The electric power distribution systems in densely-populated areas have been gradually replaced from 6kV-class systems to 20kV-class ones. It is likely that the 20kV-class systems become more common because the systems are more suitable to supply power in the cities, especially when the load demand become heavier in the future. But, in order for the 20kV-class systems to be really promoted, the reduction of the size and the cost of the 20kV-class equipments are crucial. Including this issue, we describe the problems and solutions for 20kV-class systems promotion.
Power system is a large dynamic system, which includes a lot of nonlinear elements. According to the nonlinear analyses using the Hopf bifurcation theory, it can be detected that a limit cycle exists around an operating point, which may affect the global stability of power system significantly. The authors have presented a numerical method to analyze the nonlinear characteristics in power systems by observing the power swing after some perturbation where the coefficients of nonlinear terms are determined by the least mean square method. In this paper the method is modified for the application to a longitudinally intercon-nected power system including an excitation system, and the influence of the excitation voltage limiter on the non-linear phenomena of the whole power system can be detected by some numerical analyses.
In this paper, a reasonable method has been proposed to determine the maintenance schedule of turbine in thermal power stations. Due to a fact that many failure data do not exist, we paid attention to the maintenance replacement rate, and the dimensional reduction method was used. Proper maintenance interval based on future operation rate was decided by the relationship between operation rate and maintenance interval. The results of analysis can reduce the maintenance cost, meanwhile keeping current reliability. The proposed method will be adopted by Kyushu Electric Power Co. from April in 2002 to determine the maintenance schedule of turbine in thermal power stations. Moreover, it can be also applied for any other systems.
In selection of the neutral grounding method in a three-phase four-wire 11.4kV distribution system, it is necessary to take into consideration voltage rises in lower-voltage line and induced voltage on communication line in case of ground fault. In this paper, we perform EMTP/ATP simulation to select the optimal neutral grounding method and neutral grounding resistance with consideration to above mentioned conditions. The accuracy of the modelling procedure and simulation result is confirmed through verification test performed on test distribution line. As a result, we found 1) that the 11.4kV distribution system should be associated with a resistance single grounding method; and 2) that the neutral grounding resistance should be above 20Ω.
A method for determining proper maintenance intervals for equipment based on the relationship between maintenance cost and reliability was proposed. The optimal maintenance interval decision to reduce maintenance cost was given by sacrificing system availability within allowance value. Ordering list of element availability differences between neighboring average maintenance interval types was constructed and used for optimal maintenance interval decision process. Since there were few data on failure, dimensional reduction method was adopted to approximate availabilities of elements. With the actual data of electrical elements in thermal power stations of Kyushu Electric Power Co., attractive results of the maintenance were obtained and effectiveness of the proposed method was verified.
In order to investigate effects of plasma behaviors on fluid-dynamical predictions of generator performance, local steady-state analytical calculations have been conducted. Effective Hall parameter and effective electrical conductivity have been estimated by taking a linear theory on the ionization instability into account. The numerical calculations have been compared with experimental results. Although a fully ionized seed (FIS) condition which suppresses the instability provides the highest generator performances, the FIS plasma could be realized only under the condition of high seed fraction in the experiments. The theoretical analysis implies that the instability due to the insufficient or the excessive electron temperature is a performance-limiting factor. It is possible to estimate effects of the plasma condition on the generator performance by the present simple analysis.
An expansion of the applications for 22kV equipment and facilities in underground power-distribution systems is expected in the near future when taking into consideration controls and restrictions on the construction of new transformer substations for power distribution and the effective use of existing conduit lines. Moreover, the possibility has arisen for reducing overall costs while maintaining reliability of present-day 22kV XLPE cables and their joints based on the establishment of the Standard of the Japanese Electrotechnical Committee (JEC) 3408, “High voltage tests on cross-linked polyethylene insulated cables and their accessories for rated voltages from 11kV up to 275kV”. In this report, we will focus on the development of a water-impervious straight joint that can connect existing 22kV XLPE cables with 22kV XLPE cables having a water-impervious aluminum layer. The 22kV XLPE cable having a water-impervious aluminum layer fulfills the cost-reduction objective while maintaining reliability by reducing the insulation thickness of the cable for operation in 22kV systems, the applications of which are slated to expand in the near future. Now, with the development of this heterogeneous joint, 22kV XLPE cables having a water-impervious aluminum layer can be applied in regions where existing 22kV XLPE cables are laid. Because of this, it has become possible for 22kV XLPE cables having a water-impervious aluminum layer and their corresponding joints to be put into service as standard equipment in all regions.
An expansionn of the applications for 22kV equipment and facilities in underground power-distribution systems is expected in the near future when taking into consideration controls and restrictions on the construction of new transformer substations for power distribution and the effective use of existing conduit lines. Moreover, the possibility has arisen for reducing overall costs while maintaining reliability of present-day 22kV XLPE cables and their joints based on the establishment of the Standard of the Japanese Electrotechnical Committee (JEC) 3408, “High voltage tests on cross-linked polyethylene insulated cables and their accessories for rated voltages from 11 kV up to 275 kV”. This time, as part of the effort to reduce the material costs associated with 22kV systems, we will report on the developments of water-impervious straight joints and water-impervious Y-branch joints that correspond with developments of 22kV XLPE cables that have reduced insulation thickness to 3.5 millimeters with the introduction of a water-impervious aluminum layer. For water-impervious straight joints, a cold-shrinkable method (factory expansion) that provides excellent workability for assembly has been developed, and for water-impervious Y-branch joints, an insertion method that allows both removal and reconnection has been developed. In addition, these joints have been outfitted with a water-impervious construction by incorporating water-impervious shrinkable tubes comprising of an aluminum sheet in a similar fashion to water-impervious cables.
In the case of a fuel cell system using pure hydrogen as fuel, high hydrogen utilization operation is required because the exhausted hydrogen from fuel cell is not required as a heat source for a reformer. While the hydrogen utilization operation is very high, a minor imbalance of the distributed flow causes a shortage of hydrogen gas. In order to achieve the high hydrogen utilization operation, a cell humidified at the cathode side and two block divided stack which the hydrogen gas would be fed to the first block with its exhaust hydrogen being fed to the second block were developed. 5kW class(100 cells) short stack developed with a cell humidified at the cathode and two block divided stack showed the stable operation at high hydrogen utilization operation up to 98%.
Superconducting fault current limiter (SCFCL) is expected to be a first application of high-Tc superconductors to electric power systems. We have been developing a magnetic shielding type of SCFCL. In this paper, we propose a design method for magnetic shielding type of SCFCL. The designed method is based on a newly developed computer program which can evaluate the dynamic thermal and electromagnetic behaviors of high-Tc superconducting cylinder in fault current limiter operation. As an example, the trial design of a model system, which is 6.6kV (rms) in rated voltage, 400A (rms) in rated current and 12.5kA (rms) in maximum prospective short-circuit current, is shown.
The internal voltage gradient and behavior of column on long gap DC free arcs were analyzed quantitatively. In the analysis, an image processing technique developed by us was applied to the images shot from two orthogonal views by two high-speed video cameras to estimate the three dimensional arc column paths. Objective free arcs were ignited between the rod electrodes with the current in the range of 100 to 2000A and the gap in the range of 1.6 to 3.2m. In addition, some of objective free arcs were placed in the external magnetic field brought about by a large coil. As a result, macroscopic changes with time in terms of the arc voltage gradient along the column path, the ratio of the arc column length to the gap length and the distance from the center axis between the electrodes to the furthest point were derived. All average characteristics were shown as the experimental equations. In addition, the effect of the external magnetic field on the macroscopic velocity of the expansion was studied.