Temperature distribution along superconducting power transmission lines

A system of equations, describing the refrigerant's parameters (temperature and pressure) change, during its flow along a heated pipeline, for example along a superconducting power transmission line (SPTL), is derived.The possibility of a temperature maximum existing in the SPTL is found.The possibility, in principle, of ‘isothermal’ SPTL (with constant temperature, along its length) is shown. The most favourable refrigerant parameters' region for SPTL is determined.     

Topographically modified bumper concepts for spacecraft shielding

This paper introduces the concept of Topographically Modified Bumpers, or TMB's, for spacecraft shielding. By milling geometric patterns onto the front face of a flat plate bumper, ribs are formed which create multiple contact points with a projectile during impact. As the generated shock waves travel through the projectile, they overlap causing a superposition and amplification of the shock. Several different TMB designs are considered and results from experimental tests conducted at low ( ) and medium ( ) velocities are presented. At higher velocities, a parametric study of topography dimensions is performed to show optimizing trends. It is found that TMB's with tall ribs, thin backing plate, and small rib separation should perform best.     

基于图像传输的巡线机器人远程控制

基于无线局域网络的图像传输,从提高图像传输的实时性和连贯性着手,完成了巡线机器人远程控制方案的设计和实验。图像传输采用并行的处理方式,大大提高了传输的速度;分析了影响图像传输质量的因素,提出了增强图像传输品质和连贯性的方法,实现了在监控端根据图像方便地对机器人实施控制的设计,并通过实验验证了图像传输的实时性和远程控制方案的有效性。     

Counterflow cool down of cryogenic power transmission lines

Cool down of cryogenic power transmission lines which use a counterflow cooling arrangement is investigated analytically, numerically, and experimentally. The results demonstrate that the cool down time, t_cd, is given by fx463-1 where U is the overall heat transfer coefficient between the go and return streams, l the length, and ? the mass flow rate. The accuracy and utility of a previously reported numerical cool down model are also confirmed by comparison with experiment.     

Multipole shaking field for magnetic shielding

The authors propose a novel method of magnetic shaking for enhancing the performance of ferromagnetic shielding. The method employs a new type of planar coil which generates higher-order multipole fields. Ferromagnetic shielding walls placed close to the planar coil can be suitably shaken by its field, whereas unwanted leakage of the shaking field into the shielded space can be avoided due to the highly localized nature of the multipole field. The configuration of the novel planar coil is similar to a square mesh in which alternating cells are clockwise or counterclockwise current loops. The shaking effect on the shielding performance and the leakage of the shaking field are evaluated using cylindrical shields and compared with results obtained with toroidal shaking coils     

Non-sinusoidal power theory in a power system having transmission lines with frequency-dependent resistances

Recently power systems suffer from harmonic pollution, and the resistances of transmission lines are frequency-dependent due to eddy currents and the skin effect. This frequency dependency should be taken into account, so that apparent power and the power factor accurately reflect transmission loss under non-sinusoidal conditions. A generalised power theory that is applicable to a power system whose transmission/distribution lines have frequency-dependent resistances, and where a unity power factor is given as the minimum loss condition is proposed     

Magnetic field dependence of shielding current density in Y-Ba-Cu-O rings at 77 K

A method for contactless measurement of the shielding critical current density and its dependence on the external magnetic field is described and analyzed. The obtained values are compared with those measured resistively on two different samples. It is shown that the shielding critical current densityJ _cs and the intergranular transport current densityJ _cr are identical if the measurement conditions are similar. A degradation ofJ _cs measured in the external field with AC ripple has been observed.     

Cool-down of superconducting power transmission lines with single phase helium

Numerical solutions of the one-dimensional conservation equations of fluid flow are given for the problem of the cool-down of superconducting power transmission cables. By including the continuity and momentum conservation equations, both time dependent and spatially dependent mass flow may be treated. The equations are applied to specific designs in which one stream of helium fills the bore of a cable while a second fills the space between the outside of the cable and an enclosing pipe. Both co-current flow and counter flow are treated with a variety of boundary conditions. Temperature profiles as a function of time, and total cooldown times are given.     

