Chul Kim - Academia.edu (original) (raw)

Papers by Chul Kim

AIAA Journal, May 1, 2004

Subscripts CH = HACA soot surface growth mechanism of Colket and Hall 11 eq = property for thermo... more Subscripts CH = HACA soot surface growth mechanism of Colket and Hall 11 eq = property for thermodynamic equilibrium conditions FW = HACA soot surface growth mechanism of Frenklach and Wang 12,13 and Kazakov et al. 14 0 = burner exit condition

IEEE Transactions on Applied Superconductivity, 2016

A new and novel design concept for superconducting cables that uses the gaseous cryogen as the di... more A new and novel design concept for superconducting cables that uses the gaseous cryogen as the dielectric medium is presented. This novel design addresses the challenges in developing a dielectric design of a superconducting cable that is cooled by a gaseous cryogen such as gaseous helium (GHe). This design has the added benefit of improving thermal aspects and for DC applications should reduce the possibility of space charge accumulation. The paper provides an overview of design criteria which is necessary to successfully implement this design. A proof of concept experiment was also developed to demonstrate how the voltage rating of such a design is dependent on the dielectric strength of the cryogen being used. The new design has the potential to offer a solution for high power density superconducting medium voltage DC cables that are cooled with gaseous helium and operate in the temperature range of 5-80 K.  Index Terms-Dielectric design, Gas-cooled HTS power cables, helium-hydrogen mixture, medium voltage DC, superconducting gas insulated line

IEEE Transactions on Applied Superconductivity, 2017

In this paper, the transport AC losses in a triangular, epoxy-impregnated high temperature superc... more In this paper, the transport AC losses in a triangular, epoxy-impregnated high temperature superconducting (HTS) coil made from YBCO coated conductor, intended for use in a prototype axial flux HTS electric machine, are measured using two different electrical techniques at 77 K. The first set of AC loss measurements of the coil are carried out at the University of Cambridge using a technique based on a lock-in amplifier. The coil is then measured at the Center for Advanced Power Systems (CAPS), Florida State University, using a technique based on a high accuracy data acquisition (DAQ) measurement system. The two different methods show consistent results, validating the accuracy of these two techniques for transport AC loss measurements of superconducting coils. Multiple voltage taps are utilized within the coil to study the details and distribution of the AC loss in different sections of the coil. Losses are also measured with a flux diverter made of ferromagnetic material to analyze its effect on the AC losses.  Index Terms-AC loss, critical current density (superconductivity), high-temperature superconductors, superconducting coils, transport ac loss.

IEEE Transactions on Applied Superconductivity

IEEE Transactions on Applied Superconductivity

IOP Conference Series: Materials Science and Engineering, 2020

In an attempt to eliminate solid insulation challenges in cryogenic superconducting power cables,... more In an attempt to eliminate solid insulation challenges in cryogenic superconducting power cables, a new design concept for liquid cryogen cooled superconducting power cable was investigated. The design is based on superconducting gas insulated line (S-GIL). The design used liquid cryogen as the sole insulation medium. The suitability of the design for medium voltage power cables is discussed and the benefits of eliminating a solid insulation were identified. Experiments on 1-m long model cables with insulator tubes as spacers showed that the design is suitable for cables at 50 kV or higher. The actual limits could not be identified because of the experimental limitations originated from limited standoff distances in the measurement setup used. On a fundamental level, the investigations presented in the study showed a direct correlation between the intrinsic dielectric strength of the cryogen used and the maximum tolerated voltage for a given diameter of the cable system. The results...

IEEE Transactions on Dielectrics and Electrical Insulation, 2017

This study presents a procedure that improves the dielectric strength estimation of gas mixtures ... more This study presents a procedure that improves the dielectric strength estimation of gas mixtures for superconducting applications. The procedure introduces a generalization factor into the conventional formula of ionization coefficient, which provides versatility and higher accuracy in the regression process of ionization coefficients obtained by solving the Boltzmann equation of gas mixtures with various combination ratios. Generalization factors, which provide the highest accuracy in the regression process are identified. Then, versatile models are derived for each gas mixture based on the Townsend breakdown criterion. The versatile modeling procedure not only provides improved accuracy in the dielectric strength estimation of gas mixtures, but also presents an effective way of estimating variations in dielectric strength caused by changes in gas composition and temperature. The validity of the versatile modeling procedure is confirmed by breakdown measurements conducted with a number of gas mixtures at various gas pressures and temperatures.

