This paper proposes a zer o current and zero voltage switching (ZCZVS) PWM Boost full bridge (FB) conve rter. With series inductors, the leading switches can realize zero current swit ching (ZCS) in a wide load ra...This paper proposes a zer o current and zero voltage switching (ZCZVS) PWM Boost full bridge (FB) conve rter. With series inductors, the leading switches can realize zero current swit ching (ZCS) in a wide load range using the energy of the output capacitor. Ma king use of parasitic capacitors of the lagging switches and parallel auxiliary i nductance with the primary winding of the transformer, the lagging switches can realize zero voltage switching (ZVS) under any load. Compared with the ZCS PWM Boost FB converter, the new converter has no current duty cycle loss. Operat ional principle and parameter design are analyzed. Experimental results verify the effectiveness of the proposed converter.展开更多
Single-phase low current grounding faults areoften seen in power distribution system of coal mines.These faults are difficult to reliably identify.We propose a new method of single-phase ground fault protection based ...Single-phase low current grounding faults areoften seen in power distribution system of coal mines.These faults are difficult to reliably identify.We propose a new method of single-phase ground fault protection based upon a discernible matrix of the fractal dimension associated with line currents.The method builds on existing selective protection methods.Faulted feeders are distinguished using differences in the zero-sequence transient current fractal dimension.The current signals were first processed through a fast Fourier transform and then the characteristics of a faulted line were identified using a discernible matrix.The method of calculation is illustrated.The results show that the method involves simple calculations, is easy to do and is highly accurate.It is, therefore, suitable for distribution networks having different neutral grounding modes.展开更多
The way of neutral point to earth via full compensation arc suppression coil can solve the problem of residual current compensation in coal mine power network effectively. Based on the analysis on the grounding curren...The way of neutral point to earth via full compensation arc suppression coil can solve the problem of residual current compensation in coal mine power network effectively. Based on the analysis on the grounding current detection results of Xieqiao coal mine, the conclusion that harmonic component of grounding current is dominated by higher harmonics with complex harmonic sources in coal mine power network system was obtained. The influences of harmonic source type and fault point position on harmonic voltage and harmonic current were analyzed theoretically. The influences of earthed fault feeder detection result and the estimation errors of parameters to earth on residual current compensation were analyzed. A new thought of residual current prediction and the selections of model method and control method were proposed on this basis. The simulation results prove that harmonic amplitudes of zero sequence voltage and zero sequence current are determined by harmonic source type as well as fault point position in coal mine power network, and also prove that zero sequence voltage detection can avoid the unstable problem of coal mine power network system caused by undercompensation of capacitive current. Finally, the experimental device of full compensation arc suppression coil is introduced.展开更多
The electrical characteristics of an alternating current （AC） plasma igniter were investigated for a working gas of air at atmospheric pressure. The discharge voltage and current were measured in air in both breakdo...The electrical characteristics of an alternating current （AC） plasma igniter were investigated for a working gas of air at atmospheric pressure. The discharge voltage and current were measured in air in both breakdown and stable combustion processes, respectively, and the current-zero phenomena, voltage-current （V-I） characteristics were studied for different working gas flow rates. The results indicated that the working gas between anode and cathode could be ionized to generate gas discharge when the voltage reached 8 kV, and the maximum current was 33.36 A. When the current came to zero, current-zero phenomena appeared with duration of 2 #s. At the current-zero moment, dynamic resistance between electrodes became extremely high, and the maximum value could reach 445 kf~, which was the main factor to restrain the current. With increasing working gas flow rates, the gradient of V-I characteristic curves was increased, as was the dynamic resistance. At a constant driven power, the discharge voltage increased.展开更多
