Neutral Point Shift Research Articles

Heteroepitaxial α‐Ga2O3 Thin Film‐Based X‐Ray Detector with Metal–Semiconductor–Metal Structure

This study explores the potential of 700‐nm‐thick heteroepitaxial α‐Ga2O3 thin films on c‐plane sapphire substrates for X‐ray detector applications. The crystal quality and optical bandgap of the heteroepitaxial α‐Ga2O3 thin films are comparable to those of high‐quality α‐Ga2O3 thin films. The α‐Ga2O3 thin film X‐ray detector with a metal–semiconductor–metal structure exhibits a charge neutral point shift, resulting in a short‐circuit current density of 9.07 nA cm−2 and an open‐circuit voltage of –1.2 V. The detector achieves the highest signal‐to‐noise ratio of 973 at 0 V, while the maximum sensitivity (14.7 μC Gyair−1 cm−2) occurs at 10 V. The proposed X‐ray detector demonstrates a reliable transient response and long‐term robustness, suggesting the promise of heteroepitaxial α‐Ga2O3 for low‐cost, high‐quality, large‐area X‐ray detectors.

The role of graphene patterning in field-effect transistor sensors to detect the tau protein for Alzheimer's disease: Simplifying the immobilization process and improving the performance of graphene-based immunosensors

We report the improvement in the sensing performance of electrolyte-gated graphene field-effect transistor (FET) sensors capable of detecting tau protein through a simplified, linker-free, anti-tau antibody immobilization process. For most of the graphene-based immunosensor, linkers, such as pyrenebutanoic acid, succinimidyl ester (PSE) must be used to the graphene surface, while the other side of linkers serves to capture the antibodies that can specifically interact with the target biomarker. In this study, graphene was patterned into eight different types and linker-free patterned graphene FET sensors were fabricated to verify their detection performance. The linker-free antibody immobilization to patterned graphene exhibited that the antibody was immobilized to the edge defect and had a doping-like behaviors on graphene. As the tau protein concentration in the electrolyte increased from 10 fg/ml to 1 ng/ml, the performances, charge neutral point shift and current change rate of the patterned graphene sensors without linkers were enhanced 2–3 times compared to a pristine graphene sensor with the PSE linker. Moreover, tau protein in the plasma of five Alzheimer's disease patients was measured using a linker-free patterned graphene sensor. It shows a 3–4 times higher current change rate than that of pristine graphene sensor with the PSE linker. Since the antibody is immobilized directly without a linker, a patterned graphene sensor without a linker can operate more sensitively in higher ionic concentration electrolyte.

Improved neutral shift method for fault tolerant operation of three phase MLCI

In a three-phase multilevel cascaded inverter (MLCI), when a failed module is bypassed, the number of modules in each phase becomes unbalanced. Under this condition, a method to obtain a balanced three-phase output is to move the neutral point of the MLCI. The neutral-point shift (NS) method has attracted a great deal of attention since the output of the inverter has a relatively low harmonic distortion when compared to other fault-tolerant operation methods. To move the neutral point, an offset voltage of the fundamental frequency component is injected. If the injected offset voltage exceeds the available range of the offset voltage, overmodulation occurs. Then, the output of the inverter is distorted. In this paper, the overmodulation regions of the NS method are analyzed for the fault-tolerant operation in the three-phase MLCI. In addition, a modified NS method is proposed to compensate for the overmodulation region. Both simulation and experimental results in a two-by-three MLCI demonstrate the performance of the proposed method.

Thermal Optimization of Modular Multilevel Converters With Surplus Submodule Active-Bypass Plus Neutral-Point-Shift Scheme Under Unbalanced Grid Conditions

The thermal design of highly reliable modular multilevel converters (MMCs) is significant for the voltage-source converter based high-voltage direct current (VSC-HVdc) systems, especially under the ac unbalanced fault. In this paper, the surplus submodule is employed as new control freedom to optimize the thermal behavior of the MMCs, because its number is linearly increased with the ac voltage dip. With the surplus submodule active bypassed (SSAB), the total submodules of the MMC in one arm can be taken turns to reduce the thermal stress of the hottest power device. In order to obtain equal ac voltages in three phases to improve the SSAB strategy, the neutral-point-shift (NPS) scheme is introduced under the unbalanced conditions, especially single-phase-to-ground fault. Through the balancing of the amplitude of the MMC ac voltages with the NPS scheme, the thermal stress asymmetry of theMMC under the single-phase-to-ground fault is mitigated, and the junction temperature of the most stressed power devices is significantly reduced. Finally, the simulation and experiment results show that the junction temperature rise of the most stressed power device is reduced by nearly 30% with the SSAB plus NPS scheme.