Magnetic shielding using high-temperature superconductors

Magnetic shields of various high-temperature superconductors, YBa₂Cu₃O_7−x (YBCO), YBa₂Cu₃O_7−x -Ag composites (random inclusions as well as non-random coatings) and Bi₂Sr₂Ca₂Cu₃O _x (BSCCO) were prepared by uniaxial as well as isostatic compression with various dimensions. The shielding properties were measured at 77 K for dc and ac magnetic fields in the range of frequencies from 100 Hz to 10 kHz. The critical penetration field (CPF), defined as the value of the applied magnetic field at which a detectable field was observed inside the cylinder, varied from cylinder to cylinder and also with the ageing of the cylinders in the case of YBCO shields. The highest value of CPF was 16 G at 77 K for YBCO shield prepared by isostatic compression. Even though the stability of BSCCO shields with respect to ageing is good, the CPF values are very low compared to those for YBCO. Detailed studies were performed in the case of YBCO shields. The CPF decreased as a function of time over a period of 90 days. The CPF decreased as the frequency of the applied field was increased. The wave form of the field inside the pot for a sinusoidal applied field was highly distorted and showed the presence of higher harmonics with appreciable amplitude. The wave form was Fourier-analysed to yield the field inside the shield along with the harmonics. The shields with Ag addition seem to give better performance at high fields.     

New approach to adaptive single pole auto-reclosing of power transmission lines

In this study, a novel digital algorithm is introduced for recognition of arcing (transient) faults and determination of dead time for adaptive auto-reclosing. The algorithm distinguishes between arcing and permanent faults by using the zero sequence voltage measured at the relaying point. If the fault is recognised as an arcing fault, then the third harmonic of the zero sequence voltage is used to evaluate the extinction time of secondary arc and to initiate reclosing signal. The proposed algorithm uses an adaptive threshold level and therefore no significant adjustment is needed for different transmission systems. Moreover, its performance is independent to fault location, line parameters and the system pre-fault operating conditions. The algorithm has been successfully tested for various faults and operating conditions on a 400 kV overhead line using the electro-magnetic transient program (EMTP). The test results have demonstrated validity of the algorithm in determining the secondary arc extinction time and blocking unsuccessful automatic reclosing during permanent faults.     

覆冰输电导线舞动的仿真分析

基于考虑输电导线垂向和扭转振动耦合的两自由度模型,利用多尺度法和数值分析方法,分别就系统参数和环境参数对临界风速和舞动振幅的影响进行了理论和仿真分析,并讨论了扭转振动分岔导致的可能复杂运动。利用零解的稳定性条件,得到了覆冰厚度、初始覆冰角度、来流密度、导线的初始张力和单位长度质量以及横向阻尼比对临界风速的影响曲线。基于解析和数值仿真的结果,分别给出了风速、覆冰厚度、来流密度、导线的初始张力和单位长度质量以及横向阻尼比对舞动振幅的影响曲线。对扭转振动分岔的研究表明,风速变化时,(零解失稳导致的)周期运动的分岔可导致倍周期运动和概周期运动等复杂运动形式。     

Calculation of the ion flow field of AC-DC hybrid transmission lines

A simulation method to calculate the ion flow field of AC-DC hybrid lines is proposed. The charge simulation method (CSM) and the finite element method (FEM) are applied to calculate the electric field, while a time-dependent upwind differential algorithm is applied to calculate the migration of space charges. Furthermore, a variable time-step discretisation method is introduced to accelerate the computational process. The calculation results of ground level electric field and ion current density of a reduced-scale AC-DC hybrid line model are compared with previous experimental and calculating works. Satisfactory agreement is obtained between our calculation results and the experimental ones. As an application, the ion flow field of a hybrid 1000 kV AC/800 kV DC transmission line is simulated and analysed.     

Electric field analytical formulas for single-circuit power lines with a horizontal arrangement of conductors

Analytical formulas are derived for the calculation of the electric field at any point in the vicinity of single-circuit power lines with a horizontal arrangement of conductors. The analytical formulas are appropriate for the assessment of public exposure as they allow direct calculation of the electric field. They are also suitable for assessing the dependence of the electric field on electric and geometric parameters. Initially, an accurate formula of the electric field is derived by using a proposed method. This method is based on the use of multipole expansions and double complex numbers for the representation of the electric field vector and symmetrical components analysis for the conductors' charges. Based on the accurate formula, a sensitivity analysis of the electric field is carried out with respect to the various parameters. Two simpler approximate formulas are finally developed, which provide the electric field with satisfactory accuracy in certain areas in the vicinity of the lines.     

Evaluation of long-term exposure to the magnetic field produced from power lines

For some years, the Italian Regional Agencies for Environmental Protection (ARPA) have been engaged in the evaluation of long-term exposure to 50 Hz magnetic fields for receptors close to power lines. In this study, experiences and procedures are suggested in order to estimate long-term exposure levels in the case of magnetic induction produced contemporaneously by different power lines. In the simplest cases, the influence of a single line is evaluated by verifying the correlation factor between the variation of the current in each line and the measured magnetic induction. When the prevalence of a single line is not clear and systematic, much additional data (voltage, current, phases and geometrical configuration) have to be considered. The case of a school in Livomo near two lines. placed on the same supports, is described and a range of the most probable exposure values is given, corresponding to the phase change of the two current fluxes.     

