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The spectral and spatial characteristics of three-dimensional radiation transfer across an arc column of 0.08 m long, typical in high-voltage gas blast circuit breakers, has been studied in detail. The arc column under study corresponds to an instantaneous current of 15 kA in SF6 gas at a pressure of 10 bar. Our results show that to calculate the radiative flux divergence (as a volumetric energy source), only a segment of the arc column of 0.024 m in thickness needs to be considered to attain an accuracy of better than 90%. Photons with a frequency lower than 2 × 1015 Hz (150 nm in wavelength) can travel a considerable distance (>0.02 m) with an intensity attenuation factor of 0.2–0.8. Above 2 × 1015 Hz, only photons from the continuum spectrum can travel up to 0.015 m and line emission is absorbed within a distance of typically 0.0005 m, i.e. the arcing gas is optically thick to these photons. The gas within a cross-section of the arc column can be divided into a net emission core and a net absorption zone. The 55%–75% of the radiation emitted from the core is reabsorbed. The starting position of the net absorption zone sits within the temperature interval of 75%–80% of the maximum temperature on the axis. The quantitative information from this work is expected to serve as baseline data for developing improved approximate models for radiation transfer calculation in SF6 switching arcs.

Pseudo-derivative-feedback (PDF) control is, for the first time, applied to the current control of three-phase grid-connected inverters with LCL filters, which significantly improves the transient response of the system to a step change in the reference input through the elimination of overshoot and oscillation. Two PDF controllers with different terms in the inner feedback path are developed for an inverter current feedback system and a grid current feedback system, respectively. For the inverter current feedback system, a simple PDF controller with a proportional term is used. The influence of the controller parameters on the transient performance is discussed in detail. Compared with a PI controller, the PDF controller is able to eliminate the transient overshoot and oscillation easily, while maintaining a fast response. For the gird current feedback system, a PDF controller with a proportional term and a second-order derivative is developed. The implementation and design of the PDF controller are presented. Active damping is achieved with only the feedback from the grid current, and at the same time, the system transient response is improved. Both theoretical analysis and experimental results verify the advantages of the PDF control over PI control methods.

LCL filters have been widely used for grid-connected inverters. However, the problem that how time delay affects the stability of digitally controlled grid-connected inverters with LCL filters has not been fully studied. In this paper, a systematic study is carried out on the relationship between the time delay and stability of single-loop controlled grid-connected inverters that employ inverter current feedback (ICF) or grid current feedback (GCF). The ranges of time delay for system stability are analyzed and deduced in the continuous s -domain and discrete z -domain. It is shown that in the optimal range, the existence of time delay weakens the stability of the ICF loop, whereas a proper time delay is required for the GCF loop. The present work explains, for the first time, why different conclusions on the stability of ICF loop and GCF loop have been drawn in previous studies. To improve system stability, a linear predictor-based time delay reduction method is proposed for ICF, while a time delay addition method is used for GCF. A controller design method is then presented that guarantees adequate stability margins. The delay-dependent stability study is verified by simulation and experiment.

A charge transport model based on ion drift in gases of strong electron affinity has been studied in details and validated against experimental results in air and SF 6 at different pressure and voltage levels. The size of the insulators also differs significantly. The choice of the model parameters, i.e. the solid and gas material properties, is carefully examined with information from existing literature. Results show that using the selected parameters, satisfactory agreement between predicted and measured potential distribution and surface charge density along the insulator surface can be obtained. Computational results for an 1100 kV epoxy spacer for use in HVDC wall bushing show that the concept of electrical conductivity as a material property is no longer valid in strong electric field for the gas surrounding the insulator. The polarity and density of the accumulated surface charge depend on the relative largeness of the electrical conductivity of the insulator material and the effective conductivity of the gas. The nonlinear effect due to ion generation and recombination becomes significant at very high voltage. The metallic cap providing shielding to the triple junction at the high voltage end of the insulator can induce much stronger local electric field near the insulator surface, by a factor of 1.48, if its shape is not optimized against the insulator geometry.

A study was carried out on the characteristics of the dielectric strength of gaseous working medium in a puffer circuit breaker following current interruption. A reduced critical electric field strength (E/N)cr was defined. Its value for SF6 gas within the temperature range of 300 to 3000 K and pressure range of 1 to 32 atm was obtained by solving the Boltzmann equation with most recent atomic data, which allows the determination of the critical electric field strength Ecr at different temperature and pressure. The dielectric behavioral pattern of the breaker was then characterized by applying the Ecr data to the temperature and pressure fields obtained by a 2-D magneto-hydro- dynamics model encompassing all important mechanisms operating in the arcing process. The Ecr distribution at 80 and 110 μs after current zero was then compared to the electric-field distribution to arrive at important information regarding the weakest point or regions in the design. Using a standard rate of rise of recovery voltage profile, the critical dielectric withstand level of the breaker was also predicted.

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.

The accuracy of numerical simulation on plasma behavior depends strongly on the reliability of thermophysical property data. Large number of studies for thermal plasma properties in the local thermodynamic equilibrium (LTE) exist; however, the database for thermal nonequilibrium plasmas is still far from completeness. This paper derives a general expression of total reactive thermal conductivity (TRTC) with great applicability to monatomic, diatomic, and polyatomic gases in terms of a two-temperature model. The derived formula is applied to nitrogen plasmas under thermal equilibrium and nonequilibrium conditions, considering its wide use in plasma systems and switching devices. Typical calculated results of TRTC with two different Saha equations and Guldberg-Waage equations in the temperature range of 300 K-40 000 K under different degrees of nonequilibrium are given and compared with those computed according to Brokaw and Butler's derivation for the special case of LTE plasmas, which shows excellent agreement. The influence of different expressions for Saha equations and Guldberg-Waage equations, together with different pressures of 0.1, 1, 3, 5, and 10 atm, on the TRTC evaluated by this newly developed expression is presented as well. These provide reliable reference data for use in the simulation of plasmas.