A Fault-Tolerant Control Strategy of Modular Multilevel Converter with Sub-Module Faults Based on Neutral Point Compound Shift

In view of the complex calculation and limited fault tolerance capability of existing neutral point shift control algorithms, this paper studies the fault-tolerant control method for sub-module faults in modular multilevel converters on the basis of neutral point compound shift control strategy. In order to reduce the calculation complexity of shift parameters in the traditional strategy and simplify its implementation, an improved AC side phase voltage vector reconstruction method is proposed, achieving online real-time calculation of the modulation wave adjustment parameters of each phase required for fault-tolerant control. Based on this, a neutral point DC side shift control method is proposed to further improve the fault tolerance capability of the modular multilevel converter (MMC) system by compensating the fault phase voltage with non-fault phase voltage. By means of the compound shift control strategy of the DC side and AC side of the neutral point, an optimal neutral point position is selected to ensure that the MMC system output line voltage is symmetrical and the amplitude is as large as possible after fault-tolerant control. Finally, the effectiveness and feasibility of the proposed control strategy are verified by simulation and low-power MMC experimental system testing.

Improved Fault-Tolerant Method for Modular Multilevel Converters by Combined DC and Neutral-Shift Strategy

In this paper, an improved postfault strategy is introduced for modular multilevel converters, which utilizes the remaining healthy capacity of the converter more efficiently and increases the line voltage amplitude under these circumstances. Previous works proposed to use the neutral point shift in ac side and dc side of the converter simultaneously. This paper adds a comprehensive and coherent formulation, deep analyzation, and experimental validation. The main improvement of the proposed method is maximizing the line voltage amplitude using the optimum dc component and the best neutral point position. Indeed, fewer restrictions in fault numbers, fewer restrictions in load power factor, and fewer nonsolution conditions are among other improvements of a combined dc and neutral-shift method. Comparative simulation and experimental results verify the effectiveness of the proposed method.

Transferred Voltages due to Single Phase Earth Fault on Power Transformers

In the event of an earth fault in one of the two systems of a three-winding transformer, an increase in the star-point voltage occurs in the disturbed system. This voltage is transferred to the undisturbed systems due to capacitive coupling, to affected earth faulty system and to earth. As a result, three phase voltages to earth are different in magnitude and phase position compared to their undisturbed service status.In this paper the mesh analysis method is used to calculate the neutral point shift of the faulted winding necessary to obtain values of transferred voltages to the unaffected winding (the one which is not earth faulted). The results are analyzed to investigate the influence of winding and network neutral grounding (insulated, solidly grounded, grounded over impedance) on the results.

Waverider Aerodynamics and Preliminary Design for Two-Stage-to-Orbit Missions, Part 1

The influence of the planform shape of osculating cones waveriders on the aerodynamic behavior with respect to the longitudinal motion is discussed. The inviscid flowfield around the configurations is simulated along the complete trajectory of an airbreathing two-stage-to-orbit system based on the solution of the Euler equations. Grid refinement studies, as well as wind-tunnel results, are presented to demonstrate the validation and accuracy of the code. Furthermore, special attention is drawn to the fluid-structure interaction with respect to the heating of the leading edge under hypersonic flight conditions. It is found that the modification of a gothic planform toward combined forebody-delta wing planforms allows a significant improvement of the aeordynamic efficiency L/D in sub- and transonic flows. In addition, the longitudinal stability is increased without compromising the favorable high-speed qualities. These benefits are partly diminished by an increasing neutral point shift along the trajectory. In Part 2 (Heinze, W., and Bardenhagen, A., Waverider Aerodynamics and Preliminary Design for Two-Stage-to Orbit Missions, Part 2, Journal of Spacecraft and Rockets, Vol. 35, No. 4, 1998, pp. 459-466), the impact of the aerodynamic effects on the integrated design and performance of a waverider-based two-stage.to-orbit system is presented.

A continuous current generator for high voltage

P. L. Alger: The chief interest of Mr. Bergman's paper is in the commutation of the machine, that is, the chief theoretical interest. Mr. Bergman is building a machine for proper commutation. To get the commutation we must have low reactance and we must have a very definite commutating flux to make the voltage induced by the flux exactly overcome the voltage of the changing current. Unless a machine is very perfectly compensated, you cannot hold that flux just right because when the load changes, the armature reaction makes the neutral point shift so the first thing is to obtain very perfect compensation. Mr. Bergman has done this. Another thing that helps commutation is his distributed field winding, as by this means he has a relatively gradual rise of flux on leaving the neutral, as compared with the salient pole winding.