Applied Thermal Engineering, 2018

Superconducting cables are considered a viable technology to meet the increasing global demand of... more Superconducting cables are considered a viable technology to meet the increasing global demand of electricity transmission and distribution. This paper presents a transient mathematical model to predict the thermal response of a superconducting cable contained in a flexible cryostat. The model was conceived to be computationally fast so that system response according to variations of physical properties of the materials, and operating and design parameters could be assessed for optimization purposes. A volume element method (VEM) was utilized, which resulted in a system of ordinary differential equations with time as the independent variable. The model is also space dependent, through the establishment of a mesh with a known three-dimensional distribution of the volume elements in the computational domain. Pressure drop in the gas channels and the temperature gradient with respect to space in the flow direction were taken into account. The numerically calculated DC cable heat leak rate under different environmental conditions was initially adjusted and then experimentally validated by direct comparison to actual experimental data. The final part of the study consisted of using the experimentally validated model to perform the DC cable design and operating Highlights > We introduce an HTS DC cable and cryostat transient mathematical model > HTS DC cable and cryostat transient temperature distribution is assessed > Model adjustment and experimental validation are conducted > Total power consumption is estimated based on thermodynamic losses > The model allows for system design, control and optimization

IEEE Transactions on Applied Superconductivity, 2019

The idea for this book stemmed from the symposium on 'Valued Environments' that was held at the 1... more The idea for this book stemmed from the symposium on 'Valued Environments' that was held at the 1978 Annual Meeting of the Institute of British Geographers at Hull University. The symposium brought together a wide range of papers and attracted a large audience. The lively discussion that ensued and the general interest that was aroused convinced us, as the organisers of the symposium, that the conference papers could usefully form the basis for a collection of essays. Having said that, it should be stressed that this book is decidedly not the 'proceedings' of that symposium. During the intervening period, the essays have been extensively revised and extended from the papers that were their precursors, and several new contributions have been added to the collection. The result is a volume that has developed and matured in the process of preparation, but one which we feel retains the freshness and vitality of the original event. The essays included here explore many different themes and tackle various types of valued environments-urban and rural, small-scale and regional, exalted and unsung-but all are bound together by their focus upon the meaning of place and landscape in the lives of individuals and groups. The essays themselves are by British authors and deal primarily with valued environments located in Great Britain, yet it is our firm belief that their findings, the ideas that they put forward, and the issues upon which they touch will interest all who are engaged in the study or design of places, landscapes and environments. An undertaking of this sort relies upon the goodwill and cooperation of a large number of people, most of whom receive scant reward for their labours. Certainly we consider ourselves fortunate in the quality of advice, assistance, and encouragement that we have received. In particular, we would like to thank Denis Cosgrove, Alan Jenkins and Hugh Prince for their helpful comments about the essays and the book in general. None of them, of course, should be held responsible for the outcome. Our thanks go to Jay Appleton and Nigel Haigh who acted as chairmen at the original symposium. We would also like to thank Annabel Swindells and Liz Cooke for the prompt and efficient way in which they typed the final manuscript; Alick Newman for redrawing the maps, diagrams and illustrations; and Chris Cromarty for the Ideal Home in the cover photograph. Finally, as always, we wish to thank our respective spouses, Maggie and Mike, for their support and tolerance.

Advances in Materials Science and Engineering, 2018

The idea of insulating individual high-temperature superconducting (HTS) tapes was explored as a ... more The idea of insulating individual high-temperature superconducting (HTS) tapes was explored as a dielectric design to reduce the risk and complexity of HTS cable manufacturing. Applying insulation on individual HTS tapes is amenable to continuous manufacturing processes and opens up material choices for the insulation. In this study, heat shrink insulation was selected as the material choice for exploring the possibility of this design philosophy because of its commercial availability in multiple thicknesses. A systematic set of selection criteria was developed for the selection of appropriate heat shrink for given HTS tape dimensions. The cryogenic dielectric characteristics of insulated HTS tapes were evaluated both in liquid nitrogen and gaseous helium environments at 77 K. Dielectric characteristics of tapes with a single layer of thicker insulation were compared with those insulated using multiple layers of thinner insulation to evaluate the relative merits of each method. Seve...

