Stability of long-sustained oscillations induced by electron tunneling

In this study, the DC voltage oscillation mechanism is investigated based on the linearized models of a voltage-source converter (VSC)-based multiple terminal DC (MTDC) system. First, the mechanism of DC voltage oscillation generation is revealed. DC voltage or DC voltage droop-controlled VSCs are identified as the DC voltage oscillation sources when master–slave or DC voltage droop control is separately applied to the VSC-MTDC system. Meanwhile, the active power-controlled VSC is equally regarded as a constant power source that does not provide any oscillation to the VSC-MTDC system. Subsequently, the impact of the dynamics of VSCs and MTDC network on the DC voltage oscillation is separately investigated. It is theoretically verified that the negative eigenvalues of the MTDC network conductance matrix can reduce the stability of the DC voltage oscillation. The constant power load is identified as the physical mechanism of a problematic scenario that induces DC voltage oscillation in the generalized VSC-MTDC system. Moreover, it is demonstrated for the first time that the steady-state power flow may cause DC voltage oscillations when the power flow is reversed. Finally, two VSC-MTDC systems, which individually adopt master–slave and DC voltage droop control, are presented to validate the findings using the standard MATLAB SIMULINK model.

The paper presents a multi-layer hardware-in-the-loop (HIL) based testbed for development, evaluation and testing of microgrid operation. The testbed is a configurable setup which can match any microgrid structure. The presented microgrid consists of 15 distributed energy resources (DERs). It includes 4 distributed generators (DGs): a solar plant, diesel generator, 2 constant power sources, 10 distributed loads (DLs): a direct-on-line induction machines, constant power load, 7 passive loads, a three phase rectifier with an active filter and a battery storage system (BSS). Power system and power electronics parts were simulated by a real time simulator. Controllers for the PE converters were real, industrial controllers connected to the real-time simulator. Supervisory control and data acquisition system (SCADA) for the microgrid was designed and connected to the system. The system behavior was analyzed in a number of different scenarios.

The rapid transition to green energy is changing the way that power system networks are operating. This is due to the uncertain behaviour of renewable energy sources (RES) and the drop in the dynamic influence from Synchronous Generator (SG) in the network. This thesis has documented and expanded the main concerns regarding the change of dynamics in the power network. Connecting the inverter based resource (IBR) as a constant power source has introduced new challenges for the system operators for maintaining a stable and safe operation. Reports from grid operators and researchers for projects with ancillary services provided by inverter based resource (IBR), shows that the implementation of advanced control strategies and different control modes enhance the reliability and stability of the power grid. Transitioning to a 100% inverter based resource (IBR) dominant system, Grid-forming Inverter (GFM) has been proposed as a mandatory requirement to achieve this target. Therefore, in this thesis, an optimal Dynamic Voltage Support (oDVS) has been proposed operating as a current limitation mechanism. The proposed method has been implemented into a 2-bus system modelled as a weak grid with high grid impedance with various X/R ratios. The outcome proves that the optimised coordination of fault current and tracking of the angle of displacement can enhance the Dynamic Voltage Support (DVS) improving the voltage profile in the Point of Common Coupling (PCC) even in low X/R ratio networks.

