Clay Minerals Channels Identification in the Tindikala-Boutou Area (Eastern-Cameroon) along the Kadey River Using Direct Current (DC) Method

To achieve the current study, geoelectrical surveys along six (06) profiles of 4 km long in a 100 m × 200 m grid defined according to the triangulation principle in the Tindikala-Boutou (Eastern-Cameroon) area along the Kadey River have been made through electrical sounding and profiling following Schlumberger array. The instrument is the resistive meter Syscal Junior 48 (IRIS Instrument). The data have been processed and modelled with Res2Dinv and Winsev softwares, and then interpolated with Surfer software. Investigation method used is the Direct Current (DC) method. Interpretations and analyses of results from the investigation method highlight weak zones or conductive discontinuities. The latter has been identified as shear zones within granitic structures of the Precambrian basement, according to the geologic and tectonic background of the area. The structural trend of these shear zones is E-W approximately. The mineralization characterized by conductive zones proves the presence of clay minerals disseminated in weathered quartz vein, which cross the shear zones. The intense activities of gold washers encountered in the studied area are able to attest the presence of clay minerals concentrations.

Finite element numerical simulation of 2.5D direct current method based on mesh refinement and recoarsement

To deal with the problem of low computational precision at the nodes near the source and satisfy the requirements for computational efficiency in inversion imaging and finite-element numerical simulations of the direct current method, we propose a new mesh refinement and recoarsement method for a two-dimensional point source. We introduce the mesh refinement and mesh recoarsement into the traditional structured mesh subdivision. By refining the horizontal grids, the singularity owing to the point source is minimized and the topography is simulated. By recoarsening the horizontal grids, the number of grid cells is reduced significantly and computational efficiency is improved. Model tests show that the proposed method solves the singularity problem and reduces the number of grid cells by 80% compared to the uniform grid refinement.

模块化高压大容量DC/DC变换器综述

随着新能源发电占比增加和电网规模的不断扩大,直流输电系统的容量不断提升,含有不同电压等级的直流电网是未来发展的必然趋势。在此背景下,DC/DC变换器作为直流电网关键设备,在多电压等级互联场景中发挥着重要作用。文中首先对已有的模块化高压大容量DC/DC变换器进行归纳、分类和比较,然后阐述了目前模块化高压大容量DC/DC变换器的拓扑结构、工作原理与特点,并针对其特点提出了不同变换器的适用场景,最后对未来的研究方向提出了建议。

基于双重移相控制的双向全桥DC-DC变换器最小回流功率分段优化控制

双向全桥DC-DC变换器以其重量轻、高功率密度、能量双向流动等优点成为直流微电网中不可或缺的一部分。但双向全桥DC-DC变换器在传统的单重移相控制下存在回流功率和电流应力等问题,尤其当输入电压和输出电压不匹配时,回流功率和电流应力会显著增加。针对输入电压和输出电压不匹配的情况,该文提出了一种双向全桥DC-DC变换器在双重移相下的最小回流功率控制策略。该控制策略通过对回流功率进行分段优化,得到了变换器在不同传输功率范围下的最优移相角。通过将所提控制策略和传统的双重移相控制进行对比分析,发现该文所提控制策略具有更小的回流功率和电流应力,提升了变换器的效率。最后基于所提控制策略搭建了实验样机,实验结果验证了控制策略的正确性和有效性。

Detecting and monitoring of water inrush in tunnels and coal mines using direct current resistivity method:A review

Detecting, real-time monitoring and early warning of underground water-bearing structures are critically important issues in prevention and mitigation of water inrush hazards in underground engineering. Direct current （DC） resistivity method is a widely used method for routine detection, advanced detection and real-time monitoring of water-bearing structures, due to its high sensitivity to groundwater. In this study, the DC resistivity method applied to underground engineering is reviewed and discussed, including the observation mode, multiple inversions, and real-time monitoring. It is shown that a priori information constrained inversion is desirable to reduce the non-uniqueness of inversion, with which the accuracy of detection can be significantly improved. The focused resistivity method is prospective for advanced detection;with this method, the flanking interference can be reduced and the detection dis-tance is increased subsequently. The time-lapse resistivity inversion method is suitable for the regions with continuous conductivity changes, and it can be used to monitor water inrush in those regions. Based on above-mentioned features of various methods in terms of benefits and limitations, we propose a three-dimensional （3D） induced polarization method characterized with multi-electrode array, and introduce it into tunnels and mines combining with real-time monitoring with time-lapse inversion and cross-hole resistivity method. At last, the prospective applications of DC resistivity method are discussed as follows： （1） available advanced detection technology and instrument in tunnel excavated by tunnel boring machine （TBM）, （2） high-resolution detection method in holes, （3） four-dimensional （4D） monitoring technology for water inrush sources, and （4） estimation of water volume in water-bearing structures.

