Precharging Time Research Articles

A droplet‐based electricity generator incorporating Kelvin water dropper with ultrahigh instantaneous power density

AbstractHarvesting renewable water energy in various formats such as raindrops, waves, and evaporation is one of the key strategies for achieving global carbon neutrality. The recent decade has witnessed rapid advancement of the droplet‐based electricity generator (DEG) with a continuous leap in the instantaneous output power density from 50 W/m2 to several kW/m2. Despite this, further pushing the upper limit of the output performance of DEG is still constrained by low surface charge density and long precharging time. Here, we report a DEG incorporating the Kelvin water dropper (K‐DEG) that can generate an ultrahigh instantaneous power density of 105 W/m2 upon one droplet impinging. In this design, the Kelvin water dropper continuously replenishes the high density of surface charges on DEG, while DEG fully releases these surface charges into electric output. K‐DEG with such a high output can directly light five 6‐W commercial lamps and even charge a cellphone by using falling droplets.

DC-Bus Voltage Sensorless Control of an Active Rectifier with Modular Multilevel Converter

The modular multilevel converter (MMC) is recognized as one of the promising converters for the electrification of ships and railways. Among different energy conversion stages, the MMC-based rectifier should realize stable DC output voltage and accurate input current control. However, output DC-bus voltage sensors must be installed, which are costly and bulky. This paper presents a simple but effective control method for the MMC-based rectifier, removing the traditional output DC-bus voltage sensor. An accurate evaluation model of DC-bus voltage is developed as well as the implementing method. Meanwhile, a safe and fast pre-charging scheme is presented for the MMC-based rectifier without output voltage sensors. The fault-tolerant capability of the proposed method when the failure of the output DC-bus voltage sensor occurs is examined. Simulated and testing results demonstrate that the proposed method presents not only excellent steady-state and dynamic performance but also strong fault-tolerant capability. The proposed evaluation model for DC-bus voltage has a high accuracy with an error of less than 1%. Besides, the pre-charging time is less than 0.5 s using the proposed sensorless control.

Study of the influence of current density and displacement rate on hydrogen embrittlement using small punch tests

Susceptibility to hydrogen embrittlement was studied on quenched and tempered 42CrMo steel with Small Punch Tests (SPT) using different punch displacement rates. Hydrogen was cathodically charged using an electrolyte composed of 1 M H2SO4 with 0.25 g/l As2O3, and two different cathodic current densities (0.50 and 1.00 mA/cm2). The results were analysed using different embrittlement indices: related to the energy consumed at maximum load, to the equivalent biaxial deformation at failure location and to the failed diameter. Failure location is directly related to punch displacement and, consequently, to embrittlement.All embrittlement indices increase as the punch displacement rate decreases (longer time for hydrogen diffusion) and as the cathodic current density increases (higher hydrogen concentration in equilibrium with the hydrogenated medium). On the other hand, the use of a pre-charging time before the start of the in-situ hydrogen charged test produces no change in the results, as most hydrogen enters the sample due to the high plastic deformation induced on its surface in the course of the mechanical test.

A Novel Switched Capacitor Precharging Method for Three-Phase Induction Motor Drive Applications Utilizing the Winding Inductance

Three-phase (3-φ) induction motor drives operated with switched capacitor (SC) inverter circuits offer superior harmonic performance and voltage boosting. The SC network, however, generally requires additional circuitry to precharge the capacitor(s) to prevent inrush current. To obviate the requirement of the extra circuit, an innovative SC precharging scheme is presented, which utilizes the winding inductance of the motor. The inverter circuit is so operated that the motor winding inductance first stores electrical energy from the input dc source and then releases it to the SC. These two actions are continued till the SC is precharged to a voltage equal to the magnitude of the input dc source. The circuit operation during the precharging is similar to the operation of a conventional buck-boost type dc–dc converter. The precharging action is so designed that the voltage of the SC cannot increase further. Therefore, the proposed scheme does not need any voltage sensor. The operation of the circuit during precharging is analyzed extensively to develop a mathematical background for the given method. It is shown that by manipulating the switching frequency during the precharging the precharging time can be controlled as per the user requirements. Various claims are validated using computer simulations and experiments. All the results are presented.

