papers by I. Golosnoy