The Woods Hole Oceanographic Institution (WHOI) and Dalhousie University conducted a sediment transport study at the LEO-15 observatory maintained by Rutgers University and WHOI engineers off the central New Jersey coast. This study was designed to take advantage of the unique capabilities (power and telemetry) provided at the site. The various sensing systems were attached to a bottom-mounted frame and connected by divers to the underwater-mateable guest ports on the LEO-15 node. The node supplied power for the various acoustic and electronic systems and provided telemetry for control of the remote instrumentation and acquisition of several GBytes of data per day. A shore-based support station of several PCs and workstations networked together and connected to the Internet was set up at Rutgers Tuckerton Field Station to receive, display and store this data. Instrumentation was deployed in mid-November 1999, remained active until Christmas 1999, and was recovered in January 2000. The methodology of using the LEO-15 observatory was different from previous approaches used by the WHOI and Dalhousie sediment transport teams and had some subtle problems not obvious until the equipment was set up and utilizing the observatory. A second LEO-15 node, on shore at the Tuckerton field station, facilitated testing of each subsystem and the fully integrated system before offshore deployment. One set of problems involved the node's ground fault detection system that was designed to shut down all scientific instrumentation to protect the node when a current leak to seawater was detected on any power or telemetry line. WHOI connected all their sensing systems together to provide an optimum test of any system problem. Dalhousie had isolated their instrumentation (power and communications) so the ground fault circuit only tested the cables from the LEO-15 node to their electronics. The WHOI instrumentation developed a slowly increasing ground fault in one line that resulted in some unnecessary data loss. The underwater-mateable connectors required at the LEO-15 sometimes showed a low ground fault level leakage that disappeared when they were reconnected. Because or the constant power source, the electronics on the tripod remained powered up and active during the deployment, and individual sensors were powered and burst-sampled once or twice an hour, collecting /spl sim/4 GBytes of data per day that was telemetered to shore. WHOI's high-speed link operated successfully at /spl sim/400 kbits/sec and Dalhousie's at 2 Gb/s. Dalhousie experienced some problems with their remote system suddenly stopping that may be associated with changes made to interface it with the LEO-15 observatory. All data were archived at the Tuckerton facility as the Internet link to WHOI and Dalhousie could only provide 1/3 of the required speed to transmit the data collected daily. In spite of these problems, the systems worked much of the time and collected many gigabytes of data on the suspended sediment transport and structure in relationship to bedform roughness, and wave and current forcing. Real-time sensor functionality and data acquisition operations were monitored over the Internet. The goal of plugging instruments into the LEO-15-node and obtaining continuous long time-series remotely with no interruptions is not yet fully realized. Someone had to monitor the connection to keep systems functioning optimally. However, we learned much of the methodology to move toward full remote operations in the future which will improve future observatories, such as the new Martha's Vineyard Coastal Observatory.

Despite their high efficiency, soft-switching three-phase inverters ca have disadvantages like higher conduction losses (resonant pole inverter), voltage stress and sub-harmonics (resonant DC-link inverter) or the need for additional components (auxiliary resonant commutated pole inverter). In contrast to single-phase grid connection, the three-phase grid connection provides a continuous power flow without the need for large energy storage devices. If the inverter is fed by a constant power source, it is possible to overcome these disadvantages by using the novel double 120deg flattop modulation, which results in an inverter with low conduction losses, small dc-link capacitance and soft-switching operation without the need for additional components. The developed inverter prototype in the kW-range operates with a total dc-link capacitance of only 10 muF. All these characteristics make it promising for grid-connection of renewable energy systems

High performance, corrosion resistant alloys CRA'S are specified for oilfield applications where severe environments such as general corrosion, pitting, crevice corrosion, chloride stress corrosion cracking and more importantly sulfide stress corrosion cracking are encountered. Historically, alloy 625 weld overlay has successfully been used in severe corrosive environments. INTRODUCTION The trend in oil and gas production has been towards deeper wells, which has resulted in more hostile environments due to increased temperatures, pressures and the presence of H2S, CO2 and chlorides. Many wrought CRA alloys in the cold worked, or aged condition, have been used for different components (Table 1). There exist many applications where, due to the massive and intricate nature of certain components involved, it is not practical or cost effective to use wrought products. In many cases, the materials with the appropriate corrosion resistance are limited in mechanical properties (Table 2). In such cases, the desired mechanical properties can only be achieved through cold working which are unpractical for many product configurations. For these cases, historically welding has been used to overlay the corrosion resistant alloy on low alloy steel. The alloy that has extensively been used for such applications has been alloy 625 UNS 06625. Although alloy 625 has been the primary weld overlay material used in oil field applications, there have been instances where, this alloy under severe sour corrosion conditions and temperatures, has proven to be unacceptable. Another drawback of this alloy 625 has been its limited mechanical properties. WELDING PROCESS The gas-metal arc welding (GMAW) process uses a constant power source and continuous wire feed. The wire electrode forms an arc with the base metal and is consumed in the weld pool. The weld metal is protected by shielding gases. Depending on the welding parameters selected, four different modes of metal transfer can be utilized. The four modes of metal transfer that can be used are: spray; globular; short circuiting; and pulsed arc transfer. The protective atmosphere for gas-metal-arc welding is usually a combination of argon, helium and carbon dioxide. The optimum shielding gas will vary with the type of metal transfer used. SAMPLE PREPARATION/PROCEDURE Weld overlay samples were prepared using the gas metal arc welding (GMAW) process. The substrate tubes were fully heat treated AISI 4130 with a nominal O.D. of 9" × 6" I.D. × 12" long (23 cm. × 14 cm. × 30 cm.). Welding was performed using alloy 625, alloy 59, alloy 686, alloy 725 and alloy C-22. Single layer and two layer weld deposits were applied to the inside diameter of the substrate tube. For the single layer sample, the deposited thickness was sufficient to achieve a 3 mm minimum clad thickness when the I.D. was machined concentric with the O.D. For the two pass samples, the final machined clad thickness was 9 mm. After welding, a stress relieve was performed on the alloys tubes.