Hydrogen-assisted cracking (HAC) of high-strength steels as a function of the hydrogen pre-charging time

This work demonstrates that by performing hydrogen-assisted cracking (HAC) as a function of hydrogen pre-charging time, highly symmetric circumferential cracks can be generated in circumferentially notched or grooved round bars made of high-strength steels, which can be used to study HAC in terms of fracture mechanics. Using the simple-to-perform method, control over hydrogen-assisted crack initiation is achieved by applying low hydrogen pre-charging times and by determining a threshold minimum preload to failure LHAC. The uniaxial tensile testing at LHAC under constant displacement results in a characteristic load-time-curve as well as a high-symmetry circumferential crack for each material investigated in this study. Finally, the minimum preload to failure, the time to failure and the critical crack length are each exhibited as a function of hydrogen pre-charging time and an approach to the application of fracture mechanics with the aim of determining the threshold stress intensity KIHAC is discussed.

An experimental-numerical screening method for assessing environmentally assisted degradation in high strength steels

In this work, an experimental-numerical screening method for studying the elastic–plastic properties in high strength steel subjected to environmentally assisted degradation due to hydrogen is proposed. The experiments were performed on single-edge-notch bend specimens loaded with a monotonic constant displacement rate, and the specimens were electrochemically hydrogen pre-charged and/or in-situ. A systematic investigation was conducted of the influence of current density, pre-charging time and loading rate on the fracture mechanical properties. It was found that the loading rate had the greatest effect on the J-R curves, and that the environmental ductile-to-brittle transition region was obtained in a less than a day of experimental time. In this transition region it was found from the fractography that the dominating mode of failure changed from dimple to dominating intergranular fracture.

Implementation of a novel and versatile initial rotor position estimation method on surface-mounted permanent magnet synchronous motor prototype at zero speed

This paper presents a novel initial rotor position estimation method for a surface-mounted permanent magnet synchronous motor (SPMSM) using its inherent magnetic features. Here, the initial rotor position has been estimated utilising the variation in phase inductances arising out of the unavoidable but very small saliency occurring either out of magnetic saturation of the machine or the magnet shape. Detailed investigations have been carried out to enumerate the actual magnitude of the inductances of the SPMSM with distributed armature winding and the dynamic saturation status with armature current variation. Here, 3-phase balanced voltage of higher frequency (150 Hz) is applied to the motor terminals for a short period of time (for 300 ms) and the corresponding phase currents are indirectly used to determine the rotor position. Innovative signal processing steps have been used to distinguish small differences in the 3-phase currents caused by the small differences in the phase inductances. The position is determined from the relative values of different phase currents using a novel approach. This still leads to two diametrically opposite (electrical phase) solutions for the instantaneous rotor position. To eliminate this ambiguity, two alternative methods (for pole identification) have been proposed. Extensive co-simulations of finite-element method (FEM)-based electromagnetic simulation and system simulation (for logic implementation) have been conducted. The estimation methods have been validated experimentally on a laboratory-developed prototype SPMSM that was designed and fabricated by the authors. The experimental results are found to be in excellent agreement with the FEM-based simulation results. The complete initial position estimation method takes less than 1 s of real time, which is typically less than the pre-charging time of standard commercial inverters.

Effect of Hydrogen Precharging on Mechanical and Electrochemical Properties of Pure Titanium

The electrolytic hydrogen precharging process is used to analyze the influence of hydrogen on the microstructure, corrosion resistance, and mechanical properties of pure titanium. The results demonstrate that two types of titanium hydrides (δ‐TiHx and ϵ‐TiH2) are detected by electron backscatter diffraction (EBSD), and the volume fraction of titanium hydrides, the local strain caused by hydrogen precipitation, and the thickness of hydrides layer increase with the increase in precharging time. It is noteworthy that the formation of hydrides layer has a protective effect on general corrosion, but the local strain caused by hydrides precipitation leads to the destruction of corrosion resistance, as a result the corrosion resistance of pure titanium increases first and then decreases with the increase in hydrogen precharging time. Moreover, with the precipitation of hydrides, the fracture mode changes from ductile failure to duplex‐ and multimode failure, resulting in the decrease in ultimate tensile strength (UTS) and elongation. Therefore, the mechanical and electrochemical properties of the precharged samples first increase and then decrease with the increase in precharging time, reaching their optimum values at 10 h, and the critical precharging time is determined to be 75 h when comparing those properties with pure titanium.

