Excessive Voltage Drop Research Articles

Effect on physiochemical assets of Dy added spinel ZnSm2O4 for energy storage applications

Supercapacitors are considered a potential source of energy conversion systems due to their advantages like high energy density, extensive cycle life, and quick charging and discharging. However, because of excessive voltage drops, they exhibit low energy density and durability. In this work, high content heteratom with various concentrations in a spinel system fabricated by hydrothermal technique. The materials were then analysed by different physical characterization methods including X-ray diffraction spectroscopy (XRD), Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM). Using these techniques, the impact of dysprosium ions on ZnSm2O4 structure and morphology was studied. Moreover, the samples were subjected to various electrochemical tests including a polarization curve (CV), chronoamperometry and electrochemical impedance spectroscopy (EIS). Among all high content heteroatom samples, 20% Dy-added ZnSm2O4 displayed the highest electrochemical activity. The sample shows 1908 F g−1 specific capacitance and 1.85 Wh g−1 specific energy at 0.03 A g−1 current density. The fabricated sample also showed long term stability and shows up to 95% capacitance retention after 2000 cycles. The Dy+ doping improves compound integrity that enhances electrochemical performance. These results open a new path for the enhanced electrochemical performance in the application of supercapacitor.

Impact of Electrical Vehicle Private Charging Stations on the Quality of the Low Voltage Network Supply

The appearance of new single-phase intensive and often coincident loads, such as the electric vehicle private charging stations (i.e., residential or private parking lots of shopping malls or workplaces), can affect the operation of low voltage (LV) distribution networks. Unbalances among the three-phase loading are extremely common, even if the end-users of a given LV network have similar behaviors. Due to the coincidence of such loads, excessive voltage drops may often appear, altering the quality of supply voltage provided to the LV end-users. This work evaluates the effects of residential and other charging stations located in parking lots of shopping malls or workplaces on the quality of supply voltage of LV grids. Then, a decentralized approach is proposed, based on the multi-agent system, which allows the different flexibility resources available on the grid to act together to improve the network quality. The agents cooperate to regulate the network nodal voltage, taking into account the characteristics of the resources they are controlling and the needs of their owners. A case study derived from a real portion of the Italian distribution system demonstrates the validity of the approach in solving the network operation criticalities.

(Digital Presentation) Correlative Evaluation Between Water Concentration and Voltage Fluctuation of PEFC Under Low-Humidity and Load-Change Operations Based on Laser Spectroscopy

