摘要:

AbstractThe authors have been developing extruded polymer insulated superconducting power cables. Dielectric loss in electrical insulation cannot be ignored in superconducting cables since conductor loss in the cables is minimal. Studies so far show that ethylene‐propylene rubber (EPR) is suitable as an electrical insulating material in the extruded polymer insulated superconducting cable design because it demonstrates excellent mechanical and relatively good electrical qualities at a cryogenic temperature. Widely used EPR includes some kinds of additives; however, their effect on tan δ of EPR at cryogenic temperature remains unknown.The effect of additives such as crosslinking agent and fillers on tan δ of EPR was examined at temperatures of between 4.2 K and 300 K. Thickness dependence of tan δ was also measured using EPR films of different thickness and an extruded EPR insulated cable sample. The results show that additives increase tan δ of relatively thin films of EPR even in the cryogenic temperature region; however, they do not have an intolerable dielectric loss in comparison with conductor loss and heat inflow of the superconducting cable. The remedy to tan δ increase due to the polymer contraction relative to shielding wires has been estab

ISSN:0424-7760    

DOI:10.1002/eej.4391160201

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1996

数据来源: WILEY

摘要:

AbstractAg‐sheathed Bi‐2223 superconducting oxide conductors with a critical current density of 66,000 A/cm2at 77 K and coils that have generated magnetic fields of approximately 1.5 Tesla at 4.2 K have been successfully developed. The conductors consist of piled‐up bundles of four to six sheets of element tapes with transport currents of approximately 200 Amps. The core size limit (CSL) prevented the production of single conductors that had the capacity to transport a current as high as 200 Amps at 4.2 K. Until now, it was difficult to develop a conductor having both high critical current density and the capability of transporting a large amount of current simultaneously. The role of CSL in large scale applications of high temperature superconducting oxides is disc

ISSN:0424-7760    

DOI:10.1002/eej.4391160202

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1996

数据来源: WILEY

摘要:

AbstractPlasma spraying is gradually becoming an important technique in the materials processing field, its primary feature being the ease by which the area or thickness of the deposited film can be controlled. High‐temperature plasmas to heat ceramic particles with a high melting point have in the past been generated by dc power sources. Applying a pulsed plasma using a high current of the order of tens of kA is proposed for the plasma spraying of refractory materials.Numerical analysis of the short‐time heating and acceleration of the particles showed that alumina particles 20 μ in diameter reach the complete melting state after a residence time of 83 and 9.6 μs and a flying distance of 5.8 and 0.5 mm, when the temperature of the pulsed plasma is assumed to be 10,000 and 20,000, respectively, with a constant plasma velocity of 200 m/s.Experiments are then performed using a specially developed plasma torch with a pulsed discharged current of 24 kA peak and a duration of 400 μs. This current is provided from a capacitor with a maximum stored energy of 14 kJ. The simultaneous, in‐flight measurements of the temperature, velocity, and size were taken for alumina particles injected into the pulsed plasma. Results show that the surface temperature of the alumina particles reached values as high as 2700 to 3300 K, which was considerably higher than the melting point. Particle velocity was found to be around 200 m/s, which is comparable to that of dc plasma. The experimental results confirmed the applicability of pulsed plasma for the spraying of refractory m

ISSN:0424-7760    

DOI:10.1002/eej.4391160203

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1996

数据来源: WILEY

摘要:

AbstractThis paper describes an optimal load‐shedding policy based on quadratic programming using only some power system state variables after severe generation outages. When generation outage is severe, the imbalance between supply and demand causes a declining frequency. Utilities generators cannot be operated excessively at frequencies above normal. Some of the load must be shed to prevent system damage. Optimal load‐shedding policies have been studied using all state variables of the power system. However, in real power system operations, it is difficult to obtain remote information in that fraction of a second following generation outages.An optimal load‐shedding method is constructed in this research using quadratic programming (QP) under the assumption that all power system state variables can be available. However, these state variables cannot be easily accessible (as already mentioned). A suboptimal load‐shedding scheme based on the local state variables is then studied. The proposed load‐shedding method is based on the aforementioned optimal one. The change in line‐power flows and the amount of generation power outage are used as accessible data at each load point. Incorporating this local information into the optimal loadshedding method based on the QP method, the proposed load‐shedding method is established here.The effectiveness of this proposed method is illustrated by two examples and simulation results on a model power system show that the method i

ISSN:0424-7760    

DOI:10.1002/eej.4391160204

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1996

数据来源: WILEY

摘要:

AbstractA new power control method is proposed which is suitable for the MHD power generation system which consists of nonequilibrium disk generator and line‐commutated inverter. The thermal input of the generator is controlled by changing the inlet stagnation pressure, whereas the seed mass flow rate instead of the inlet stagnation pressure is utilized as a manipulated variable of power control system. It is possible that the proposed method can realize both high performance for part load operation and fast output power control of which the time constant is much shorter than the response time of thermal input. Numerical simulations are carried out for the MHD power generation system connected to infinite bus and then it is confirmed that the system is stable and shows excellent power control performanc

ISSN:0424-7760    

DOI:10.1002/eej.4391160205

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1996

数据来源: WILEY

摘要:

AbstractA converter‐fed adjustable‐speed generator‐motor for a pumped storage power plant has been developed in order to contribute to the automatic frequency control on an electric power system during pumping operations, as well as to improve the efficiency of turbines during generating operations together with stability. This system consists of a reversible pump turbine, a generator motor, cycloconverter and control units. The rotor of the generator motor, which is coupled directly with the turbine runner, normally is operated at a rotating speed which is equal to the difference between the frequency of the electric power system and that of the rotor current. In the state that a fault has occurred in the primary circuit of the generator motor, the adjustable‐speed generator‐motor fault current differs from that of a conventional synchronous generator motor.This paper describes the fault current analysis results for the condition when a sudden short‐circuit is made on the primary side of the generator motor, and collates the results with the field test and EMTP simulat

ISSN:0424-7760    

DOI:10.1002/eej.4391160206

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1996

数据来源: WILEY

摘要:

AbstractA method of assessing dynamic stability of large‐scale power system by Rayleigh's quotient is proposed. One‐machine infinite‐bus systems show damping torque characteristics similar to diagonal components of operational transfer matrices for original multimachine systems, which means that design of PSS with one‐machine systems controls those components. An expression for damping constants of oscillation modes is derived based on an energy function and its time derivative for a simplified system representation. This expression demonstrates that oscillations do not necessarily become unstable even if there are some generators with negative damping; and the effect of damping torque is determined by eigenvectors. The expression is generalized with Rayleigh's quotient, and a method of estimating eigenvalues of large‐scale power systems is proposed. With this method, approximate eigenvalues are refined to accurate eigenvalues. Only a specified number of eigenvalue analyses are required irrespective of the number of generators, hence much calculation is saved. Finally, this method is applied to a 107‐machine system to verify its ef

ISSN:0424-7760    

DOI:10.1002/eej.4391160207

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1996

数据来源: WILEY

摘要:

AbstractIt is necessary to take into account the effects of mutual couplings when unbalanced faults are to be calculated in transient stability study.When the method based on symmetrical coordinates is applied to faults calculation, it has been necessary in the conventional method to construct the zero sequence network, the configuration of which is more complex than other networks so far. Then, the application of faults calculation including mutual couplings to the analysis on the bulk power system model has been considerably difficult for a long time.This report presents a newly developed method to calculate the effects of mutual coupling in multifaults calculation under the same configuration and with no change for all sequence networks. The validity of the proposed method was verified through the application of transient stability analysis for power system models.

ISSN:0424-7760    

DOI:10.1002/eej.4391160208

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1996

数据来源: WILEY

摘要:

AbstractThis paper proposes an application of genetic algorithm (GA) to contingency screening in power systems. The contingency selected by the GA‐based screening method is the double line outage which has the risk of causing transient instability. Generally, the contingency screening problem including multiple outage can be interpreted as the combinatorial optimization one for searching the combination of single outages which makes the system insecure. Therefore, GA which is one of the probabilistic searches for combinatorial problems, is applicable for such contingency screening problems. In the GA‐based contingency screening method, a double line outage is represented as a chromosome. The fitness of the chromosome for environment is defined by using the transient energy function of power systems. The new procedure for avoiding the loss of important outage during contingency screening is developed and embedded in the proposed method by using the theorem of schema for GA. The validity of the proposed method is confirmed by applying it to a 6‐machine 30‐bus 40‐line system. The result shows that the GA‐based contingency screening has the potential for pr

ISSN:0424-7760    

DOI:10.1002/eej.4391160209

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1996

数据来源: WILEY

摘要:

AbstractExternal magnetic disturbance influences the quality of the image obtained with a magnetic resonance imaging (MRI) system because it causes magnetic resonance frequency drift. When a superconducting magnet in persistent current mode operation is exposed to external magnetic disturbance coil current varies to keep linkage flux constant. Conventional analysis is usually formulated with constant current and does not provide correct evaluation of drift of magnetic field in the magnet. This paper proposes a method of numerical analysis of magnetic field considering conservation of linkage flux in a superconducting magnet under a persistent current mode operation. This method is applied to evaluate drift of the magnetic field in a superconducting magnet for magnetic resonance imaging due to external magnetic disturbance. Analysis results show good agreement with the experimental results.

ISSN:0424-7760    

DOI:10.1002/eej.4391160210

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1996

数据来源: WILEY