It is well known that the critical current density of a superconductor depends on its size, shape, nature of doping and the manner of preparation. It is suggested here that the collective effect of such differences fo...It is well known that the critical current density of a superconductor depends on its size, shape, nature of doping and the manner of preparation. It is suggested here that the collective effect of such differences for different samples of the same superconductor is to endow them with different values of the Fermi energy—a single property to which may be attributed the observed variation in their critical current densities. The study reported here extends our earlier work concerned with the generalized BCS equations [Malik, G.P. (2010) Physica B, 405, 3475-3481;Malik, G.P. (2013) WJCMP, 3,103-110]. We develop here for the first time a framework of microscopic equations that incorporates all of the following parameters of a superconductor: temperature, momentum of Cooper pairs, Fermi energy, applied magnetic field and critical current density. As an application of this framework, we address the different values of critical current densities of Bi-2212 for non-zero values of temperature and applied magnetic field that have been reported in the literature.展开更多
零序电流互感器(current transformer,CT)测量精度问题一直是限制配电网故障选线准确率的关键性问题,现有研究方法在理论层面已较为成熟,但在现场使用中受限于零序CT测量精度,耐过渡电阻能力往往不佳。灵活接地系统是一种新型接地方式,...零序电流互感器(current transformer,CT)测量精度问题一直是限制配电网故障选线准确率的关键性问题,现有研究方法在理论层面已较为成熟,但在现场使用中受限于零序CT测量精度,耐过渡电阻能力往往不佳。灵活接地系统是一种新型接地方式,系统感知永久性接地故障后,于中性点投入并联小电阻,使得配电网接地方式发生转变,从而导致同一电气信号量呈现两种完全不同的故障特征,故障信息量增加了一倍,综合利用两阶段故障特征有望突破现有故障选线装置性能极限,摆脱选线装置对零序CT测量精度的依赖。为此,该文首先分析了灵活接地系统不同阶段各电气量的故障特征,之后结合灵活接地系统特点提出了一种基于零序电流幅值比的高阻接地故障选线方法,最后通过合理设计选线算法,剔除误差较大数据,最大程度降低保护对零序CT测量精度的依赖。实时数字仿真系统(real time digital simulation system,RTDS)以及低压实验平台实验结果表明,该方法不受零序CT极性反接与相位测量误差影响,且无需故障信息间的横向比较,具有耐过渡电阻能力强、可靠性高、对零序CT测量精度要求低、易实现等优点。展开更多
The aim of this paper is to present an analytical expression for the streamwise velocity distribution in a non-uniform flow in the presence of waves; the correlation between the horizontal and vertical velocity compon...The aim of this paper is to present an analytical expression for the streamwise velocity distribution in a non-uniform flow in the presence of waves; the correlation between the horizontal and vertical velocity components has been compreheusively examined. Different from previous researches which attributed the deviation of velocity from the classical log-law to the wave Reynolds stress, i.e. - ρ uv^- only, this study demonstrates that the momentum flux caused by mean velocities, i.e., u^- and v^-, is also responsible for the velocity deviation, and it is found that the streamwise velocity for a flow in the presence of non-zero wall-normal velocity does not follow the classical log-law, but the modified log-law proposed in this study based on simplified mixing-length theorem. The validity of the modified log-law has been verified by use of available experimental data from published sources for combined wave-current flows, and good agreement between the predicted and observed velocity profiles has been achieved.展开更多
文摘This paper proposes a zer o current and zero voltage switching (ZCZVS) PWM Boost full bridge (FB) conve rter. With series inductors, the leading switches can realize zero current swit ching (ZCS) in a wide load range using the energy of the output capacitor. Ma king use of parasitic capacitors of the lagging switches and parallel auxiliary i nductance with the primary winding of the transformer, the lagging switches can realize zero voltage switching (ZVS) under any load. Compared with the ZCS PWM Boost FB converter, the new converter has no current duty cycle loss. Operat ional principle and parameter design are analyzed. Experimental results verify the effectiveness of the proposed converter.
基金Project 50504015 supported by the National Natural Science Foundation of Chinathe Youth Science and Technology Research Program of China University of Mining and Technology (0C060996)
文摘Single-phase low current grounding faults areoften seen in power distribution system of coal mines.These faults are difficult to reliably identify.We propose a new method of single-phase ground fault protection based upon a discernible matrix of the fractal dimension associated with line currents.The method builds on existing selective protection methods.Faulted feeders are distinguished using differences in the zero-sequence transient current fractal dimension.The current signals were first processed through a fast Fourier transform and then the characteristics of a faulted line were identified using a discernible matrix.The method of calculation is illustrated.The results show that the method involves simple calculations, is easy to do and is highly accurate.It is, therefore, suitable for distribution networks having different neutral grounding modes.
