High-speed current limiters

Current limiters based on the high-field saturation of electron-drift velocity in germanium have been fabricated by making two closely spaced ohmic contacts to a surface n-layer diffused into Ge. In operation, current flows from contact to contact through the n-layer. Current saturation begins at a voltageV_{s} = E_{s} d, where d is the contact spacing and Esis the field for saturation of electron drift velocity. Generally, in order to work at reasonable signal levels (volts and milliamperes) a structure small in all dimensions is required. A diffused-layer structure provides a convenient way of meeting this requirement and, at the same time, has, first, a high surface-to-volume ratio which facilitates heat removal and, second, a p-n junction which minimizes conductivity modulation by removing avalanche-generated holes. Limiters withd cong 2µ, and 10 µ × 10 µ contacts, were fabricated on Sb-diffused layers of 10³-Ω sheet resistance and ∼10¹⁷cm^-3surface concentration. Limiting current was ∼2.5 mA and limiting extended from ∼2 to 16 V. The conductance in the limiting range was ∼40 µmho and a contact-to-contact capacitance was ∼0.1 pF.     

Analysis of a passive superconducting fault current limiter

An analysis was carried out to predict the forced response of the equivalent circuit of an innovative superconducting fault current limiter (FCL). The FCL employs two superconducting coils with differing critical currents wound noninductively. The analysis shows that to reduce the voltage drop under normal operating conditions, the coupling coefficient should be kept fairly high (k>0.90). For a given coil configuration (ωL¯=constant), the limiting capability of the device increases with the resistance R of the trigger coil up to a certain value of R, then further increase in R changes very little the limiting capability of the FCL. However, further increase in R can reduce the heat generation rate in the device which will help alleviate the problem of relatively long recovery time. The fault current predicted from estimated values of R compares fairly well with results of an experiment reported in the literature. The discrepancy between the predicted and measured current is due mainly to the uncertainty in the estimated resistance R, because both the purity of Cu and the percentage of Cu in CuNi are not known. By varying the ratio of NbTi-Cu-CuNi in the matrix, the purity of Cu, and the percentage of Cu in CuNi various values of R, can be achieved, which should help to alleviate the problem of excessive Joule heating and recovery time     

Sidegating in GaAs current limiters

Potentials of several volts or less applied to the sidegate contact of an ion-implanted GaAs current limiter cause a decrease in the channel current, in both the linear and Saturated modes of operation. Larger sidegate potentials have the opposite effect due to the addition of a large substrate leakage current, and cause an apparent shift in the impact-ionization threshold of the channel current.     

Transforming transformers [superconducting windings]

Use of high-temperature superconducting (HTS) windings may soon turn power transformers into compact high-performers on good terms with the environment. The potential for HTS transformers is being examined in major design and hardware development programs by several teams of engineers and scientists worldwide. The team to which the authors belong is led by Waukesha Electric Systems. The Waukesha-led team has conducted a series of reference designs concentrating mostly on a 30-MVA, 138-kV/13.8-kV transformer rating. This rating is representative of a medium power transformer class foreseen as comprising about half of all US power transformer sales in the next two decades. Two of these designed are compared with a 30-MVA conventional oil-filled transformer typical of this class. Each uses a different commercially successful and reliable type of refrigeration. Since reliable operation is paramount, both transformers are designed to have a several-day on-board refrigeration reserve that lets them operate continuously with no interruption of service to customers     

Low-frequency noise in GaAs current limiters

The theory of the one-dimensional diffusion noise is modified and applied to explain the low frequency noise in GaAs current limiters. The devices show noise spectra with $\text{f}{}^{\mathrm{<mtext>?</mtext><mtext>1</mtext><mtext>2</mtext>}}$ and $\text{f}{}^{\mathrm{<mtext>?</mtext><mtext>3</mtext><mtext>2</mtext>}}$ branches. The turn-over frequency increases with the applied voltage and temperature increase. The diffusion constant depends exponentially on temperature and the activation energy varies with electric field because of Poole-Frenkel effect. The experimental data confirms the temperature and voltage dependence of noise. The calculated activation energy is very close to the value reported in connection with leakage current in GaAs MESFET. Since the diffusion constant differs by many orders of magnitude from diffusion constant of electrons or ions one-dimensional diffusion of ions along dislocation lines is proposed. However, the mechanism of diffusion is not fully understood yet.     

The evolution of quenches in superconducting parallel bifilarwinding by fault current

A superconducting parallel bifilar winding with one strand in each branch was tested and analyzed. When quenched by a fault current, it showed the so-called “fast quench” and the experimental results explained the increase of resistance. Also the experiments confirm that the current redistribution of magnetically coupled strands develops new normal zones after the initial quench, and that the size of normal zones at an early stage increases almost linearly as dI/dt increases. By these results, we could explain the resistance rise of the superconducting parallel bifilar winding when quenched by a current above the critical value     

Possible ballistic effects in GaAs current limiters

Two-terminal N+NN+ current limiters have been fabricated in GaAs by selective ion-implantation of the N+ and N regions into undoped substrate material. Voltage-current characteristics have been measured at 77°K, 300°K and 400°K and reveal current limiting properties above about 0.8 to 1.0 volts at all three temperatures. In the voltage range below 0.7 volts a relation of I ~ V^{3/2}was observed for the devices operating at 77°K. Such a variation has been suggested as being indicative of ballistic motion of electrons when taking into consideration that the distance between the N+ regions is in the range of 0.5 to 0.8µm and the doping concentration of the N region is about 10¹⁶cm^-3.     

