J. W. Spencer

Thermophysical properties of nitrogen plasmas under thermal equilibrium and non-equilibrium conditions

Calculated thermophysical properties of nitrogen plasmas in and out of thermal equilibrium are presented. The cut-off of the partition functions due to the lowering of the ionization potential has been taken into account, together with the contributions from different core excited electronic states. The species composition and thermodynamic properties are determined numerically using the Newton–Raphson iterative method, taking into account the corrections due to Coulomb interactions. The transport properties including diffusion coefficient, viscosity, thermal conductivity, and electrical conductivity are calculated using the most recent collision interaction potentials by adopting Devoto’s electron and heavy particle decoupling approach, expanded to the third-order approximation (second-order for viscosity) in the framework of Chapman–Enskog method. Results are presented in the pressure range of 0.1 atm–10 atm and in electron temperature range from 300 to 40 000 K, with the ratio of electron temperature t...

Study of Multisampled Multilevel Inverters to Improve Control Performance

A classic digitally controlled power converter usually uses the same sampling and switching frequency. The technique is known as uniform sampling. As the performance of digital signal processors is increasing dramatically and the price is decreasing, applying high sampling frequency becomes increasingly feasible. The multisampling technique is developed to reduce switching delay. However, the control gains are still limited by the switching frequency. To demonstrate an alternative way of improving the control performance without increasing the switching frequency, this paper discusses multisampled multilevel inverters. The example of a voltage controlled multilevel inverter with cascaded control loops is provided. The filter current ripple frequency is increased by the phase-shifted pulsewidth modulation. The multisampling is synchronized to the peaks of the phase-shifted carriers. The small-signal z-domain model is derived to analyze the multisampled multilevel inverter. Compared to the bipolar switched inverter, the multisampled multilevel inverter is characterized by the capability of achieving higher feedback control gains, which improves the control performance. An experimental prototype based on a 10-kHz switching frequency, 80-kHz sampling frequency five-level single-phase H-bridge inverter is tested to demonstrate the validity of the analysis.

Thermophysical properties of carbon–argon and carbon–helium plasmas

The calculated values of thermodynamic and transport properties of mixtures of carbon and argon, and carbon and helium, at high temperatures are presented in this paper. The thermodynamic properties are determined by the method of Gibbs free energy minimization, using standard thermodynamic tables. The transport properties including electron diffusion coefficients, viscosity, thermal conductivity and electrical conductivity are evaluated using the Chapman–Enskog method expanded up to the third-order approximation (second order for viscosity). Collision integrals are obtained using the most accurate cross-section data that could be located. The calculations, which assume local thermodynamic equilibrium, are performed for atmospheric pressure plasmas in the temperature range from 300 to 30 000 K for different pressures between 0.1 and 10 atm. The results are compared with those of previously published studies. Good agreement is found for pure argon and helium. Larger discrepancies occur for carbon and mixtures of carbon and argon, and carbon and helium; these are explained in terms of the different values of the collision integrals that were used. The results presented here are expected to be more accurate because of the improved collision integrals employed.

Analysis of Boost PFC Converters Operating in the Discontinuous Conduction Mode

As power factor correction (PFC) converters for low-power applications usually operate in the discontinuous conduction mode (DCM), operating in continuous conduction mode (CCM) will produce input current distortion. This distortion can be observed in a few switching cycles of one line cycle. Asynchronous switching maps are derived to obtain the time-domain waveforms of input current and output voltage. It can be seen that the cause of the distortion is the change in the current conduction mode. A model for PFC converters operating in DCM with fixed switching frequency and duty-ratio is developed, which can predict the converter operation mode under practical circumstances. Analysis of the output voltage and power limitations is provided based on the proposed model. Simulation and experimental results are presented to verify the proposed method.

Theoretical investigation of the decay of an SF 6 gas-blast arc using a two-temperature hydrodynamic model

The behaviour of a decaying SF6 arc, which is representative of the approach to the final current-zero state of switching arcs in a high-voltage circuit breaker, is theoretically investigated by a two-temperature hydrodynamic model, taking into account the possible departure of the plasma state from local thermodynamic equilibrium (LTE). The model couples the plasma flow with electromagnetic fields in a self-consistent manner. The electrons and heavy species are assumed to have different temperatures. The species composition, thermodynamic properties and transport coefficients of the plasma under non-LTE conditions are calculated from fundamental theory. The model is then applied to a two-dimensional axisymmetric SF6 arc burning in a supersonic nozzle under well-controlled conditions; for this configuration, experimental results are available for comparison. The effect of turbulence is considered using the Prandtl mixing-length model. The edge absorption of the radiation emitted by the arc core is taken into account by a modified net emission coefficient approach. The complete set of conservation equations is discretized and solved using the finite volume method. The evolution of electron and heavy-particle temperatures and the total arc resistance, along with other physical quantities, is carefully analysed and compared with those of the LTE case. It is demonstrated that the electron and heavy-particle temperature diverge at all times in the plasma–cold-flow interaction region, in which strong gas flow exists, and further in the transient current-zero period, in which case the collision energy exchange is ineffective. This study quantitatively analyses the energy exchange mechanisms between electrons and heavy particles in the high-pressure supersonic SF6 arcs and provides the foundation for further theoretical investigation of transient SF6 arc behaviour as the current ramps down to zero in gas-blast circuit breakers. (Some figures may appear in colour only in the online journal)