Multiple linear system techniques for 3D finite element method modeling of direct current resistivity

The strategies that minimize the overall solution time of multiple linear systems in 3D finite element method (FEM) modeling of direct current (DC) resistivity were discussed. A global stiff matrix is assembled and stored in two parts separately. One part is associated with the volume integral and the other is associated with the subsurface boundary integral. The equivalent multiple linear systems with closer right-hand sides than the original systems were constructed. A recycling Krylov subspace technique was employed to solve the multiple linear systems. The solution of the seed system was used as an initial guess for the subsequent systems. The results of two numerical experiments show that the improved algorithm reduces the iterations and CPU time by almost 50%, compared with the classical preconditioned conjugate gradient method.

On-line Measurement for Ohmic Resistance in Direct Methanol Fuel Cell by Current Interruption Method

Electrochemical impedance spectroscopy (EIS) is widely used in fuel cell impedance analysis. However, for ohmic resistance (R Ω), EIS has some disadvantages such as long test period and complex data analysis with equivalent circuits. Therefore, the current interruption method is explored to measure the value of RΩ in direct methanol fuel cells (DMFC) at different temperatures and current densities. It is found that RΩ decreases as temperature increase, and decreases initially and then increases as current density increases. These results are consistent with those measured by the EIS technique. In most cases, the ohmic resistances with current interruption (R iR ) are larger than those with EIS (R EIS ), but the difference is small, in the range from –0.848% to 5.337%. The errors of R iR at high current densities are less than those of R EIS . Our results show that the R iR data are reliable and easy to obtain in the measurement of ohmic resistance in DMFC.

功率阶跃型LCC-HVDC频率支撑控制策略

随着大量常规机组被直流馈入和新能源发电替代,系统惯量水平下降,调频资源减少,频率稳定性问题日益突出。基于电网换相型换流器的直流输电(line-commutatedconverterbasedhigh-voltagedirect-current,LCC-HVDC)应用广泛,利用其功率快速调节能力为系统提供支撑是改善频率稳定性问题的有效手段。该文基于功率阶跃型频率控制的基本框架,考虑LCC的出力限制,以出现功率缺额后频率最低点最高为目标建立了LCC-HVDC系统的出力时序优化模型。基于序列二次规划法对优化问题进行求解,确定最佳的LCC附加功率与支撑时间。基于MATLAB/Simulink的仿真验证了所提频率支撑控制策略的有效性。

高效率DC/DC转换器在通信电源中的应用研究

文章旨在评估高效率直流/直流(Direct Current/Direct Current,DC/DC)转换器在通信电源中的应用,着重考察最小输出电压、效率、功耗以及瞬态响应等关键性能参数。通过深入研究TPS54160芯片,揭示在不同工作频率下该芯片的性能特点。最后,通过实验验证TPS54160芯片在通信电源中的性能表现,旨在为通信系统的电源设计提供参考和指导。

基于DCPD的CT试样裂纹扩展数值模拟标定方法

为保证核电结构裂纹扩展监测结果的准确性,建立有效的标定方法至关重要。该文根据直流电位降(DCPD)法的原理和有限元分析软件建立电位场模型,直观、定量地获得电位场变化数据,分析紧凑拉伸(CT)试样在不同裂纹长度下的电位降,并从监测环节考虑多种因素对标定曲线的影响,最后结合裂纹监测平台进行实验验证。结果表明,裂纹主电位接线点的位置、不同材料和几何尺寸对标定曲线产生微伏级的影响,仿真与实验数据分析结果的最大误差为4.9%。该标定曲线具有一定的可行性,可为后续动态裂纹扩展监测实验及验证工作提供参考。

