ZERO-CURRENT AND ZERO-VOLTAGE SWITCHING PWM BOOST FULL-BRIDGE CON VERTER

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.

Zero-sequence Circulating Current Reduction for Three-phase Three-level Modular Photovoltaic Grid-connected Systems

针对共享正、负母线和交流母线的三相三电平模块化光伏并网系统的零序环流问题,建立三电平模块化系统零序环流的等效模型,并根据激励源的不同将零序环流分成3类,提出一种共享正、负母线和中线的并联方案来抑制I类零序环流,及一种基于改进型LCL滤波器的并联方案来抑制II、III类零序环流的高频分量,和采用零序环流控制器来抑制II、III类零序环流的低频分量。最后,通过10 kW的三相三电平模块化光伏并网系统的仿真和实验,证明了零序环流模型分析和抑制方法的正确性。

Vacuum Dielectric Recovery Characteristics of a Novel Current Limiting Circuit Breaker Base on Artificial Current Zero

为了保证新型强迫换流型真空直流限流断路器关断短路电流的可靠性，对该型断路器分断过程的真空介质恢复特性进行研究。设计了与断路器关断过程等效的介质恢复试验方案，通过等效试验结果和理论推演公式的拟合，得到了新型强迫换流型限流断路器真空灭弧室触头打开过程的动态介质强度恢复规律。研究结果表明：减小燃弧能量、提高触头运动速度可提高真空灭弧室介质的临界击穿电压；综合考虑燃弧时间与燃弧能量及触头开距的关系，随着燃弧时间的增加，真空灭弧室临界击穿电压先减小后增大。所得介质恢复规律可以作为新型断路器优化设计的参考依据。

Application of fractal theory in detecting low current faults of power distribution system in coal mines

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.

Residual current compensation for single-phase grounding fault in coal mine power network

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.

Cross correlation mediated by distant Majorana zero modes with no overlap

Existing studies via shot noise calculation conclude that the cross correlation between the currents in the two leads connected by a pair of Majorana zero modes(MZMs)vanishes when their coupling energy𝜖∈_(M)→0.Motivated by the intrinsic nature of nonlocality of the MZMs,we revisit this important problem and propose an experimental scheme to demonstrate the nonvanishing cross correlation even at the limit𝜖∈_(M)→0.The proposed scheme employs the Andreevprocess-associated branch circuit currents,which are theoretically obtained by applying a decomposition analysis for the total currents while are accessible directly in practical measurement.For different bias voltage setup,we find intriguing results of both negative and positive correlations and carry out simple physical understanding using a quantum jump technique.Importantly,combining together with the evidence of the zero-bias-peak of conductance,the nonlocal cross correlation predicted in this work can help to confirm the existence of the nonlocal MZMs.

A Fermi Energy-Incorporated Framework for Dealing with the Temperature- and Magnetic Field-Dependent Critical Current Densities of Superconductors and Its Application to Bi-2212

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.

Electrical Characteristics of an Alternating Current Plasma Igniter in Airflow

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.

柔性低频输电系统的主变稳态负序差动保护

为了解决常规纵差保护难以适用于低频系统的问题,通过分析低频系统的序分量抑制策略,并根据低频系统故障进入稳态后单侧有负序注入的特征,提出一种基于稳态负序分量的主变差动保护方法。为防止该差动保护在区外故障时因电流互感器饱和出现误动,通过对比分析三类制动电流计算方式的制动性能,得到了差动保护的制动方程和制动系数。为解决励磁涌流、电流互感器断线可能导致误动的问题,提出了相电压突变量的差动保护开放条件。仿真分析表明,该方法能够灵敏反映区内金属性故障及轻微故障,在区外故障伴随电流互感器饱和时有可靠的制动裕度。

Interaction of Streamwise and Wall-Normal Velocities in Combined Wave-Current Motion

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.