IEEE Transactions on Applied Superconductivity, 2017

Thermal network models (TNM) with coupled electrical and cryogenic thermal aspects of a gaseous h... more Thermal network models (TNM) with coupled electrical and cryogenic thermal aspects of a gaseous helium cooled high temperature superconducting cable loop are developed. The cable system loop consisted of the terminations, a 30 m long HTS cable, and a cryocooler based cryogenic helium gas circulation system. The utility of the models is demonstrated through simulations of cryogenic thermal response of the cable loop under several operational scenarios. Temperature profile of the complete cable system has been modelled for different helium gas mass flow rates and with DC and AC current up to 3 kA. Transient response of the cable loop under some potential contingencies was simulated to demonstrate the utility of the thermal network model tools.

IEEE Transactions on Applied Superconductivity, 2017

A comparative study of cryogenic dielectric and mechanical characteristics of electrical insulati... more A comparative study of cryogenic dielectric and mechanical characteristics of electrical insulation materials is described. Breakdown strengths and tensile strengths of Cryoflex, Kapton, PPLP, and Teflon were measured at 77 K in liquid nitrogen. Breakdown strengths of Cryoflex, Kapton, PPLP were also measured in gaseous helium at 2 MPa and 77 K. The relative breakdown strengths of Cryoflex and Kapton vary between liquid nitrogen and gaseous helium. Kapton performed better than Cryoflex both for dielectric and mechanical properties. Among the materials studied, Teflon exhibited the least breakdown strength and mechanical properties.  Index Terms-GHe cooled HTS devices, Insulation materials, Cryogenic dielectric properties, Cryogenic mechanical properties. Automatically generated dates of receipt and acceptance will be placed here; authors do not produce these dates.

2021 IEEE Electrical Insulation Conference (EIC)

2018 12th International Conference on the Properties and Applications of Dielectric Materials (ICPADM), 2018

The influence of magnetic field on the dielectric strength of cryogenic helium is discussed. The ... more The influence of magnetic field on the dielectric strength of cryogenic helium is discussed. The effect of magnetic field that is perpendicular to the electric field, as encountered in high-temperature superconducting (HTS) power cables, on the dielectric aspects was investigated. Experiments measuring the breakdown strength of helium gas with and without the magnetic field were performed, and the results were further analyzed based on the developed models. The experimental results showed no significant effect of the magnetic field on the dielectric strength of gaseous helium.

2018 12th International Conference on the Properties and Applications of Dielectric Materials (ICPADM), 2018

The influence of temperature on the dielectric strength of gaseous cryogens has been studied over... more The influence of temperature on the dielectric strength of gaseous cryogens has been studied over a wide range of temperature and pressure. Electric breakdown measurements were designed to evaluate if there is any temperature effect after adjusting the pressure to equate the mass density of the gas medium. Breakdown measurements were conducted at 77 K and 293 K with the appropriately selected gas pressure to maintain the equivalent particle number density of helium. A chamber compatible for breakdown experiments at cryogenic temperature was used for the measurements at 77 K. A second chamber was specially designed for breakdown measurements at high-pressure at 293 K. The measurements at both temperatures were conducted over a range of pressures. It was observed that the dielectric strength of the gaseous medium scales with the mass density but no temperature effect was observed. It was also observed that a liner relation exists between the dielectric strength and mass density. These results provide an effective practical method to study the dielectric behavior of gases at a temperature of convenience and use the data to estimate the dielectric properties at a different set of operating temperature and pressure in any application.

IEEE Transactions on Applied Superconductivity, 2016

A versatile method to model combined electrical and thermal behavior of superconducting power dev... more A versatile method to model combined electrical and thermal behavior of superconducting power devices is introduced. The methodology and computational tools of thermal network models (TNM) that use MATLAB with a Simulink toolbox primarily intended for power electronics simulations (“PLECS”) are presented. The utility of the modeling technique is demonstrated with a case study of superconducting cable termination that is cooled with gaseous helium circulation. The temperature profile in the termination due to the heat leak from the ambient and the Joule heating in the bushing and at the interfaces between copper leads and superconducting cable has been modeled. The temperature profile resulted from the TNM compared well with values obtained from experimental investigations. Suggestions for expanding the capabilities of the model are described.