Adrenaline for the grid [superconducting synchronous condenser]

This paper presents a novel superconducting device that provides essential voltage support. A superconducting synchronous condenser is basically an electric generator that is optimized and operated to act like a capacitor or inductor, providing the grid with reactive power, an abstruse but absolutely vital element in any power system relying on alternating current. The condenser quickly provides large quantities of reactive power to support grid voltage - as much as double the rating for up to a minute from a single unit and to supply controllable continuous-duty reactive compensation for optimizing grid-transmission effectiveness. It promises to do the job more effectively, and possibly more economically, than other solution, especially for industrial voltage flicker applications and voltage regulation of transmission lines with large isolated nodes.     

The ties that bind [back story]

Some 2000 years ago, North Africa was an integral part of the Roman Empire. Those early ties with Europe are apparent from the Roman ruins that remain there, including those of Leptis Magna, in Libya. The spectacular theater of this ancient city was one of the few tourist sites that Contributing Editor Peter Fairley [above] was able to take in during his journeys.     

Operational characteristics of a flux-lock-type high-T/sub c/ superconducting fault current limiter with a tap changer

The authors investigate the operational characteristics of a flux-lock-type high-T/sub c/ superconducting (HTSC) fault current limiter (SFCL) with a tap changer which could adjust the number of turns of the third winding. In the case of conventional flux-lock-type SFCL, the phase adjusting capacitor is connected in series with magnetic field coil to adjust the magnetic field applied to the HTSC element in phase with the current flowing through the HTSC element during a fault time. However, the current flowing at the third winding, which is connected with magnetic field coil, affects the fault current limiting characteristics. To analyze the influence of current flowing at the third winding on the fault current limiting characteristics, the fault current limiting characteristics of the flux-lock type SFCL, whose inductance of coil 3 could be adjustable through a tap changer, are investigated through the experiments and the computer-aided simulations. The relation of line currents flowing into the flux-lock-type SFCL during a fault time and numbers of turns of a tap changer is drawn.     

Solid-state circuit breakers and current limiters for medium-voltage systems having distributed power systems

State-of-the-art mechanical circuit breakers in medium-voltage systems allow a safe handling of short-circuits if the short circuit power of the grid is limited. Using delayed turn-off times, the circuit breakers can be coordinated with lower level protection gear. Hence, a high availability of the grid can be guaranteed. However, during a short-circuit a significant voltage sag can be noticed locally in the medium-voltage grid. Sensitive loads such as computers will fail even if the voltage returns within a few seconds. A semiconductor circuit breaker, however, is able to switch fast enough to keep voltage disturbance within acceptable limits. The optimization and selection of power electronic switch topologies is critical. In this paper, different semiconductors are briefly compared considering the requirements of a solid-state switch integrated into a 20-kV medium-voltage grid. Based on these semiconductor characteristics, various switch topologies are developed, which are compared under technical and economical aspects. It is shown that solid-state circuit breakers offer significant advantages when compared to present solutions and can be used in today's medium-voltage power systems.     

Find that leak [digital signal processing approach]

Digital signal processing can help cut losses from leaking water mains. The authors explain how one new development, sponsored by Thames Water, is helping it in the battle to locate leaks more quickly and reliably. From the outset their aim was to develop a `virtual' instrument approach based on a standard personal computer (PC). In this way, as the knowledge base increases and further improvements are made, software upgrades can be implemented on the same hardware platform. The use of a standard commercially available sound card fits in with this approach. In practice the software development proceeded in two main stages: an initial programme of algorithm development, followed by real-time algorithm implementation and graphical user interface (GUI)     

Light that acts like `natural bits' [solitons]

Solitons, ultrastable light pulses that could revolutionize long-haul, fiber-optic communications and perhaps even optical switching, are discussed. Two classes of solitons are described. Temporal solitons, pulses of light that resist a medium's chromatic dispersion, or tendency to broaden them in time, have been known for a while. They offer the possibility of all-optical communication systems transmitting signals thousands of kilometers. Erbium-doped fiber amplifiers provide an all-optical form of soliton amplification that brings such lightwave systems closer to reality. Spatial solitons, which are actually light beams that travel through a planar waveguide without self-focusing or spreading, were observed for the first time last year and might eventually prove useful for optical switching     

Simulation studies of biomagnetic computed tomography [current flowidentification]