This study is concerned with the quantised stabilisation of linear discrete systems with packet dropout. Based on the zoom strategy and Lyapunov theory, for a given packet dropout rate, a sufficient condition is given for the closed-loop system to be mean square stable. A numerical simulation is presented to show the effectiveness of the main result.

A direct grid-current control method is proposed to attenuate low-order current harmonics in three-phase LCL-filtered grid-connected inverters. This method allows the use of a distorted reference current, thus the reference can be generated directly from grid voltage or by a simple phase-locked loop (PLL). It is found that the direct control is necessary for effectively attenuating the current harmonics caused by the distortion in the grid voltage. Active damping is achieved with an inner inverter-current feedback loop, which is found to be superior to the widely used capacitor-current feedback damping in improving system stability. A systematic controller design procedure is proposed to optimize system performance. Experimental results confirm the improved harmonic attenuation ability of the proposed method in comparison to that of conventional control methods.

The thermal and fluid dynamic disturbances caused by the insertion of a Langmuir probe into a flowing thermal plasma are examined. Attention is paid to a 5-mm-long, free burning 200-A argon plasma at atmospheric pressure. It has been found that the disturbances caused by the probe are so severe that probe measurements cannot refer directly to an undisturbed plasma. The difficulties in interpreting the probe voltage-current (V-I) characteristics are discussed. The use of a Langmuir probe as a general diagnostic tool for the plasma conditions investigated is not yet a practical proposition.

In circuit breakers, arc heaters and arc welding apparatus, metal vapor resulting from electrode erosion is inevitably injected into the arc plasma. The arc then burns in a mixture of the working gas (SF/sub 6/ in the case of circuit breakers) and electrode vapor, whose properties are substantially different from those of pure SF/sub 6/. The present work is a computational investigation into the effects of electrode vapor on the behavior of a supersonic nozzle arc under dc conditions. The arc and electrodes are treated as a coupled system. The arc plasma is assumed to be in local thermodynamic equilibrium (LTE). The thin non-LTE layer between the electrode and the arc is not included in the computation but its effects are taken into account in the energy balance at the electrode surface. A conservation equation for the electrode vapor is solved together with the governing equations for mass, momentum, and energy of the gas mixture. Comparisons were made between the results with and without electrode vaporization. Results show that vaporization of the electrode material (copper in the present investigation) has a cooling effect at the arc center and broadens the arc column. The arc voltage is, however, not sensitive to the presence of electrode vapor for the gas discharge conditions investigated.

We have designed a low-cost and reliable 2.45-GHz waveguide-based applicator to generate a microwave plasma jet (MPJ) at atmospheric pressure. The MPJ system consists of a 1-6 kW magnetron power supply, a circulator, a water-cooled matched load and the applicator. The applicator includes a tuning section, which is required to reduce the reflected power and the nozzle section. The plasma is formed by the interaction of the high electrical field, generated by the microwave power, between the waveguide aperture and the gas nozzle. A variety of gasses have been used to produce the plasma including argon, helium, and nitrogen. A 2-kW 2.45-GHz MPJ constructed using a rectangular waveguide WG9A (WR340) has been investigated. An MPJ has been used for material processing applications including cutting, welding, glass vitrification, and quartz/ceramic processing. This paper discusses the design parameters and the potential of the MPJ for industrial applications and how the jet can be tailored to suit different tasks, by adjusting the various parameters such as the type of gas, the flow rate, the input power, and the nozzle design.

Supersonic nozzles are commonly used in modern high voltage circuit breakers. The thermodynamic and electrical behavior of arcs burning in such nozzles is closely related to the nozzle geometry and the pressures at the nozzle inlet and exit. Computational investigation has been carried out on the thermal interruption capability of a supersonic nozzle at different inlet stagnation pressures and at different exit pressure to inlet stagnation pressure ratios (referred to as pressure ratios). The critical rate of rise of recovery voltage (RRRV), which is the most important parameter determining the thermal interruption capability of a circuit breaker, was predicted for five pressure ratios at three inlet stagnation pressures. The predicted RRRV values are compared with the test results of Campbell et al. (1976).

The transport properties including electron diffusion coefficient, viscosity, thermal conductivity, and electrical conductivity of CF 4 , C 2 F 6 , and C 3 F 8 plasmas are obtained by using the Chapman-Enskog method expanded up to the third-order approximation (second-order for viscosity). The most recent data on potential interactions and elastic differential cross sections for interacting particles were utilized. The calculations, which assume local thermodynamic equilibrium, are performed at atmospheric pressure in the temperature range from 300 to 30 000 K. Our results can provide reliable reference data for the further simulation to study the relevant plasmas behavior.

Intelligent simulation tools are developed within the Integrated Simulation and Evaluation Environment using Visual Studio C++ and object oriented programming. Such an environment should result in huge savings in time and cost for switchgear manufacturers. Conventional k-epsilon turbulence model has been modified to take into account of steep radial temperature gradient at the arc edge with much improved results. Investigations of computational convergence and the effects of initial conditions show that solutions independent of initial arc conditions with sufficient accuracy can be achieved with the Liverpool arc model.