IOP Conference Series: Materials Science and Engineering, 2020

In an attempt to eliminate solid insulation challenges in cryogenic superconducting power cables,... more In an attempt to eliminate solid insulation challenges in cryogenic superconducting power cables, a new design concept for liquid cryogen cooled superconducting power cable was investigated. The design is based on superconducting gas insulated line (S-GIL). The design used liquid cryogen as the sole insulation medium. The suitability of the design for medium voltage power cables is discussed and the benefits of eliminating a solid insulation were identified. Experiments on 1-m long model cables with insulator tubes as spacers showed that the design is suitable for cables at 50 kV or higher. The actual limits could not be identified because of the experimental limitations originated from limited standoff distances in the measurement setup used. On a fundamental level, the investigations presented in the study showed a direct correlation between the intrinsic dielectric strength of the cryogen used and the maximum tolerated voltage for a given diameter of the cable system. The results...

IEEE Transactions on Dielectrics and Electrical Insulation, 2017

This study presents a procedure that improves the dielectric strength estimation of gas mixtures ... more This study presents a procedure that improves the dielectric strength estimation of gas mixtures for superconducting applications. The procedure introduces a generalization factor into the conventional formula of ionization coefficient, which provides versatility and higher accuracy in the regression process of ionization coefficients obtained by solving the Boltzmann equation of gas mixtures with various combination ratios. Generalization factors, which provide the highest accuracy in the regression process are identified. Then, versatile models are derived for each gas mixture based on the Townsend breakdown criterion. The versatile modeling procedure not only provides improved accuracy in the dielectric strength estimation of gas mixtures, but also presents an effective way of estimating variations in dielectric strength caused by changes in gas composition and temperature. The validity of the versatile modeling procedure is confirmed by breakdown measurements conducted with a number of gas mixtures at various gas pressures and temperatures.

AIAA Journal, May 1, 2004

Subscripts CH = HACA soot surface growth mechanism of Colket and Hall 11 eq = property for thermo... more Subscripts CH = HACA soot surface growth mechanism of Colket and Hall 11 eq = property for thermodynamic equilibrium conditions FW = HACA soot surface growth mechanism of Frenklach and Wang 12,13 and Kazakov et al. 14 0 = burner exit condition

IEEE Transactions on Applied Superconductivity, 2016

A new and novel design concept for superconducting cables that uses the gaseous cryogen as the di... more A new and novel design concept for superconducting cables that uses the gaseous cryogen as the dielectric medium is presented. This novel design addresses the challenges in developing a dielectric design of a superconducting cable that is cooled by a gaseous cryogen such as gaseous helium (GHe). This design has the added benefit of improving thermal aspects and for DC applications should reduce the possibility of space charge accumulation. The paper provides an overview of design criteria which is necessary to successfully implement this design. A proof of concept experiment was also developed to demonstrate how the voltage rating of such a design is dependent on the dielectric strength of the cryogen being used. The new design has the potential to offer a solution for high power density superconducting medium voltage DC cables that are cooled with gaseous helium and operate in the temperature range of 5-80 K.  Index Terms-Dielectric design, Gas-cooled HTS power cables, helium-hydrogen mixture, medium voltage DC, superconducting gas insulated line

IEEE Transactions on Applied Superconductivity, 2017

In this paper, the transport AC losses in a triangular, epoxy-impregnated high temperature superc... more In this paper, the transport AC losses in a triangular, epoxy-impregnated high temperature superconducting (HTS) coil made from YBCO coated conductor, intended for use in a prototype axial flux HTS electric machine, are measured using two different electrical techniques at 77 K. The first set of AC loss measurements of the coil are carried out at the University of Cambridge using a technique based on a lock-in amplifier. The coil is then measured at the Center for Advanced Power Systems (CAPS), Florida State University, using a technique based on a high accuracy data acquisition (DAQ) measurement system. The two different methods show consistent results, validating the accuracy of these two techniques for transport AC loss measurements of superconducting coils. Multiple voltage taps are utilized within the coil to study the details and distribution of the AC loss in different sections of the coil. Losses are also measured with a flux diverter made of ferromagnetic material to analyze its effect on the AC losses.  Index Terms-AC loss, critical current density (superconductivity), high-temperature superconductors, superconducting coils, transport ac loss.