Low-humidity and load-change operations of polymer electrolyte fuel cells (PEFCs) often tend to induce the excessive voltage drop called “undershoot”. Especially when the load current rapidly increases in steps, the strong electro-osmotic drag dehydrates the electrolyte membrane and deteriorates the proton conductivity, resulting in a temporal increase in ohmic loss. To identify a factor for causing undershoot, it is necessary to precisely grasp the transient water behavior within PEFCs operated under load-change conditions. In the previous works, several researchers vigorously explored the water vapor distribution along flow fields of working cells with micro gas chromatography (μGC) [1]. Although GC techniques provide more accurate species concentrations, it takes a long time (at least two minutes) to collect a gas sample and identify a species. As an alternative approach, the authors have developed a single-ended fiber-optic sensor to measure the in-situ concentration of water vapor within narrow channels of fuel cells at high speed and with high accuracy based on tunable diode laser absorption spectroscopy (TDLAS) [2]. TDLAS is one of the high-speed and high-sensitive absorption spectroscopy techniques which detect the absorbance of a chemical species and identify its concentration using infrared diode laser. This study became able to monitor the transient variation of water vapor concentration in the anode channel of a PEFC during low-humidity and load-change operations using the TDLAS-based fiber-optic sensor and discussed the correlation between the water behavior and the voltage fluctuation. Furthermore, the effects of load current and compression pressure were also investigated.Figs. 1(a) and 1(b) show the dynamic responses of cell voltage and the transient water concentrations in the anode channel of the operating PEFC assembled under two different compression pressures of 1.0 and 2.0 MPa. The electrode area of the experimental cell is 50 mm2. Two separators on both sides have a single-straight channel (1.0 mm width, 1.0 mm depth, 6.0 mm length). Hydrogen (stoichiometry: 2.3@1.0A/cm2, 30%RH) and oxygen (stoichiometry: 9.1@1.0A/cm2, 30%RH) were fed to the anode and cathode channel in the co-flow arrangement. The fiber-optic probe for water detection was positioned at the fractional location of x/L=0.5 on the anode side and the modulated beam emitted from a DFB laser diode (wavelength: 1392.5 nm) was directly injected into the channel. The cell temperature is 70oC. The repetitive current cycling was performed between 0.2 A/cm2 for 23 s and 0.6 A/cm2 for 23 s. In the case of the compression pressure of 1.0 MPa, when the current density increases steeply from 0.2 to 0.6 A/cm2, the strong electro-osmotic drag temporarily reduces the water content on the anode side of the electrolyte membrane and increases the ohmic loss, resulting in the voltage undershoot. After a step increase in the current density, the temporal drop of water vapor concentration in the anode channel can also be observed. When the compression pressure is increased to 2.0 MPa, the porosities of the diffusion media are reduced and the water concentration becomes high especially under the anode rib. Thus, even if the current density is increased stepwise after the repeated current cycles, the water shortage in the anode and the undershoot are not caused. It was noted that the strong compression effectively improves the water retaining on the anode side under low-humidity operations and alleviate the voltage undershoot.

Normally-Off p-GaN Gate AlGaN/GaN Transistor with a New Schottky Second Gate

This study presents a normally-off dual-gate AlGaN/GaN high-electron-mobility transistor. The second gate is located between the p-GaN gate and the drain and is connected to the source. The optimized thickness and length of the AlGaN layer under the second gate next to the p-GaN significantly impact the maximum drain current and the off-state breakdown conditions. The reverse conduction characteristic is also improved because the freewheeling path of the reverse current is established between the second gate and the drain to prevent excessive voltage drop and conduction losses when the device is negatively biased. Compared with conventional HEMT, the proposed method shows a promising way to achieve normally-off GaN-based HEMTs with excellent forward ad reverse conduction performance.

VAR Compensation on Load Side using Thyristor Switched Capacitor and Thyristor Controlled Reactor

Generally, AC loads are the inductive loads which are reactive in nature. These loads, thus, demand and draw reactive power from the supply source. If these loads draw large lagging current from the source, this will cause excessive voltage drop in the line, which can even cause the voltage collapsing in the line itself if the drop in the line is excessively high. VAR compensation means efficient management of reactive power locally to improve the performance of AC power systems. In this paper, Static VAR Compensator, using TSC (Thyristor Switched Capacitor) and TCR (Thyristor Controlled Reactor), is designed and simulated in MATLAB to maintain the power factor of power system nearly to unity at all times. TSC and TCR are basically shunt connected capacitors and inductor respectively whose switching (of capacitors) and firing angle control (of inductor) operations are carried out using thyristors. The purpose of capacitors is to supply lagging VAR as per the demand by the connected loads and the overcompensation due to excess VAR generated by the discrete set of turned on capacitors are absorbed by the adjustable inductive reactance of the inductor in TCR branch through firing angle control mechanism.