An approximate method is developed for calculating the instability boundaries, as well as the frequencies and amplitudes, of acoolant’s autooscillations self-excited during its motion in a vertical channel exposed to locally applied heat from a constant power source. Unlike in the classical generators of thermoacoustic oscillations such as the Rijke tube and Lehmann’s plant, it is assumed that the main stationary flow is collinear with the gravity force. In order to exclude the surging nature of the self-oscillations, it is assumed that the flow is created by a supercharger with a monotonically decreasing head-capacity characteristic. The resulting mathematical model makes it possible to theoretically determine the self-excitation conditions of the considered autooscillations and also to evaluate the effect of various parameters on their shape.

Using a first-order k ⋅ p model for electron intersubband scattering in quantum wells (QW), our group re-analyzed pump-probe experiments performed in the mid-nineties that investigated the intersubband and intrasubband electron dynamics in two InGaAs/AlInAs QWs grown in the same molecular beam epitaxy (MBE) laboratory. This work confirms that the measured electron lifetime can only be interpreted through a rigorous k ⋅ p approach that not only considers the hyperbolic subband dispersion but also band-mixing’s k-dependence, which slows down all scattering mechanisms. The latter includes the enhanced wavefunction confinement with k. Concomitantly, this analysis investigated the range of interface roughness parameters (IFR) and the potential for alloy disorder (AD), whose values are very important for designing many existing/new quantum devices. Just by using the one-band model, the overall 2–1 scattering rate is ∼66% overestimated in a QW tuned at λ 21 ∼ 5 µm, while using the eight-band model, good agreement with the experimental data is obtained by properly choosing these material parameters. However, due to the inherent time resolution of the pump-probe experiments and the fact that AD and IFR scatterings are processes of the same nature, these material parameters are fitted with a large uncertainty. The suggested IFR parameters are in line with what is typically used in quantum cascade laser simulations, but the AD potential is smaller. Pump-probe experiments are not sufficient to capture the material parameters with sufficient accuracy and, therefore, should be associated with other material characterization techniques, such as ultra-high vacuum scanning tunneling microscope or electron tomography. The quantum devices’ simulation tools could benefit from the IFR and AD material parameters derived in this work, but above all, this analysis reaffirms that for modeling and optimizing intersubband devices, it is crucial to consider not only the correct dispersion but, most of all, the band-mixing, which is strongly k-dependent.

Transcatheter intravascular electrocoagulation for therapeutic occlusion of vessels has been experimentally developed and successfully used clinically as an alternative to, and in conjunction with, more routine methods of vessel occlusion. A constant current power source and a steelguide wire anode are used for the occlusion. The subject is grounded with a standard bovie plate insulated from the skin by a highly lubricated sponge. Permanent occlusion of strategic vessels at accurately controlled sites has been achieved with little risk of embolization of non-target areas. There has been minimal damage to vessel walls, and complete occlusion has been possible despite heparinization and/or thrombocytopenia. One limitation is the inconstant ability to position the anode precisely at the desired site of occlusion. Also, a long time may be required (up to one hour) to occlude larger vessels with presently available anodes. Wider use must await refinements in development of the anode material and in its delivery system.