IEEE Transactions on Applied Superconductivity

IEEE Transactions on Applied Superconductivity

IOP Conference Series: Materials Science and Engineering, 2020

In an attempt to eliminate solid insulation challenges in cryogenic superconducting power cables,... more In an attempt to eliminate solid insulation challenges in cryogenic superconducting power cables, a new design concept for liquid cryogen cooled superconducting power cable was investigated. The design is based on superconducting gas insulated line (S-GIL). The design used liquid cryogen as the sole insulation medium. The suitability of the design for medium voltage power cables is discussed and the benefits of eliminating a solid insulation were identified. Experiments on 1-m long model cables with insulator tubes as spacers showed that the design is suitable for cables at 50 kV or higher. The actual limits could not be identified because of the experimental limitations originated from limited standoff distances in the measurement setup used. On a fundamental level, the investigations presented in the study showed a direct correlation between the intrinsic dielectric strength of the cryogen used and the maximum tolerated voltage for a given diameter of the cable system. The results...

IEEE Transactions on Dielectrics and Electrical Insulation, 2017

This study presents a procedure that improves the dielectric strength estimation of gas mixtures ... more This study presents a procedure that improves the dielectric strength estimation of gas mixtures for superconducting applications. The procedure introduces a generalization factor into the conventional formula of ionization coefficient, which provides versatility and higher accuracy in the regression process of ionization coefficients obtained by solving the Boltzmann equation of gas mixtures with various combination ratios. Generalization factors, which provide the highest accuracy in the regression process are identified. Then, versatile models are derived for each gas mixture based on the Townsend breakdown criterion. The versatile modeling procedure not only provides improved accuracy in the dielectric strength estimation of gas mixtures, but also presents an effective way of estimating variations in dielectric strength caused by changes in gas composition and temperature. The validity of the versatile modeling procedure is confirmed by breakdown measurements conducted with a number of gas mixtures at various gas pressures and temperatures.

Applied Thermal Engineering, 2018

Superconducting cables are considered a viable technology to meet the increasing global demand of... more Superconducting cables are considered a viable technology to meet the increasing global demand of electricity transmission and distribution. This paper presents a transient mathematical model to predict the thermal response of a superconducting cable contained in a flexible cryostat. The model was conceived to be computationally fast so that system response according to variations of physical properties of the materials, and operating and design parameters could be assessed for optimization purposes. A volume element method (VEM) was utilized, which resulted in a system of ordinary differential equations with time as the independent variable. The model is also space dependent, through the establishment of a mesh with a known three-dimensional distribution of the volume elements in the computational domain. Pressure drop in the gas channels and the temperature gradient with respect to space in the flow direction were taken into account. The numerically calculated DC cable heat leak rate under different environmental conditions was initially adjusted and then experimentally validated by direct comparison to actual experimental data. The final part of the study consisted of using the experimentally validated model to perform the DC cable design and operating Highlights > We introduce an HTS DC cable and cryostat transient mathematical model > HTS DC cable and cryostat transient temperature distribution is assessed > Model adjustment and experimental validation are conducted > Total power consumption is estimated based on thermodynamic losses > The model allows for system design, control and optimization