Direct hybridization of PEMFC and supercapacitors: Effect of excess hydrogen on a single cell fuel cell durability and its feasibility on fuel cell stack

The present paper focuses on the effect of reduced hydrogen stoichiometry on the dynamic behavior of a PEMFC directly hybridized with SC or non-hybridized, using the European standard cycling protocol (FC-DLC) for transport applications. Two gas stoichiometry factors have been considered, at 1.2 and 1.1. Whereas direct hybridization of a single cell enables to reduce the hydrogen overconsumption from 28% to 20% at hydrogen stoichiometry of 1.2, the overconsumption was further reduced to 10% by reducing the stoichiometry to 1.1.The durability of a single fuel cell has been investigated by comparing the long-term cycling operation depending on the hydrogen stoichiometric conditions, for both hybridized and non-hybridized fuel cell. The non-hybridized fuel cell, at hydrogen stoichiometry of 1.2, was operated for 687 h (end of life), whereas the non-hybridized fuel cell at 1.1, could not be operated due to the sudden, excessive voltage drop in the high current density period of the cycle. On the contrary, the hybridized fuel cell was successfully submitted to cycling for more than 1000 h at both conditions of hydrogen stoichiometry. The GDL appears to be responsible for the failure of non-hybridized fuel cell performance, whereas no particular degradation was evidenced with hybridized configurations. Finally, the technique has been successfully applied to a three-cell stack, with very comparable voltage profiles of the individual cells over the cycling protocol.

Improved Voltage Drop Compensation Method for Hybrid Fuel Cell Battery System

In this paper, a voltage drop compensation method for hybrid hydrogen fuel cell battery system, with a hydrogen recirculation powering a forklift, is studied. During recirculating hydrogen fuel to recycle hydrogen that has not reacted enough at the system, impurities can be mixed with the hydrogen fuel. This leads to low hydrogen concentration and a drop in the output voltage of the fuel cell system. In excessive voltage drop, the fuel cell system can be shutdown. This paper proposes a voltage drop compensation method using an electrical control algorithm to prevent system shutdown by reducing voltage drop. Technically, voltage drop is typically caused by three kinds of factors: (1) The amount of pure hydrogen supply; (2) the temperature of fuel cell stacks; and (3) the current density to catalysts of the fuel cell. The proposed compensation method detects voltage drop caused by those factors, and generates compensation signals for a controller of a DC–DC converter connecting to the output of the fuel cell stack; thus, the voltage drop is reduced by decreasing output current. At the time, insufficient output current to a load is supplied from the batteries. In this paper, voltage drop caused by the abovementioned three factors is analyzed, and the operating principle of the proposed compensation method is specified. To verify this operation and the feasibility of the proposed method, experiments are conducted by applying it to a 10 kW hybrid fuel cell battery system for a forklift.

FPGAhammer: Remote Voltage Fault Attacks on Shared FPGAs, suitable for DFA on AES

With each new technology generation, the available resources on Field Programmable Gate Arrays increase, making them more attractive for partial access from multiple users. They get increasingly adopted as accelerators in various application domains, embedded in shared Systems on Chip or remote cloud services. Thus, some recent works have already explored Denial-of-Service and side-channel attacks, where an FPGA fabric is shared among multiple users. In this work, we show how fault attacks can be launched within an FPGA, through software-provided bitstreams alone. Excessive voltage drops can be generated from legitimate logic mapped into the FPGA to cause timing faults, reaching from spatially and logically isolated partitions of one to another user of the FPGA fabric. To cause this voltage drop, we first show how specific patterns to activate Ring Oscillators can cause timing failures in simple test designs on various FPGA boards. Subsequently, we analyze and adapt an existing fault model for the Advanced Encryption Standard to match the accuracy of our fault attack. In the same multi-user scenario, we show as a proof-of-concept how a successful Differential Fault Analysis attack on an AES module can be launched. We perform experiments on three FPGA boards of the same model and confirm that the attack adapts to all systems and is successful under process variation, but with different susceptibility to faults. The paper is concluded by validating the attack on another platform, and analyzing the vulnerability based on a timing analysis, proving the applicability to different devices.