Hydrogen embrittlement of X80 pipeline steel in H2S environment: Effect of hydrogen charging time, hydrogen-trapped state and hydrogen charging–releasing–recharging cycles

This study investigated the susceptibility of X80 pipeline steel to hydrogen embrittlement given different hydrogen pre-charging times and hydrogen charging–releasing–recharging cycles in H2S environment. The fracture strain of the steel samples decreased with increasing hydrogen pre-charging time; this steel degradation could almost be recovered after diffusible hydrogen was removed when the hydrogen pre-charging time was <8 d. However, unrecoverable degeneration occurred when the hydrogen pre-charging time extended to 16–30 d. Moreover, nanovoid formation meant that the hydrogen damage to the steel under intermittent hydrogen pre-charging–releasing–recharging conditions was more serious than that under continuous hydrogen pre-charging conditions. This study illustrated that the mechanical degradation of steel is inevitable in an H2S environment even if diffusible hydrogen is removed or visible hydrogen-induced cracking is neglected. Furthermore, the steel samples showed premature fractures and exhibited a hydrogen fatigue effect because the repeated entry and release of diffusible hydrogen promoted the formation of vacancies that aggregated into nanovoids. Our results provide valuable information on the mechanical degradation of steel in an H2S environment, regarding the change rules of steel mechanical properties under different hydrogen pre-charging times and hydrogen charging–releasing–recharging cycles.

Hydrogen embrittlement susceptibility of a Ni-16Mo-7Cr base superalloy

This study investigated the hydrogen embrittlement sensitivity of a Ni-16Mo-7Cr base superalloy under different hydrogen pre-charging states. Detailed electromicroscopy characterization has been employed on the hydrogen pre-charged alloy after tensile testing to understand the mechanism of hydrogen-assisted cracking. The results show that the tensile strength of the alloy is remarkably decreased with the increasing of hydrogen pre-charging time, whereas the yield strength remains stable all the time. The fractographic study combined with ECCI analysis by Scanning Electron Microscopy demonstrates that the cracks propagated predominantly along the grain boundary in the presence of hydrogen, and a few can propagate along planar dislocation slip bands (DSBs) especially along the intersections between nonparallel DSBs. The proposed hydrogen embrittlement mechanism for the Ni-16Mo-7Cr base superalloy is that the coalescence and widening of the nano-voids, which are resulted from the interaction between the hydrogen and dislocations, cause the crack initiation and propagation along the grain boundary, and further cause the alloy intergranular fracture.

Effect of ferrite on the hydrogen embrittlement in quenched-partitioned-tempered low carbon steel

This article proposes a novel method for the reduction of the hydrogen embrittlement susceptibility in a low carbon steel by the introduction of the ferrite phase into the quenched-partitioned-tempered (Q-P-T) specimen. Taking the Q-P-T specimen without ferrite as the reference, the CQ-P-T and QCQ-P-T specimens containing ferrite were obtained by intercritical annealing before the Q-P-T treatment and by pre-quenching before the CQ-P-T treatment to further refine the microstructure. The susceptibility of the three kinds of specimens toward hydrogen embrittlement was evaluated by the slow strain rate tensile test. The results showed that the elongation and area reduction of the three kinds of specimens continuously decrease with the greater electrochemical precharging time, but the three kinds of specimens exhibited different susceptibility index values. Fractographic observation showed that the primary fracture modes gradually evolved from typical dimple to quasi-cleavage, quasi-cleavage mixed with intergranular cracking and then complete intergranular manner, but the evolution rate of the QCQ-P-T specimen is the lowest while that of the Q-P-T specimen is the highest. It is suggested that the ferrite may reduce the stress concentration caused by the martensitic transformation and retard the crack propagation during deformation. Refining microstructure further improves the hydrogen embrittlement resistance by trapping more hydrogen atoms at the interphase boundary, lowering their density in the lattice.