IEEE Transactions on Applied Superconductivity, 2019

The idea for this book stemmed from the symposium on 'Valued Environments' that was held at the 1... more The idea for this book stemmed from the symposium on 'Valued Environments' that was held at the 1978 Annual Meeting of the Institute of British Geographers at Hull University. The symposium brought together a wide range of papers and attracted a large audience. The lively discussion that ensued and the general interest that was aroused convinced us, as the organisers of the symposium, that the conference papers could usefully form the basis for a collection of essays. Having said that, it should be stressed that this book is decidedly not the 'proceedings' of that symposium. During the intervening period, the essays have been extensively revised and extended from the papers that were their precursors, and several new contributions have been added to the collection. The result is a volume that has developed and matured in the process of preparation, but one which we feel retains the freshness and vitality of the original event. The essays included here explore many different themes and tackle various types of valued environments-urban and rural, small-scale and regional, exalted and unsung-but all are bound together by their focus upon the meaning of place and landscape in the lives of individuals and groups. The essays themselves are by British authors and deal primarily with valued environments located in Great Britain, yet it is our firm belief that their findings, the ideas that they put forward, and the issues upon which they touch will interest all who are engaged in the study or design of places, landscapes and environments. An undertaking of this sort relies upon the goodwill and cooperation of a large number of people, most of whom receive scant reward for their labours. Certainly we consider ourselves fortunate in the quality of advice, assistance, and encouragement that we have received. In particular, we would like to thank Denis Cosgrove, Alan Jenkins and Hugh Prince for their helpful comments about the essays and the book in general. None of them, of course, should be held responsible for the outcome. Our thanks go to Jay Appleton and Nigel Haigh who acted as chairmen at the original symposium. We would also like to thank Annabel Swindells and Liz Cooke for the prompt and efficient way in which they typed the final manuscript; Alick Newman for redrawing the maps, diagrams and illustrations; and Chris Cromarty for the Ideal Home in the cover photograph. Finally, as always, we wish to thank our respective spouses, Maggie and Mike, for their support and tolerance.

Advances in Materials Science and Engineering, 2018

The idea of insulating individual high-temperature superconducting (HTS) tapes was explored as a ... more The idea of insulating individual high-temperature superconducting (HTS) tapes was explored as a dielectric design to reduce the risk and complexity of HTS cable manufacturing. Applying insulation on individual HTS tapes is amenable to continuous manufacturing processes and opens up material choices for the insulation. In this study, heat shrink insulation was selected as the material choice for exploring the possibility of this design philosophy because of its commercial availability in multiple thicknesses. A systematic set of selection criteria was developed for the selection of appropriate heat shrink for given HTS tape dimensions. The cryogenic dielectric characteristics of insulated HTS tapes were evaluated both in liquid nitrogen and gaseous helium environments at 77 K. Dielectric characteristics of tapes with a single layer of thicker insulation were compared with those insulated using multiple layers of thinner insulation to evaluate the relative merits of each method. Seve...

IEEE Transactions on Applied Superconductivity, 2017

Thermal network models (TNM) with coupled electrical and cryogenic thermal aspects of a gaseous h... more Thermal network models (TNM) with coupled electrical and cryogenic thermal aspects of a gaseous helium cooled high temperature superconducting cable loop are developed. The cable system loop consisted of the terminations, a 30 m long HTS cable, and a cryocooler based cryogenic helium gas circulation system. The utility of the models is demonstrated through simulations of cryogenic thermal response of the cable loop under several operational scenarios. Temperature profile of the complete cable system has been modelled for different helium gas mass flow rates and with DC and AC current up to 3 kA. Transient response of the cable loop under some potential contingencies was simulated to demonstrate the utility of the thermal network model tools.

IEEE Transactions on Applied Superconductivity, 2017

A comparative study of cryogenic dielectric and mechanical characteristics of electrical insulati... more A comparative study of cryogenic dielectric and mechanical characteristics of electrical insulation materials is described. Breakdown strengths and tensile strengths of Cryoflex, Kapton, PPLP, and Teflon were measured at 77 K in liquid nitrogen. Breakdown strengths of Cryoflex, Kapton, PPLP were also measured in gaseous helium at 2 MPa and 77 K. The relative breakdown strengths of Cryoflex and Kapton vary between liquid nitrogen and gaseous helium. Kapton performed better than Cryoflex both for dielectric and mechanical properties. Among the materials studied, Teflon exhibited the least breakdown strength and mechanical properties.  Index Terms-GHe cooled HTS devices, Insulation materials, Cryogenic dielectric properties, Cryogenic mechanical properties. Automatically generated dates of receipt and acceptance will be placed here; authors do not produce these dates.