Optimal Charging Strategy Based on Model Predictive Control in Electric Vehicle Parking Lots Considering Voltage Stability

Recently, electric vehicles (EVs) using energy storage have gained attention over conventional vehicles using fossil fuels owing to their advantages such as being eco-friendly and reducing the operation cost. In a power system, an EV, which operates through the energy stored in the battery, can be used as a type of load or energy source; hence, an optimal operation of EV clusters in power systems is being extensively studied. This paper proposes an optimal strategy for charging EVs in parking lots. This strategy is based on the model predictive control (MPC) framework due to the uncertainty of loads, renewable energy sources, and EVs, and considers the voltage stability of the distribution systems. EV chargers in the parking lot charge EVs to minimize the charging cost, which results in a sudden increase in charge load at a certain time. As a result, an excessive voltage drop may occur in the power system at that time. Therefore, we need to minimize the charging cost of EVs while preventing an excessive voltage drop in the power system. The parking lot is stochastically modeled to consider EV uncertainty under the MPC framework. In the MPC framework, the charging schedule of an EV charger in the parking lot is optimized by considering both voltage stability and charging cost minimization in real time. The charging constraints on voltage stability are updated through parameters that change in real time, and thus, errors caused by uncertainty can be reduced. Subsequently, this charging strategy is applied to multiple chargers through Monte Carlo simulation. The proposed charging strategy is verified based on MATLAB/Simulink.

Dynamics of microvortices induced by ion concentration polarization

We investigate the coupled dynamics of the local hydrodynamics and global electric response of an electrodialysis system, which consists of an electrolyte solution adjacent to a charge selective membrane under electric forcing. Under a dc electric current, counterions transport through the charged membrane while the passage of co-ions is restricted, thereby developing ion concentration polarization (ICP) or gradients. At sufficiently large currents, simultaneous measurements of voltage drop and flow field reveal several distinct dynamic regimes. Initially, the electrodialysis system displays a steady Ohmic voltage difference (ΔV_{ohm}), followed by a constant voltage jump (ΔV_{c}). Immediately after this voltage increase, microvortices set in and grow both in size and speed with time. After this growth, the resultant voltage levels off around a fixed value. The average vortex size and speed stabilize as well, while the individual vortices become unsteady and dynamic. These quantitative results reveal that microvortices set in with an excess voltage drop (above ΔV_{ohm}+ΔV_{c}) and sustain an approximately constant electrical conductivity, destroying the initial ICP with significantly low viscous dissipation.

Efficient Stacked OLED processed by Organic Vapor Phase Deposition (OVPD)

ABSTRACTThe development of efficient large-area organic light emitting diodes (OLED) requires reliable and easily processable charge generation layers (CGL) with low excess voltage drop and high optical transparency. OVPD offers the advantage of a precise control of layer morphology, composition and thickness and is a powerful method for the deposition of advanced OLED designs. In this work, electrical doping of organic semiconductors using OVPD is investigated and applied to stacked OLED utilizing inorganic/organic CGL. The organic p-type dopant NDP-9 of Novaled GmbH is used for doping the hole transport material N,N‘-diphenyl-N,N‘-bis(1-naphthylphenyl)-1,1‘-biphenyl-4,4‘-diamine (α-NPD) in an AIXTRON OVPD tool. A doping concentration of 8 vol.% of NDP-9 in α-NPD is found optimal for hole injection as well as conductivity. This dopant concentration was employed in CGL with the structure: electron transport material/LiF/Al/α-NPD:8 vol.% NDP-9. External quantum efficiencies (EQE) of 15%, 35% and 50% and luminous efficiencies of 37 lm/W, 45 lm/W and 45 lm/W at 1000 cd/m2 are demonstrated for single, double- and triple-unit green phosphorescent OLED, respectively.

Asymmetric Spin Accumulation Induced by the Rashba Spin-Orbit Effect in a Domain Wall Inside a Magnetic Nanowire

We have investigated the spin accumulation effect in a Bloch-type magnetic domain wall in the presence of the Rashba spin-orbit interaction. The investigation has been done to study the excess voltage drop due to spin accumulation across the domain wall using the kinetic equation in Wigner space. It has been demonstrated that the Rashba interaction extremely increases the spin accumulation effect in the domain wall which in turn results in up to two orders of magnitude enhancement in the excess voltage drop.