Inductive coupler for contactless power transmission

To provide power to a moving vehicle without using contacts, a transformer is presented, consisting of a fixed primary winding inductively coupled to a moving secondary winding. The primary winding along the whole trajectory of motion is configured such that its magnetic stray field and its self inductance are limited. This winding transfers power to the moving coupler containing the yoke and the secondary winding. The secondary winding is a resonant circuit, with a rectifier and the motor of the vehicle as load. To describe the energy transmission system, a numerical model is developed that combines an electrical circuit with a 2D nonlinear finite element model. This model is used for an optimisation of the design. For the optimised configuration, an experimental setup is built to validate the model by measurements. A sensitivity analysis is carried out concerning the primary current, the supply frequency, the position of the secondary winding relative to the primary winding and the number of vehicles coupled with the primary winding.     

Basic power system reliability concepts

The basic function of an electric power system is to satisfy the system load requirements as economically as possible and with a reasonable assurance of continuity and quality. In order to achieve the required degree of reliability, power system managers, designers, planners and operators have utilized a wide range of criteria in their respective areas of activity. Initially all of these criteria were deterministically based and many of these criteria and associated techniques are still in use today. The basic weakness of deterministic criteria is that they do not respond to nor do they reflect the probabilistic or stochastic nature of system behavior, of customer demands, or of component failures. These factors can be incorporated in a probabilistic approach to electric power system reliability assessment and a wide range of techniques and criteria are available. This paper presents a summary of basic power system reliability concepts and briefly illustrates their application in system planning and design.     

Integrated transmission lines for magnetic thin film memories

The pulse transmission properties of integrated strip lines are examined for a structure that has been proposed for application in a large capacity, high-speed magnetic film memory. The lines are vacuum-deposited onto an insulated ground plane and are characterized by low impedances. Previous analyses, such as those given by Eastman and Chang, McQuillan, and Harloff, examine the effects of skin-depth losses in lines which are assumed to be perfectly terminated. In general, this type of analysis is insufficient for lines which are lossy and which have resistive terminations. The present analysis considers skin-effect losses in the ground plane and in the strip conductors separately and also includes the effect of reflections from resistive terminations. The influence of heavy magnetic loading of the transmission line due to the presence of magnetic film devices is examined in detail. A computer program, similar to the one described by Bertin, has been used to obtain numerical results for the pulse response of the transmission lines. Data are presented which demonstrate the influence of line geometry and material properties.     

Application of fiber-optic bragg grating sensors in monitoring environmental loads of overhead power transmission lines

We demonstrate the capability of using fiber-optic sensors for measurements on environmental loads on a high-power, overhead transmission line. A trial system with three Bragg gratings, including a temperature reference, was installed on a 160-m span of a 60-kV line. An interrogation system with a tunable distributed Bragg reflector laser source was used. Several measurements of the induced loads on a conductor were recorded in various wind conditions. In particular, aeolian vibrations were frequently observed, and several measurements of this phenomenon were made. The results correlate well with simple theoretical predictions and visual observations.     

Hypervelocity testing of advanced shielding concepts for spacecraft against impacts to 10 km/s

Experiments have been performed on NASA state-of-the-art hypervelocity impact shields using the Sandia Hypervelocity Launcher (HVL) to obtain test velocities greater than those achievable using conventional two stage light-gas sun technology. The objective of the tests was to provide the first experimental data on the advanced shielding concepts for evaluation of the analytical equations (shield performance predictors) at velocities previously unattainable in the laboratory, and for comparison to single Whipple Bumper Shileds (WBS) under similar loading conditions. The results indicate that significantly more mass is required on the back sheet of the WBS to stop an approximately flat-plate particle impacting at 7 km/sec and at 10 km/sec that the analytical equations (derived from spherical particle impact data) predicted. The Multi-Shock Shield (MSS) consists of four ceramic fabric bumpers, and is lighter in terms of areal density by up to 33%, but is as effective as the heavier WBS under similar impact conditions at about 10 km/s. The Mesh Double Bumper shield (MDB) consists of an aluminum wire mesh bumper, followed by a sheet of solid aluminum and a layer of Kevlar^® fabric. It provides a weight savings in terms of areal density of up to 35% compared to the WBS for impacts of around 10 km/s.