A Generalized Precharging Strategy for Soft Startup Process of the Modular Multilevel Converter-Based HVDC Systems

The modular multilevel converter (MMC) has become the most attractive converter technology for medium/high-power applications, specifically for high-voltage dc (HVdc) transmission systems. One of the technical challenges associated with the operation and control of the MMC-based system is to precharge the submodule (SM) capacitors to their nominal voltage during the startup process. In this paper, considering various SM circuits, a generalized precharging strategy is proposed for the MMC-based systems, which can implement soft startup from the dc or ac side. The proposed precharging strategy can be applicable for various SM circuits and MMC configurations. The proposed startup strategy does not require extra measurements and/or auxiliary power supplies. The charging current is controlled by adjusting the changing rate of the number of blocked and bypassed SM capacitors. Based on the proposed startup strategy, the startup processes of MMC/MMC-HVdc systems with various SM circuits are analyzed and a generalized startup procedure for various MMC-HVdc systems is proposed. In addition, the uncontrollable steady-state SM capacitor voltages of various MMC-based systems are analyzed and the associated precharging time is also investigated. Performance of the proposed strategy for various MMC-HVdc systems is evaluated based on time-domain simulation studies in the PSCAD/EMTDC software environment and experimental results are based on a scaled-down prototype.

Effect of Degree of Order on Hydrogen Diffusion in (Fe,Ni)&lt;sub&gt;3&lt;/sub&gt;V Alloys

The effect of hydrogen on the mechanical properties of disordered and ordered (Fe,Ni)3V alloys has been investigated. The diffusion behaviors of hydrogen in the disordered and ordered (Fe,Ni)3V alloys were reported. The results showed that the depth of intergranular (IG) fracture on a surface of tensile fracture of (Fe,Ni)3V alloy increased linearly with increasing pre-charging temperature at the same pre-charging time. An apparent hydrogen diffusion coefficient was calculated by the time lag method. The apparent hydrogen diffusion coefficient in the disordered (Fe,Ni)3V alloy was greater than that in the ordered (Fe,Ni)3V alloy. The relationship between the apparent hydrogen diffusion coefficient and pre-charging temperature in (Fe,Ni)3V alloy agreed with Arrhenius equation. The activation energies of apparent hydrogen diffusion in the disordered and ordered (Fe,Ni)3V alloys were 34.6 kJ/mol and 42.2 kJ/mo1, respectively.

Hydrogen induced cracking and pitting of brass heat exchanger tube

Tin brass heat exchanger tube specimens were precharged with hydrogen at different cathodic charging conditions. Cathodic hydrogen charging was found to induce severe cracking and blistering on the surface of brass heat exchanger tube without externally applied stress. Microscopic observations revealed that the hydrogen cracks were intergranular (IG) or transgranular (TG) or mixed mode of cracking, i.e. IG and TG, while the blisters were observed to be spherically shaped. The severity of hydrogen induced cracking and blistering increased with the cathodic current density. Free corrosion immersion experiments were conducted on the hydrogen precharged tin brass heat exchanger tube specimens in 0·5M NaCl solution to investigate the role of hydrogen in pitting corrosion. It was found that hydrogen significantly promoted pit initiation and pit growth. Moreover, the pit formation and growth increased with charging time. The corrosion potential measurements revealed that the hydrogen precharged tin brass heat exchanger tube specimens exhibited lower corrosion resistance as compared with the non-precharged specimen. The corrosion potential of hydrogen precharged specimens reached a steady state after a longer immersion time. Furthermore, with increasing precharging time, the immersion time required to reach a steady state potential increased.

Investigation of corrosion behaviour of hydrogenated 7075-T6 aluminum alloy

The corrosion behaviour of high strength aluminium alloy type 7075-T6 precharged with hydrogen for various charging times was investigated using electrochemical polarization and free corrosion potentials measurement techniques. The results showed that the precharged 7075-T6 aluminium alloy with hydrogen exhibited lower open-circuit (OCP) and breakdown potentials in deaerated 0.5 M NaCl than those for the non-hydrogenated material. Moreover, the OCP and breakdown potentials decreased with increasing the precharging time. Microscopic observations of the exposed surfaces revealed that intergranular attack was formed in non-hydrogenated specimens whereas mixed mode of attack (i.e. intergranular and transgranular) were formed in the precharged ones during potentiostatic polarization. Moreover, the pits formed during the free corrosion potential testing were observed to be larger on the exposed surfaces of hydrogen precharged specimens. The severity of attack and pitting was observed to increase with increasing the precharging time. Consequently, the corrosion resistance of hydrogen precharged of 7075-T6 aluminium alloy with hydrogen was lower than that for non-hydrogenated material.