2021 IEEE Electrical Insulation Conference (EIC)

2018 12th International Conference on the Properties and Applications of Dielectric Materials (ICPADM), 2018

The influence of magnetic field on the dielectric strength of cryogenic helium is discussed. The ... more The influence of magnetic field on the dielectric strength of cryogenic helium is discussed. The effect of magnetic field that is perpendicular to the electric field, as encountered in high-temperature superconducting (HTS) power cables, on the dielectric aspects was investigated. Experiments measuring the breakdown strength of helium gas with and without the magnetic field were performed, and the results were further analyzed based on the developed models. The experimental results showed no significant effect of the magnetic field on the dielectric strength of gaseous helium.

2018 12th International Conference on the Properties and Applications of Dielectric Materials (ICPADM), 2018

The influence of temperature on the dielectric strength of gaseous cryogens has been studied over... more The influence of temperature on the dielectric strength of gaseous cryogens has been studied over a wide range of temperature and pressure. Electric breakdown measurements were designed to evaluate if there is any temperature effect after adjusting the pressure to equate the mass density of the gas medium. Breakdown measurements were conducted at 77 K and 293 K with the appropriately selected gas pressure to maintain the equivalent particle number density of helium. A chamber compatible for breakdown experiments at cryogenic temperature was used for the measurements at 77 K. A second chamber was specially designed for breakdown measurements at high-pressure at 293 K. The measurements at both temperatures were conducted over a range of pressures. It was observed that the dielectric strength of the gaseous medium scales with the mass density but no temperature effect was observed. It was also observed that a liner relation exists between the dielectric strength and mass density. These results provide an effective practical method to study the dielectric behavior of gases at a temperature of convenience and use the data to estimate the dielectric properties at a different set of operating temperature and pressure in any application.

IEEE Transactions on Applied Superconductivity, 2016

A versatile method to model combined electrical and thermal behavior of superconducting power dev... more A versatile method to model combined electrical and thermal behavior of superconducting power devices is introduced. The methodology and computational tools of thermal network models (TNM) that use MATLAB with a Simulink toolbox primarily intended for power electronics simulations (“PLECS”) are presented. The utility of the modeling technique is demonstrated with a case study of superconducting cable termination that is cooled with gaseous helium circulation. The temperature profile in the termination due to the heat leak from the ambient and the Joule heating in the bushing and at the interfaces between copper leads and superconducting cable has been modeled. The temperature profile resulted from the TNM compared well with values obtained from experimental investigations. Suggestions for expanding the capabilities of the model are described.

IOP Conference Series: Materials Science and Engineering, 2020

In an attempt to eliminate solid insulation challenges in cryogenic superconducting power cables,... more In an attempt to eliminate solid insulation challenges in cryogenic superconducting power cables, a new design concept for liquid cryogen cooled superconducting power cable was investigated. The design is based on superconducting gas insulated line (S-GIL). The design used liquid cryogen as the sole insulation medium. The suitability of the design for medium voltage power cables is discussed and the benefits of eliminating a solid insulation were identified. Experiments on 1-m long model cables with insulator tubes as spacers showed that the design is suitable for cables at 50 kV or higher. The actual limits could not be identified because of the experimental limitations originated from limited standoff distances in the measurement setup used. On a fundamental level, the investigations presented in the study showed a direct correlation between the intrinsic dielectric strength of the cryogen used and the maximum tolerated voltage for a given diameter of the cable system. The results...

IEEE Transactions on Dielectrics and Electrical Insulation, 2017

This study presents a procedure that improves the dielectric strength estimation of gas mixtures ... more This study presents a procedure that improves the dielectric strength estimation of gas mixtures for superconducting applications. The procedure introduces a generalization factor into the conventional formula of ionization coefficient, which provides versatility and higher accuracy in the regression process of ionization coefficients obtained by solving the Boltzmann equation of gas mixtures with various combination ratios. Generalization factors, which provide the highest accuracy in the regression process are identified. Then, versatile models are derived for each gas mixture based on the Townsend breakdown criterion. The versatile modeling procedure not only provides improved accuracy in the dielectric strength estimation of gas mixtures, but also presents an effective way of estimating variations in dielectric strength caused by changes in gas composition and temperature. The validity of the versatile modeling procedure is confirmed by breakdown measurements conducted with a number of gas mixtures at various gas pressures and temperatures.