CMOS circuits consume great dynamic power in switching. It has been proposed that energy transfer through a rising Vdd dissipates small amounts of energy. In typical power gate circuits, the high-performance PMOS transistors (PSW) that connect the circuit blocks to the power supply reduce leakage power by shutting off outer power (Vdd) to the idle blocks. We expand this technique by utilizing active PSW, which are turned on and off by clock signal. The PSW are fully turned on only for half of each clock cycle. This means that sufficient Vdd is provided to the circuit continuously for half of each clock cycle. In this manner, the circuit charge and discharge actions are cycle occur in different phases, and ramp Vdd is supplied to the designed circuit; we name this technique "CKVdd." CKVdd is a clock-controlled self-stabilized voltage technique, which generates stable ramp voltage to suppress the currents effectively. It is proposed to reduce dynamic power dissipation in conventional CMOS digital circuits. As compared to the conventional circuit, the circuits using CKVdd technique possesses several characteristics that differ from those of the current circuits using constant Vdd power source. First, CKVdd technique combines the power source and clock signal; it is an efficient low power technique. Second, CKVdd propose a feasible method to generate ramp-Vdd and low-Vdd. This technique would be convenient used to design generic low power digital circuits. Third, normal CMOS circuits show the dynamic power consumption increase proportional to the clock frequency. CKVdd results in a lower-than-usual frequency dependency, it is suitable used to design high clock speed circuits. In investigating constant Vdd for MPEG VLD decoders, CKVdd-circuit reduces 48% of the usual power dissipation and 88% of the usual peak current with small delay penalty.

Biological ion transport processes in proteins have inspired the development of bio-sensors, actuators, photoelectric, and chemoelectric energy conversion devices. These bio-inspired devices use ion transport through a protein energized by biochemical reactions in the protein’s sub-units to perform their engineering function. In an effort to advance the use of biological processes in synthetic systems, a chemoelectrical energy conversion device is demonstrated in this article that uses hydrolysis of adenosine triphosphate (ATP) in ATPase enzyme to generate electrical power. The ATPase enzyme in the device is reconstituted in a bilayer lipid membrane (BLM) and supported on a porous substrate. ATP is dissolved in pH7 buffer and added to one of the chambers in this bicameral device. The transmembrane gradient established by proton transport, resulting from hydrolysis of ATP in the enzyme, is converted into electron flow in an external circuit via silver-silver chloride electrodes placed in the buffer solution on both the sides of the membrane. The chemoelectrical energy conversion of ATP is demonstrated in this article using electrical impedance spectroscopy and load characterization experiments on BLMs supported in the pore(s) of a 25% porous polycarbonate membrane and single-pore silicon nitride chip. In electrical impedance spectroscopy, the change in conductance states of the membrane quantified by specific resistance is used to demonstrate protein activity. The mean electrical impedance of the BLM with ATPase supported on a single pore silicon nitride chip drops from 7 kΩ cm2 to 250 Ω cm2 on adding ATP to one side of the membrane. This change in ionic conductance of the BLM with ATPase in the presence of ATP demonstrates protein activity in the membrane. Impedance analysis of the membranes (BLM, BLM with ATPase) supported in multi-pore polycarbonate substrate demonstrates similar trend confirming ion transport and energy conversion in the membrane. In load characterization experiments, a resistive load of known magnitude is connected to the device and voltage across the membrane and current through the circuit are measured. The current−voltage characteristics of the device resembles a constant current power source and the slope of the response represents the internal resistance of the device. The current through the membrane supported on the single pore substrate is below the range of our data acquisition equipment and hence the device with the porous polycarbonate membrane is used in load characterization experiments. This polycarbonate membrane-based device has an open-circuit voltage of 87.5(±7.5) mV with a specific power output of 1.85 μW/cm2 and an internal resistance of 30.5k(±8.5k)Ω. The theoretical maximum specific power available from the membrane supported on the multi-pore polycarbonate membrane and the single pore silicon-nitride substrate are estimated from the current−voltage response to be 7.65 and 18 μW/cm2.