Study on hydrogen absorption of pipeline steel under cathodic charging

The hydrogen electrochemical oxidation method was used to determine the total amount of hydrogen absorbed by pipeline steels under cathodic charging. The oxidation currents were obtained for the hydrogen pre-charged specimen maintained at a certain positive potential in dilute NaOH electrolyte. The area under the current curve demonstrated a relationship with the hydrogen concentration. Hydrogen concentrations in pre-charged X70 and 16Mn steels were estimated using this method. It is shown clearly that hydrogen concentration in specimens increase as the applied current density increased, but the sensitivities of different materials to hydrogen are different. The effect of solution pH and pre-charging time on absorbed hydrogen is also studied.

Hydrogen embrittlement of duplex stainless steel in concentrated sodium chloride solution

The cracking behavior of 2205 duplex stainless steels in 26 wt% NaCl of pH 2 under cathodic polarization condition was investigated. Dramatic reductions in ultimate tensile strength and ductility were found, indicating its susceptibility to hydrogen-assisted fracture at an applied potential of −1500 mV (SCE). The effect of hydrogen-assisted cracking became more pronounced as the stain rate decreased, and/or as the precharging time increased. Microstructural examination showed that surface crack could initiate either in austenite phase or in ferrite phase. Under cathodic charging condition internal cracks, resulted from hydrogen embrittlement of ferrite phase, could be arrested by austenite.

Influence of Potential, Chlorides, pH, and Precharging Time on Embrittlement of Cathodically Polarized Prestressing Steel

Abstract Corrosion of prestressing steel in concrete has become a major technological problem in highways, buildings, and pipeline structures. While cathodic protection is recognized as an appropriate technique to mitigate corrosion of reinforcing steel in concrete, the possibility of environmental cracking (hydrogen embrittlement) in the case of prestressing tendon has limited application to this usage. To establish the appropriateness of cathodic protection for prestressing steel, constant extension rate testing was pertormed on smooth and notched wire specimens in deaerated Ca(OH)2 solutions as a function of potential, [Cl−], pH, and precharging time. Results indicated potential is the most important of these variables, and a threshold value of −0.90 VSCE was identified below which embrittlement is enhanced. Notched specimens, which may best simulate the geometry of corroded tendon, particularly were susceptible when compared to smooth tendon. Failure of some tendons in this condition could occur upon ...

Hydrogen diffusion and its relevance to intergranular cracking in nickel

This investigation examines the effect of hydrogen precharging upon the fracture mode of pure nickel at 77 K. Specific attention is given to the diffusion coefficient of hydrogen along grain boundaries,D g , the critical hydrogen concentration to cause intergranular fracture, C g * , and the binding energy of grain boundaries with hydrogen,E b . Both scanning electron microscope (SEM) observations and a newly developed grain boundary diffusion model indicate that the fracture mode changes from transgranular (TG) to intergranular (IG) when the hydrogen concentration in grain boundaries reaches a critical value, in the range of 6.5 to 9.8 at. pct. The experimental results further show that the observed increment of IG cracking depth with the precharging time could be accounted for by lattice diffusion alone, thus implying that hydrogen transport in this material is not enhanced by grain boundaries. Finally, the binding energy of grain boundaries with hydrogen is found to be 11.3 to 12.3 kJ/mol.

A 45-ns 256K CMOS static RAM with a tri-level word line

A 32K words by 8-bit static RAM fabricated with a CMOS technology is described. The key feature of the RAM is a tri-level word-line, in which an automatic power down by a pulsed word-line in the READ cycle and a power saving by a middle-level word-line in the WRITE cycle are combined. This circuit technique minimizes bitline swing, shortens the precharging time, and depresses the transient current. An improved address transition detection circuit reduces the chip select access time. The sense amplifier uses internally synchronized signals for improved operation. The RAM has a typical access time of 45 ns with an active power dissipation of 7 mW. The peak transient current is less than 40 mA. A double-level polysilicon technology with a 1.3-/spl mu/m design rule allowed layout of the NMOS memory cell in an area of 116.0 /spl mu/m/SUP 2/ and the die in 49.6 mm/SUP 2/.