3 µm diode lasers grown on (Al)GaInSb compositionally graded metamorphic buffer layers

Diode lasers operating at 3 µm in continuous wave mode at room temperature were fabricated using metamorphic molecular beam epitaxy. The laser heterostructures have a lattice constant 1.3–1.6% bigger than that of the GaSb substrates. The mismatch between the epi-structure and the substrate lattice constants was accommodated by a network of misfit dislocations confined within linearly compositionally graded buffer layers. Two types of the buffers were tested—GaInSb and AlGaInSb. The laser heterostructures with Al-containing buffer layers demonstrated better surface morphology and produced devices with lower threshold and higher efficiency. At the same time the use of Al-containing buffers caused an excessive voltage drop across the laser heterostructure. Thus, a maximum continuous wave output power of 200 mW was obtained from lasers grown on GaInSb buffers, while only 170 mW was obtained from those grown on AlGaInSb buffers.

Fabrication Study on Rectangular Luminous Layer for Field Emission Display Panel

Using carbon nanotube as field emitter, the field emission display (FED) panel with rectangular luminous layer was fabricated. The etched indium tin oxide (ITO) film was used as the ITO electrodes, and the phosphor powder was prepared to form the phosphor layer, so the rectangular luminous layer was fabricated on the anode back plane. In one anode pixel, the three anode sub-pixels would be included, which would reside on the same ITO electrode surface. The insulation layer would separate the adjacent anode sub-pixels and the adjacent ITO electrode, and the fabricated anode extension line was mainly used to reduce the excessive voltage drop. The FED panel with rectangular luminous layer possessed good field emission characteristic and high image brightness.

Distribution network phase load balancing as a combinatorial optimization problem using fuzzy logic and Newton–Raphson

The electrical network system is to ensure that an adequate supply is available to meet the estimated load of the consumers in both the near and more distant future. In order to avoid excessive voltage drop and minimize loss, it may be economical to install apparatus to balance or partially balance the loads. It is believed that the technology to achieve an automatic load balancing lends itself readily for the implementation of different types of algorithms for automatically rearranging the connection of consumers on the low voltage end of a feeder for optimal performance. In this paper the combination of the fuzzy logic, which addresses the linear solution, with Newtown Raphson as optimization method for the nonlinear behavior of the load is developed. A study case is presented for a radial feeder of 150 loads.

The analysis of contact resistivity between a p-type GaN layer and electrode in InGaN MQW laser diodes

The individual elements attributing to the excess voltage drop in the nitride-based laser diodes were investigated. The specific contact resistivity ρ c to p-type GaN was estimated by transmission line model (TLM) method and was estimated to slightly vary from 3.2×10 −3 to 6×10 −4 Ω cm 2 in the current range of 1–10 kA cm −2. It is found that a low specific contact resistivity was obtained by optimizing both the acceptor density of p-type GaN and the contact metals. The respective voltage drop at the p-side contact in our laser diodes was found to be 5.6 V for a device with a threshold current density of 6.7 kA cm −2 (Ith=100 mA) at 14 V under pulsed current injection at room temperature. It is found that the excess voltage drop in these devices is mainly due to the p-side contact resistance.

Formation and control of MBE-ZnSe/GaAs heterointerface and regrown homointerface

A combination of ex-situ HF-pretreatment and in-situ Se-pretreatment, ( HF + Se)-pretreatment, is successfully applied to GaAs surface to control MBE-ZnSe/GaAs (100) heterointerface. c–v analysis of the interface indicates that the minimum state density, N ss,min( E), is considerably reduced to 1 × 10 11 cm −2 eV −1, being one order of magnitude smaller than that for the standard chemically etched sample. The excess voltage drop across the n-ZnSe/n +-GaAs interface for current conduction is consistently reduced by the pretreatment, and the behavior of the i–v characteristics can be theoretically fitted by using a previously proposed model considering interface charge and obtained N ss( E) shapes. The quality of the ZnSe epilayer, which is assessed from X-ray diffraction and Raman scattering measurements, is also improved by the ( HF + Se)-pretreatment. Air-exposing and regrowth process is found to introduce a relatively high potential barrier at the ZnSe/ZnSe homointerface even when the regrowth is undertaken after a slight etching of the first grown epilayer.