Simulation studies were performed on phantom models of electrical sources. As a first step toward the development of an imaging algorithm, the simplified problem of identifying the shape and direction of current flow in a planar surface was addressed. The problem was formulated by identifying a space in which the image was to be reconstructed. The space was segmented into a grid. Each grid space represented a current element. The magnetic field at a sampling point due to the current elements was computed using the Biot-Savart law. Since there were many more current elements than sample points, the problem was underdetermined and had an uncountable number of solutions. The projection theorem was used to define an analytic solution for the magnitude and orientation of the current elements in the grid space. The accuracy of the resulting image was determined by comparing it with the known location of the sources shows that shape of the filamentary current flow can be imaged with this technique. The resolution of images based on the sampling of the field, number of voxels in the reconstruction space, and noise is also analyzed     

Subthreshold current reduction for decoded-driver by self-reversebiasing [DRAMs]

Analytical expressions are presented for subthreshold current reduction in a decoded-driver by self-reverse biasing, which is inherently required for low-voltage, low-power, high-speed DRAM's for portable equipment. The scheme involves inserting a switching MOS transistor between the driver circuits and its power supply line. The subthreshold current of the decoded-driver is reduced to the order of 10 ^-3 in the practical temperature range (250-350 K) with 254 mV of self-reverse biasing voltage, while the delay time is only 3% more than in conventional schemes. The transition time of 1 ms from the operating state to the low subthreshold current state is sufficient to reduce the subthreshold current. The rapid recovery time of 1 ns from the low subthreshold current state does not interrupt the start of normal operation. The subthreshold current reduction was confirmed experimentally using a test chip fabricated with 0.25-μm technology     

Pulling the plug on the current drain [threshold switching]

Power management is one of the main challenges to continued development of large-scale integrated circuits. In particular, the offstate, or standby leakage, is becoming a significant fraction of total power consumption as gate dimensions continue to shrink. A review of the literature identified many possible solutions in the application of threshold-switching devices. This data has apparently never been summarized in one place, suggesting a need for this article. This article discusses the capabilities of classical amorphous semiconductor switches, and more recent advances in silicon, III-V materials, and organic semiconductors that all exhibit threshold-switching properties. Applications and future prospects for the development of more energy-efficient devices are discussed. The long-term vision is that conductors themselves can be engineered to dynamically sense and adapt their conductivity to active or passive states as required.     

Magnetic-shield-type fault current limiter equivalent circuit

In order to investigate how a superconducting fault current limiter (SFCL) can enhance the performance of a power system, an accurate circuit model of the device needs to be introduced in power system simulators. In this paper, we present a finite-element numerical model to calculate the time evolution of the voltage across a magnetic-shield-type SFCL, when it is connected to an external circuit. The calculation of the voltage is carried out by using the energy conservation law, and requires the calculation, at any instant, of the current density distribution inside the superconducting tube and magnetization distribution inside the ferromagnetic core of the device. These distributions are determined by means of two coupled equivalent electric and magnetic circuits, whose topology and components are obtained through the spatial integration of quasi-static form of Maxwell equations. Comparisons between numerical and experimental results are shown.     

故障电流限制器中故障传感器的改进设计与实现

故障电流限制器电流迅速提升,会产生短路问题,需要通过故障传感器检测电流限制器中的短路故障,确保故障电流限制器的顺利运行。设计并实现故障电流限制器中的故障传感器,给出故障信号采集模块设计过程,通过引进LM293差分比较器的整形电路进行整形,再传递到MSC1210单片机进行变换,并通过故障诊断模块进行分析,将分析结果反馈到上位机中,对电流限制器的故障进行处理。给出了软件设计过程。实验结果表明,所设计的传感器可及时获取反映故障电流限制器的故障信息信号,故障信号采集效率和空间性能都较高。     

An instrument that isn't really [Laboratory Virtual InstrumentEngineering Workbench]

The concept and development of LabVIEW (Laboratory Virtual Instrument Engineering Workbench), a virtual instrument (VI) that provides a simple, graphical way to set up and run numerous instruments on a Mackintosh PC, are described. The first concept basic to LabVIEW had its roots in a large test system comprised of programmable signal sources compatible with IEEE-488 interface specifications, and switching matrices and measurement instruments controlled by a minicomputer. The test system's strength was its flexibility, which it owed to its several levels of user interfaces. This flexible configuration was refined into the notion of instrumentation as a hierarchy of virtual instruments in which all VIs at all levels had the same type of construction. Another characteristic was that each VI have its own user interface. The final conceptual element was the use of loop data-flow structures as the technique used by an engineer to construct and run his or her own VIs. Problems encountered in the development of LabVIEW and the solutions used are discussed     

The atomic fortress that time forgot [plutonium production plant]

The future of the world's first full-scale nuclear reactor, called the B Reactor, remains uncertain in the face of the US Dept. of Energy's effort to clean up the radioactive and chemical contamination at Hanford. Established by the Manhattan Project during World War II, the B Reactor is part of the Hanford site, a 1500-square kilometer plutonium production complex in the state of Washington. A study has been commissioned to assess the possibility of converting some of the Manhattan Project's historic sites into parks and museums in order for future generations to learn about the Project and its impact on world history.