Thermodynamic and transport properties of two-temperature SF6 plasmas

This paper deals with thermodynamic and transport properties of SF6 plasmas in a two-temperature model for both thermal equilibrium and non-equilibrium conditions. The species composition and thermodynamic properties are numerically determined using the two-temperature Saha equation and Guldberg-Waage equation according to deviation of van de Sanden et al. Transport properties including diffusion coefficient, viscosity, thermal conductivity, and electrical conductivity are calculated with most recent collision interaction potentials by adopting Devoto’s electron and heavy particle decoupling approach but expanded to the third-order approximation (second-order for viscosity) in the frame of Chapman–Enskog method. The results are computed for various values of pressures from 0.1 atm to 10 atm and ratios of the electron temperature to the heavy particle temperature from 1 to 20 with electron temperature range from 300 to 40 000 K. In the local thermodynamic equilibrium regime, results are compared with avail...

Investigation on critical breakdown electric field of hot sulfur hexafluoride/carbon tetrafluoride mixtures for high voltage circuit breaker applications

Sulfur hexafluoride (SF6) gas, widely used in high-voltage circuit breakers, has a high global warming potential and hence substitutes are being sought. The use of a mixture of carbon tetrafluoride (CF4) and SF6 is examined here. It is known that this reduces the breakdown voltage at room temperature. However, the electrical breakdown in a circuit breaker after arc interruption occurs in a hot gas environment, with a complicated species composition because of the occurrence of dissociation and other reactions. The likelihood of breakdown depends on the electron interactions with all these species. The critical reduced electric field strength (the field at which breakdown can occur, relative to the number density) of hot SF6/CF4 mixtures corresponding to the dielectric recovery phase of a high voltage circuit breaker is calculated in the temperature range from 300 K to 3500 K. The equilibrium compositions of hot SF6/CF4 mixtures under different mixing fractions were determined based on Gibbs free energy mi...

Thermophysical Properties of High-Temperature Reacting Mixtures of Carbon and Water in the Range 400–30,000 K and 0.1–10 atm. Part 1: Equilibrium Composition and Thermodynamic Properties

This paper is devoted to the calculation of the chemical equilibrium composition and thermodynamic properties of reacting mixtures of carbon and water at high temperature. Equilibrium particle concentrations and thermodynamic properties including mass density, molar weight, entropy, enthalpy and specific heat at constant pressure, sonic velocity, and heat capacity ratio are determined by the method of Gibbs free energy minimization, using species data from standard thermodynamic tables. The calculations, which assume local thermodynamic equilibrium, are performed in the temperature range from 400 to 30,000 K for pressures of 0.10, 1.0, 3.0, 5.0 and 10.0 atm. The properties of the reacting mixture are affected by the possible occurrence of solid carbon formation at low temperature, and therefore attention is paid to the influence of the carbon phase transition by comparing the results obtained with and without considering solid carbon formation. The results presented here clarify some basic chemical process and are reliable reference data for use in the simulation of plasmas in reacting carbon and water mixtures together with the need of transport coefficients computation.

Sensitivity improvement of an optical current sensor with enhanced Faraday rotation

A sensitivity improvement technique is proposed for a class of bulk-glass optical current sensors that employ a ferromagnetic field concentrator. The total effective optical path length is demonstrated theoretically to be an invariant regardless of the bulk glass thickness and consequently independent of the size of the concentrator gap opening. Thus, if the magnetic field is increased by reducing the gap size, the eventual Faraday rotation for a given electric current can be increased proportionally, leading to an improved device sensitivity. The dependence of the gap magnetic field on gap size is calculated with an equivalent circuit model, and this analytical treatment is compared with a dedicated finite element computer package. By taking account of various types of optical power losses present in the bulk glass, the above formulated gap dependence of magnetic field is used to aid a realistic assessment of device sensitivity and this serves as a tool to design and analyze practical bulk-glass optical current sensors. A detailed experimental study to confirm the proposed sensitivity improvement technique is also reported.

Investigation of SF 6 Arc Characteristics Under Shock Condition in a Supersonic Nozzle With Hollow Contact

Arc-shock interaction and its influence on steady and dynamic characteristics of SF6 arcs in a supersonic nozzle with hollow contact, which is representative of switching arcs in a gas blast high-voltage circuit breaker, are computationally investigated using the magneto-hydro-dynamic theory with fixed inlet stagnation pressures and at five different exit pressure to inlet stagnation pressure ratios (referred to as pressure ratios). The significant quantity in determining the thermal interruption capability of a circuit breaker, i.e., the critical rate of rise of recovery voltage, was predicted and compared with available test results. The deterioration of thermal interruption capability of a supersonic nozzle under shock conditions, which was observed in previous tests is verified and discussed by current arc model. It was found that the close coupling between the shock and the surrounding gas flow can greatly influence the aerodynamic and electrical behavior of a nozzle arc and hence the thermal recovery behavior of current interruption. Additionally, the velocity deceleration caused by the shocks and the enhanced turbulent cooling brought by the sucked gas and arc interaction both pay a significant role in the determination of thermal interruption capability.