考虑铁心结构的三相变压器励磁涌流和角型绕组环流分析

基于三相组式、五柱式和三柱式铁心结构变压器的磁路特征和运行方程,首先,提出运用星形侧零序电流和电压来拟合角型绕组环流的数值方法,并进一步研究不同铁心结构变压器的角型绕组环流和励磁涌流的特点;然后,进行软件仿真和现场录波以验证该数值方法的有效性,可判断三相组式和五柱式变压器的各相绕组差流即为对应相的励磁涌流,而三柱式变压器某相的绕组差流并不是对应相的励磁电流;最后,进一步的理论分析与仿真验证表明,在3类铁心结构的变压器从星形侧空载合闸时,星形侧任意两相的线电流之差可以认为是这两相等效磁路的励磁涌流之差。研究结论可以为不同铁心结构三相变压器的励磁涌流识别提供依据。

基于主动零序电流注入的新型漏电流控制方法

非隔离型光伏并网系统因缺少隔离级,导致漏电流流入交流电网,引起并网电流畸变等问题。针对此问题,提出一种基于主动零序回路的非隔离型并网变换器拓扑,并提出了一种基于主动零序电流注入的新型漏电流控制方法,该方案可以低成本、高效率的实现漏电流抑制、共模电压稳定等功能。最后使用StarSim设备搭建实验平台,验证了所提拓扑结构和所提控制方法对漏电流抑制和共模电压稳定的有效性。

高压SF_(6)断路器电流零区测量研究综述

电流零区测量对标定评价断路器开断性能、改进断路器结构设计至关重要,设计一套高精度、便携、可靠、安全的零区测量系统一直是行业的追求,国内外研究者在测量方法和测量装置研制方面做了很多创新性工作。从高压断路器灭弧原理出发,梳理提出了零区测量的难点和关键点,综述了国内外研究者提出的测量方案及效果。综合考虑多维度产业需求,研制了一套便携性和动态特性好的零区测量系统,并在实际断路器产品试验中进行了验证。结果表明:该系统抗干扰性和重复性好,能够满足产品研发需求。结合零区测量系统研究及使用经验,提出了零区测量系统的未来展望,为后续交直流开关的研发提供参考。

基于谐振变换器的串联储能单元均压控制策略

储能系统中通常通过串联多个储能单体得到更高的电压,但由于每个储能单体电压的不一致性会降低储能单元的利用率。针对以上问题,提出一种基于谐振变换器的串联储能单元均压控制策略。该均衡电路通过开关阵列与谐振变换器相互配合,实现任意单体之间和任意多单体之间的能量传递,且这两种能量传递模式均实现了零电流开关,减少了电路损耗。为获得最优均衡速度和均衡效率,采用K-Means聚类复合控制策略,优先对分组单元进行均衡,然后再进行单体间的均衡。最后,在Matlab/Simulink平台上对6个串联储能单体进行均衡仿真实验,实验结果表明,在静置均衡实验下,电压在24 min左右达到均衡,且均衡精度达到5 mV,均衡效率达到98.98%。与传统均压方法相比,该方法具有更快的均衡速度和更高的均衡效率。

基于变频重构S/SP拓扑的无线电能传输系统恒流恒压研究

针对电动汽车无线充电系统耦合系数和负载变化导致系统输出电流和电压剧烈波动的问题,提出一种由交错叠放式线圈结构和变频重构控制策略组成的无线电能传输系统,通过重构补偿拓扑,并切换工作频率,实现了系统的恒流恒压输出。首先,分析了拓扑重构前后系统的恒流、恒压输出特性和零相角特性,对比恒流恒压工作模式下补偿电容的大小,给出了系统参数调整方法。其次,剖析了交错叠放式线圈结构模型,提出基于耦合系数波动最小原则的线圈结构优化策略。所提出的系统补偿元件较少,两个工作频率点符合国际标准。最后,搭建一套500 W的实验样机,验证系统的可行性和有效性。实验结果表明,交错叠放式线圈在X轴方向偏移发射线圈外径的55%(187mm)或Y轴方向偏移120mm时,耦合系数波动率小于5%。系统在恒流模式下电流波动率为3.58%,在恒压模式下电压波动率为4.83%,满足无线充电系统恒流恒压需求。