基金The financial support from the National Natural Science Foundation of China (No. 51107143)
文摘The way of neutral point to earth via full compensation arc suppression coil can solve the problem of residual current compensation in coal mine power network effectively. Based on the analysis on the grounding current detection results of Xieqiao coal mine, the conclusion that harmonic component of grounding current is dominated by higher harmonics with complex harmonic sources in coal mine power network system was obtained. The influences of harmonic source type and fault point position on harmonic voltage and harmonic current were analyzed theoretically. The influences of earthed fault feeder detection result and the estimation errors of parameters to earth on residual current compensation were analyzed. A new thought of residual current prediction and the selections of model method and control method were proposed on this basis. The simulation results prove that harmonic amplitudes of zero sequence voltage and zero sequence current are determined by harmonic source type as well as fault point position in coal mine power network, and also prove that zero sequence voltage detection can avoid the unstable problem of coal mine power network system caused by undercompensation of capacitive current. Finally, the experimental device of full compensation arc suppression coil is introduced.
基金supported by National Natural Science Foundation of China(Nos.50776100,51106179)
文摘The electrical characteristics of an alternating current （AC） plasma igniter were investigated for a working gas of air at atmospheric pressure. The discharge voltage and current were measured in air in both breakdown and stable combustion processes, respectively, and the current-zero phenomena, voltage-current （V-I） characteristics were studied for different working gas flow rates. The results indicated that the working gas between anode and cathode could be ionized to generate gas discharge when the voltage reached 8 kV, and the maximum current was 33.36 A. When the current came to zero, current-zero phenomena appeared with duration of 2 #s. At the current-zero moment, dynamic resistance between electrodes became extremely high, and the maximum value could reach 445 kf~, which was the main factor to restrain the current. With increasing working gas flow rates, the gradient of V-I characteristic curves was increased, as was the dynamic resistance. At a constant driven power, the discharge voltage increased.
文摘It is well known that the critical current density of a superconductor depends on its size, shape, nature of doping and the manner of preparation. It is suggested here that the collective effect of such differences for different samples of the same superconductor is to endow them with different values of the Fermi energy—a single property to which may be attributed the observed variation in their critical current densities. The study reported here extends our earlier work concerned with the generalized BCS equations [Malik, G.P. (2010) Physica B, 405, 3475-3481;Malik, G.P. (2013) WJCMP, 3,103-110]. We develop here for the first time a framework of microscopic equations that incorporates all of the following parameters of a superconductor: temperature, momentum of Cooper pairs, Fermi energy, applied magnetic field and critical current density. As an application of this framework, we address the different values of critical current densities of Bi-2212 for non-zero values of temperature and applied magnetic field that have been reported in the literature.
文摘零序电流互感器(current transformer,CT)测量精度问题一直是限制配电网故障选线准确率的关键性问题,现有研究方法在理论层面已较为成熟,但在现场使用中受限于零序CT测量精度,耐过渡电阻能力往往不佳。灵活接地系统是一种新型接地方式,系统感知永久性接地故障后,于中性点投入并联小电阻,使得配电网接地方式发生转变,从而导致同一电气信号量呈现两种完全不同的故障特征,故障信息量增加了一倍,综合利用两阶段故障特征有望突破现有故障选线装置性能极限,摆脱选线装置对零序CT测量精度的依赖。为此,该文首先分析了灵活接地系统不同阶段各电气量的故障特征,之后结合灵活接地系统特点提出了一种基于零序电流幅值比的高阻接地故障选线方法,最后通过合理设计选线算法,剔除误差较大数据,最大程度降低保护对零序CT测量精度的依赖。实时数字仿真系统(real time digital simulation system,RTDS)以及低压实验平台实验结果表明,该方法不受零序CT极性反接与相位测量误差影响,且无需故障信息间的横向比较,具有耐过渡电阻能力强、可靠性高、对零序CT测量精度要求低、易实现等优点。
文摘The aim of this paper is to present an analytical expression for the streamwise velocity distribution in a non-uniform flow in the presence of waves; the correlation between the horizontal and vertical velocity components has been compreheusively examined. Different from previous researches which attributed the deviation of velocity from the classical log-law to the wave Reynolds stress, i.e. - ρ uv^- only, this study demonstrates that the momentum flux caused by mean velocities, i.e., u^- and v^-, is also responsible for the velocity deviation, and it is found that the streamwise velocity for a flow in the presence of non-zero wall-normal velocity does not follow the classical log-law, but the modified log-law proposed in this study based on simplified mixing-length theorem. The validity of the modified log-law has been verified by use of available experimental data from published sources for combined wave-current flows, and good agreement between the predicted and observed velocity profiles has been achieved.