基于功率补偿的DC-DC变换器电流应力控制方法

针对双有源桥DC-DC(dual active bridge,DAB)变换器中存在较大的电流应力现象,提出一种基于功率补偿的电流应力改进控制方法。通过建立DAB的电流应力、传输功率与相移量之间的函数关系,利用拉格朗日乘数法建立电流应力的优化数学模型,从而推导出最优占空比组合,降低电流应力至最小值。通过引入一个虚拟分量对传输功率进行实时补偿控制,以满足系统在输入、输出突变情况下的快速响应能力,提高DAB的动态特性。最后,通过搭建Matlab/Simulink仿真模型进行验证,通过仿真结果表明:基于功率补偿的电流应力优化控制方法的动态响应特性和ZVS特性明显优于传统控制方法,且在减小电流应力的同时也降低了回流功率,进一步验证了优化控制方法的正确性和优越性。

Evidence of Iron Mineralization Channels in the Messondo Area (Centre-Cameroon) Using Geoelectrical (DC &IP) Methods: A Case Study

A geophysical survey was conducted in the Kelle-Bidjocka village, Messondo subdivision, in the Centre Region, Cameroon. The data acquisition was made by combining Schlumberger profiling and electrical soundings along six (06) profiles of 1500 m in length for a total of 64 geoelectrical stations’ survey conducted through a variable mesh 100 m × 200 m, or 100 m × 300 m. The equipment used is the DC resistivimeter Syscal Junior 48 (Iris Instrument). Processing and modelling of field data are made by using the Res2Dinv, Qwseln and Surfer software. The investigation methods used are electrical resistivity (DC) and induced polarization (IP) methods. The analyses and interpretations have helped to highlight areas of weakness or conductive discontinuities (fractures, faults, shear zones, etc.) in Precambrian gneiss formations, sometimes undergoing weathering processes. They identify the weathering or mineralogical accumulation horizons, the most promising is a mineralization channel identified in the NE-SW direction. The highlighted mineralization is characterized by strong gradients of chargeability or polarization. Samples and other geological evidences observed in the area are used to associate the most polarizable structures with ferriferous formations. Weakly polarizable and particularly conductive backgrounds identified by the inverse pseudo-sections are thought to be sulphate minerals or groundwater targets for future hydrogeological studies.

A Fault Current Limiting Hybrid DC Circuit Breaker

Due to the low impedance characteristic of the high voltage direct current(HVDC)grid,the fault current rises extremely fast after a DC-side fault occurs,and this phenomenon seriously endangers the safety of the HVDC grid.In order to suppress the rising speed of the fault current and reduce the current interruption requirements of the main breaker(MB),a fault current limiting hybrid DC circuit breaker(FCL-HCB)has been proposed in this paper,and it has the capability of bidirectional fault current limiting and fault current interruption.After the occurrence of the overcurrent in the HVDC grid,the current limiting circuit(CLC)of FCL-HCB is put into operation immediately,and whether the protected line is cut off or resumed to normal operation is decided according to the fault detection result.Compared with the traditional hybrid DC circuit breaker(HCB),the required number of semiconductor switches and the peak value of fault current after fault occurs are greatly reduced by adopting the proposed device.Extensive simulations also verify the effectiveness of the proposed FCL-HCB.

Design and implementation of a position sensorless control method for brushless DC motors

Because brushless direct current(BLDC) motors have the advantages of a compact size, high power density, high efficiency, and long operating life time, they are widely used in many industrial products and electric traction systems. It is known that the BLDC motors have no brushes for commutation. They are commutated with electronically commutation. So, the rotor position information of the BLDC motors must be known to understand which winding will be energized according to the energizing sequence. In most of the existing BLDC motor drivers, rotor position information is detected by Hall effect sensors. This kind of mechanical position sensors will bring additional connections and costs, reliability decrease and noise increase. In order to improve the control performance and extend the range of speed regulation for BLDC motors, a position sensorless control method is proposed in this paper. In the proposed control method, rotor position information of the BLDC motors is detected from the back electromagnetic forces(back-EMFs) which are estimated by an unknown-input observer with line to line currents and line to line voltages. For the purpose of verifying the effectiveness of the proposed control method, a model is built and simulated on the Matlab/Simulink platform. The simulation results show that the speed regulation performance of BLDC motors is improved compared with using Hall effect sensors. At the same time, the reliability of the BLDC motors is improved and the costs of them are reduced because the position sensor is eliminated.