一种低电流纹波高增益软开关直流变换器

提出了一种应用于新能源发电系统的低电流纹波高增益软开关直流变换器。在传统交错Boost变换器基础上,该变换器通过引入耦合电感和二极管、电容升压单元提高电压增益,耦合电感在整个开关周期过程中都传递能量,提高了磁芯利用率。输入侧工作在交错模式,两相电感电流纹波可以相互抵消,从而获得较低的输入电流纹波。由于耦合电感自身漏感的存在,减轻了整流二极管反向恢复问题,同时采用有源钳位电路回收利用漏感能量,实现了所有开关管零电压软开关,抑制了开关管关断电压尖峰,提高了变换器转换效率。详细分析了变换器的工作原理、电路特性以及软开关实现方法。最后,搭建了一台200 W的试验样机验证了理论分析的正确性。

限制调频范围的不对称多模式宽输出LLC变换器

频率控制的传统LLC谐振变换器往往受限于开关频率的有效调节范围,难以实现宽输出电压范围,为此,研究了一种限制调频范围的不对称多模式宽输出LLC谐振变换器。采用双谐振腔且对应两变压器变比不同的不对称结构,能够根据原边开关组合的不同,使得双谐振腔分别工作在单半桥、双半桥和半桥+全桥3种不同的模式,从而获得3种不同的电压增益,并且保证每种模式之间归一化增益调节范围不超过1.5,可以在窄开关频率范围内实现宽输出电压范围。建立300 W的实验样机,验证了所提变换器可实现1~3倍的宽输出电压范围,并且实现了原边开关管的零电压开通和副边二极管的零电流关断,具有良好的软开关性能,验证了变换器的可行性。

基于低频暂态零序电流畸变率的灵活接地系统高阻接地保护

灵活接地系统中,基于稳态故障特征构成的单相接地保护普遍存在耐过渡电阻能力低的问题,并联小电阻投入后该问题进一步突出。文中研究了并联小电阻投入后故障线路与非故障线路低频暂态零序电流中暂态直流分量比值关系,发现暂态直流分量比值仅与对地电容有关。分析了故障线路与非故障线路低频暂态零序电流畸变率之间比值关系,进一步提出了一种基于低频暂态零序电流畸变率的灵活接地系统单相高阻接地故障选线方法。通过比较特定时间窗口内各馈线低频暂态零序电流畸变率的大小实现故障选线,并可准确区分母线故障与出线故障。该方法能够避免零序电压互感器断线与零序电流互感器极性反接的影响,且受故障距离影响小。同时,该方法采用暂态故障特征构成选线判据,有利于克服并联小电阻投入后稳态故障分量微弱的问题,对互感器精度要求不高。最后,通过实时数字仿真和实验平台分别验证了所述方法的准确性和有效性。

二次侧附加电容磁复位正激变换器电气性能分析及参数设计

由于二次侧附加电容磁复位正激变换器具有结构简单等优点,因此有着较好的应用前景,但目前尚未见附加电容对电气性能影响及其设计方法的研究报道。为此,该文通过深入分析附加电容C_(b)对变换器工作模式和能量传输过程的影响,指出变换器存在四种组合工作模式,且励磁电感断续导通模式(DCM)/正激电感连续导通模式(CCM)最佳。C_(b)不仅能将励磁能量传输到负载,还可调节开关管电压应力,在负载不太大时实现续流二极管零电流关断(ZCS)和开关管低电压导通。通过分析C_(b)对变换器电气性能的影响,指出开关管电压应力随C_(b)增大而降低,开关管低电压导通性能随C_(b)和正激电感的减小而提升,并提出了一种确保变换器工作于最佳模式且可最大程度提升电气性能的参数设计方法。实例和实验结果验证了理论分析的正确性和设计方法的可行性。

基于DAB的蓄电池最小电流应力控制策略研究

双有源桥(DAB)变换器是一种适用于蓄电池充电的双向DC/DC变换器。为减小蓄电池充电过程中的功率损耗并提高传输效率,提出了双重移相(DPS)最小电流应力控制策略,分析DAB变换器全部开关管的零电压开通(ZVS)的电感电流约束条件,求解得到ZVS区域内电流应力最小时的移相比组合,能够减小蓄电池的充电损耗,提高充电效率,并通过理论对比验证了优化效果。仿真与实验结果均表明,所提优化策略可以实现开关管ZVS以及最小电流应力,从而减小损耗,提高变换器的性能。