Microbes, like Geobacters, have inhabited the seafloors around the world since the early days of earth. Such regions are anaerobic and they gain energy by using the widely prevalent iron oxides and organic matters. Because they appear to colonize conducting surfaces that act as sinks of electrons, microbial fuel cells have been shown to convert organic matter to electricity. A microbial fuel cell system has been deployed in Narragansett Bay in Newport, Rhode Island for a year. Currently, the cathode and anode areas are of the order of that of a small wind mill. Measurements have been carried out to determine the marine scaling laws of power harvesting in passive benthic microbial fuel cells. The focus has been on the ocean engineering aspects such as marine scaling laws and the integration of the biochemical and the electronic systems. The characteristics examined are: the relationship of electrode surface area and power produced, the stabilization rates of ionic paths, that is, the effects of location depth of cathodes on stabilization after deployment, the effects of solar and lunar cycles in the Narragansett Bay on the dynamic components of power produced, and the hysteresis effects between periods of active power harvesting and dormancy; the effects of 'on sediment surface' versus 'in sediment' anode deployment have been examined for smaller electrode areas so far. A capacitance model of power consumption and harvesting has been proposed for the marine environment. It is assumed that the primordial benthic microbe laden layer of the earth acts like a giant capacitor. In the microbial fuel cell, this charged benthic layer acts in series with a smaller constant voltage DC power source. This giant benthic capacitance is a result of untapped accumulated charge from the microbes while the DC source originates from the real-time production due to the microbes. Finally, the microbial fuel cell is integrated with a power conversion system to intermittently energize a small incandescent lantern in the NUWC Stillwater Basin located in Narragansett Bay in Rhode Island.

Effects of ion species and their concentration on the iontophoretic transport of benzoic acid through an artificial membrane [poly(vinyl acetate)] were investigated using a 2-chamber diffusion cell equipped with platinum electorodes and a constant current power source. The cathode side of the cell was filled with sodium benzoate solution, and the anode side with pottassium chloride, lithium chloride or teraethylammonium bromide solution. When the molar concentration of sodium benzoate in the cathode side (0.21 M) was the same as the potassium chloride in the anode side, the amounts of benzoate anion and potassium cation permeated through the membrane were greater with increasing current. With an increase in the concentration of benzoate anion in the cathode side and a constant concentration of potassium cation in the anode side, the amount of the former that permeated was proportional to the concentration applied, while permeation of potassium cation remained almost constant at a constant current of 0.2mA. Conversely, an increase in the concentration of potassium cation in the anode side with a constant concentration of benzoate anion in the cathode side resulted in an increase of the permeation of potassium cation and a decrease of that of benzoate anion, respectively, at the same constant current. When potassium chloride in the anode side was replaced by tetraethylammonium bromide, the amount of benzoate anion permeated was increased at a constant current of 0.2mA. These results may be explained by the following theory on the transport number of each ion through the artificial membrane : Current density calculated from ion flux throutgh the membrane was almost the same as that measured by observed current density. The results appeared to indicate that not only an ion species and its concentration in the donor solution but also ion species in the receiver solution should be considered when evaluating the iontophoretic transport phenomena.

The SCR chopper circuit has a very wide range of application in regulating a dc power, and its merits are universally appreciated. Because of its merits, the circuit is most suitable for a nonfailure power source that supplies power from a set of rectifier-batteries to a communication equipment or any other electronic equipment, which requires a dc power source of several output voltages. For such purposes, the output voltage of the circuit should be smooth, accurately regulated, and should respond quickly; at the same time, the efficiency must be high and reliability excellent. This paper offers, firstly, some improvements on the Morgan circuit in order to make it peculiarly suitable for different kinds of practical applications; namely, the smoothing filter is made as small as possible to make the response of the circuit rapid, while the efficiency is kept high. Secondly, data, obtained through the analysis of the operation in the case of the small filter, are presented. Thirdly, the paper shows, as the result of such analysis, that if a saturable current transformer (SCT) is designed properly, the circuit will behave as a self-regulating constant voltage power source. Lastly, it proposes a practical application which has a suitable protecting circuit composed of some simple circuit elements providing against the failure of commutation which causes a severe overvoltage and overcurrent to the load.

A 256-Mbit flash memory has been developed using a NAND cell structure with a shallow trench isolation (STI) process. A tight bit-line pitch of 0.55 /spl mu/m is achieved with 0.25-/spl mu/m STI. The memory cell is shrunk to 0.29 /spl mu/m/sup 2/, which realizes a 130-mm/sup 2/, 256-Mbit flash memory. Peripheral transistors are scaled with memory cells in order to reduce fabrication process steps. A voltage down converter, which generates 2.5-V constant internal power source, is applied to protect the scaled transistors. An improved bit-line clamp sensing scheme achieves 3.8-/spl mu/s first access time in spite of long and tight pitch bit-line. A 1-kbyte page mode with 35-ns serial data out realizes 25-Mbyte/s read throughput. An incremental step pulse with a bit by bit verify scheme programs 1-k cells in 1-V Vt distribution within 200 /spl mu/s. That realizes 4.4-Mbyte/s programming throughput.