Characterization of Epitaxial ZnSe/GaAs (100) Interface Properties and Their Control by (HF+Se)-Pretreatment

Interface properties of molecular beam epitaxy (MBE)-ZnSe/GaAs (100) substrate systems formed after various pretreatments of the substrate surface, including standard chemical etching, and (NH4)2Sx -, HF-, Te-, Se- and (HF+Se)-pretreatments, have been characterized by C–V, deep level transient spectroscopy (DLTS) and I–V measurements. A comparative study of the quality of the grown epilayer has also been made by X-ray diffraction (XRD) and Raman scattering measurements. It is found that the (HF+Se)-pretreatment gives superior interface with minimum interface state density, N ss,min, which is smaller than 1×1011 cm-2 eV-1 below the midgap of GaAs. The value is about a thirtieth of the standard chemically etched sample. The influence of interface states on I–V characteristics across the interface is also analyzed on the basis of a model which takes interface charge into consideration. The excess voltage drop at the interface is considerably reduced for the Se-pretreated sample, which is in agreement with the present model. The quality of the ZnSe epilayers assessed by XRD and Raman spectra is firmly correlated with the interface quality.

Analysis and prevention of DRAM latch-up during power-on

The occasional power-on latch-up phenomenon of DRAM modules with a data bus shared by multiple DRAM chips on different modules was investigated and the circuit techniques for latch-up prevention were presented. Through HSPICE simulations and measurements, the latch-up triggering source was identified-to be the excessive voltage drop at the n-well pick-up of the CMOS transmission gate of read data latch circuit due to the short-circuit current which flows when the bus contention occurs during power-on. By extracting the HSPICE Gummel-Poon model parameters of the parasitic bipolar transistors of DRAM chips from the measured I-V and C-V data, HSPICE simulations were performed for the power-on latch-up phenomenon of DRAM chips. Good agreements were achieved between measured and simulated voltage waveforms. In order to prevent the power-on latch-up even when the control signals (RAS, GAS) do not track with the power supply, two circuit techniques were presented to solve the problem. One is to replace the CMOS transmission gate by a CMOS tristate inverter in the DRAM chip design and the other is to start the CAS-BEPORE-RAS (CBR) refresh cycle during power-on and thus disable all the Dout buffers of DRAM chips during the initial power-on period.

Effect of GaAs surface pretreatment on electrical properties of MBE-ZnSe/GaAs substrate interfaces

Interface properties of MBE-grown ZnSe/GaAs substrate systems formed on variously pretreated GaAs surfaces, which include standard chemically etched (5H2SO4:1H2O2: 1H2O), (NH4)2Sx-, NH4I-, and HF-pretreated surfaces, are investigated by capacitance-voltage (C-V) and deep level transient spectroscopy (DLTS) measurements. A HF-pretreated and annealed ZnSe/p-GaAs sample showed marked reduction of interface state density, Nss, with Nss,min below 4 x 1011cm-2 eV-1 near Ec- EFS= 1.0 eV. The value is about one order of magnitude smaller than that of the standard chemically etched interface, and comparable to (NH4)2Sx- pretreated interface. Nevertheless, C-V characteristics of ZnSe/nGaAs samples, which were measured for the first time, indicate that interface Fermi level, EFS, is not completely unpinned due to the interface states located above the midgap. A consistent result was obtained by DLTS method in determining EFS position. The influence of Nss distribution on vertical current conduction is also analyzed. It is found that U-shaped interface states with Nss(E) > 1 x 1013 cm-2 eV-1 above the midgap may cause an excess voltage drop larger than a few volts at the interface.