Gold Mineralization Channels Identification in the Tindikala-Boutou Area (Eastern-Cameroon) Using Geoelectrical (DC &IP) Methods: A Case Study

To achieve the current study, geoelectrical surveys along six (06) profiles of 4 km long in a 100 m × 200 m grid defined according to the triangulation principle in the Ngoura area (Tindikala-Boutou villages) have been made through electrical sounding and profiling following Schlumberger array. The instrument is the resistivimeter Syscal Junior 48 (IRIS Instrument) which uses the electrical current. The data have been processed and modelled with Res2Dinv and Winsev softwares then interpolated with Surfer software. Electrical methods used are the Direct current (DC) and the Induced Polarization (IP). Interpretation and analyses of results from each investigation method highlight weak zones or conductive discontinuities and mineralized zones. Conductive zones have been identified as shear zones within granitic structures of the Precambrian basement, according to the geologic and tectonic background of the area. The structural trend of these shear zones is E-W. The mineralization within it is N-S and characterized by high values of chargeability, essentially in the eastern part of the area under study. This mineralization proves the presence of metalliferous or sulphide heaps disseminated in weathered quartz veins which cross shear zones. Also, the poor mineralization and conductive structures in shear zones characterize the groundwater zones. The intense activities of gold washers encountered in the area enable to link that mineralization to gold within quartz veins. The near surface gold mineralization is eluvial or alluvial, and in depth this mineralization is primary.

双有源桥DC-DC变换器线性自抗扰控制

针对双有源桥DC-DC变换器单重移相控制电流应力大,采用传统PI控制动态特性差等问题,为同时优化变换器电流应力和动态特性,在扩展移相控制下,提出一种线性自抗扰控制结合直接功率控制的电流应力优化方案。通过建立扩展移相各个状态下的数学模型,推导出电流应力的全局最优解,将线性自抗扰控制器引入电压环,与直接功率控制相结合,输出传输功率标幺值至电流应力优化算法中,计算对应移相角。仿真和实验结果表明:新优化控制策略能在减小电流应力的同时提高变换器的动态响应能力。

Analysis on the Magnetic Shielding Effectiveness of DC Current Comparator

There are magnetic interference problems in the applications of DC current comparator. Analysis on the magnetic effectiveness which is applied by the external magnetic field is introduced in this paper. The effectiveness is proved by the actual results which are compared with the magnetic- circuit method and the finite element method. In addition, the reference comment is given which can be used in the practical work of DC current comparator shield design.

Combined Geoelectrical Approach DC and IP Methods in the Identification of the Mineralized Bodies Parallel to the NE-SW Tectonic Line of Kadei River: Case of Quartz or Pegmatite Gold Bearing Veins of Ngoura Subdivision (East Cameroon)

A geophysical investigation of subsurface structures using the Syscal Junior 48 resistivity-meter was conducted in Ngoura subdivision (East Cameroon) following a combined geoelectrical direct current (DC) approach involving Resistivity and IP methods. This investigation was allowed to collect data on forty-five (45) profiling lines at three acquisition levels (AB = 100 m, MN = 10 m;AB = 200 m, MN = 20 m and AB = 500 m, MN = 50 m) and two electric panels L1 and L4, using the Schlumberger array. Processing, modeling and interpretation of data collected using the Winsev, Res2Dinv and Surfer software helped in highlighting a conductive and strongly mineralized discontinuity in granite formations, which lined up with the NE-SW Kadei tectonic line. It extends beyond 100 m depth over an average width of 600 m. The mineralization associated with this discontinuity is identified by a high concentration of disseminated metalliferous minerals in quartz or pegmatite veins. The mining reconnaissance works in the study area and those of several authors have characterized this anomaly to a lode gold quartz or large pegmatite. The results of this study correlate with geological and tectonic data for the region marked by NE-SW Kadei tectonic line. Therefore, they confirm the reliability of a geoelectrical DC investigation method combining Resistivity and IP to the identification of ore bodies.