Since unstable phenomena that load bus voltages collapse rapidly after a large disturbance occurred in a bulk power system, it is becoming necessary to develop a fast and precise evaluation method of what is called transient voltage stability. Though many indices have so far been proposed for static voltage stability assessment, there are few evaluation methods but digital simulation analysis for the transient voltage stability assessment considering dynamic characteristics of generators, loads etc. This paper presents the transient voltage stability evaluation system which consists of two artificial neural networks (NNs). The first NN judges whether the system is stable or not for given operating condition, load composition and fault location from a viewpoint of the transient voltage stability. If it is judged to be unstable, the second NN estimates critical operating time of emergency control action as a severity index of the transient voltage instability. Here, closing of a bus tie which is switched off in the normal state is adopted as the emergency control for the voltage stabilization. Numerical examples for 26-bus model system are shown in order to check the effectiveness of the proposed evaluation system.
In this paper, autoparametric resonance in longitudinal power systems is investigated. The resonance occurs in narrow area of parameters, and even if it occurs, it is swiftly suppressed by generator dampings in the previously studied power systems. This paper shows that the autoparametric resonance occurs very naturally in longitudinal power systems. First, it is shown through simulations that a low-frequency oscillation con-spicuouly changes its magnitude, which gets larger with fault-period, and finally causes a system-separation. Next, the above phenomenon is reproduced by strictly solving the mode equations, and it is clarified that the resonance occurs between two modes with the lowest and the second lowest frequencies. Furthermore, it is shown with the Mathieu diagram that the resonance gets stronger with system size, and accordingly it becomes an important factor to determine the system size. The stable system size gets larger with effects of generator dampings and AVR, but gets smaller with generator capacity, which means that the resonance gets stronger with system capacity, too. Lastly, the resonance potentially exists in a real-size system, but it is not actualized owing to AVR and generator dampings.
A novel pulse generator with a current pulse shaping circuit has been developed for the excitation of a large-bore copper vapor laser. The pulse generator completely restrains the wasteful discharge current that flows after the laser oscillation. We have proved that the pulse generator is very effective for the increase of the laser output power by applying it to an 80mm bore copper vapor laser. The laser output power increased about 12% for the 510nm line and 9% for the total lasing lines.
Thermal units must be maintained periodically as prescribed by the electric utility industry law. As time to execute maintenance works increases with thermal unit capacity, maintenance scheduling has a great influence on the reliability and economy of a power system. In the recent amendment in the law, three inspection rankings have been introduced and scheduling over several consecutive years becomes mandatory, thus making maintenance scheduling extremely difficult. Reflecting a recent stringent supply capability, the emphasis is laid on security rather than a minimum operating cost, having been the primary objective in determining the schedules. Therefore, this study aims to level the spinning reserve at each period under study in the maintenance scheduling while taking into consideration all the amendments in the law. Although rigorous methods such as integer programming and branch and bound method can solve small scale problems, large size problems are beyond of these techniques due to an exponential explosion in the number of possible combinations. The prime objective of this paper is to investigate the capability of the Hopfield neural network (FINN) in solving the newly formulated maintenance scheduling problem. The scheduling problem has been mapped on the HNN with slight problem relaxations to facilitate the implementation. A small scheduling problem that determines the maintenance schedules of 3 generators over 3 years (divided to 78 periods) has been solved by the neural network simulator. It has made clear from simulation results that the proposed approach is very promising in handling a complicated combinatorial optimization problem.
New technologies such as power electronics have made it possible to change continuously the impedance of a power system not only to control power flow but also to enhance stability. A power system incorporating variable impedance apparatus such as variable series capacitor (VSrC) and high speed phase shifter (HSPS) is called VIPS (Variable Impedance Power System) by the authors. This paper proposes a novel control method of VIPS apparatus such as VSrC and HSPS installed at an interconnecting point for stabilizing inter-area unstable and/or oscillatory modes. The proposed design method of the control system is a kind of hierarchical decentralized control method of a large-scale power system based on Lyapunov function. Under the proposed control scheme, each subsystem can be stabilized independently by local controllers such as AVR, speed governor and PSS, and then the whole interconnected system can be stabilized by VIPS apparatus taking into account interactions between subsystems. Effectiveness and robustness of the VIPS apparatus control are shown by numerical examples with model systems including a large-scale power system.
Distribution system loss minimization re-configuration is a 0-1 planning problem, and the number of combinations requiring searches is extremely large when dealing with typical system scales. For this reason, the application of a genetic algorithm (GA) seems attractive to solve this problem. Although genetic algorithms are a type of random number search method, they incorporate a multi-point search feature and are therefore superior to one-point search techniques. The efficiency of GAs for solving large combinational problems has received wide attention. Further, parallel searching can be performed and the optimal solution is more easily reached. GAs offer the possibility of searching the parameter space intelligently to find regions of interest, hence, they were selected for this application. In this paper, in order to apply a GA successfully to this distribution system loss minimization re-configuration problem, a new method that automatically amends the crossover and mutation rate of the Dynamic Parameters Modification (DPM) is proposed. Simulations were carried out using a model system to check this method's validity.
Many efforts towards a settlement of the CO2 problem have already been made through internationally organized meetings. It is not unlikely that some targets will be set for the reduction of CO2 emissions. If a stringent reduction target should be imposed on CO2 emitted from a national energy system, this can possibly have as severe an impact upon electric power sectors as the oil price crises experienced in the seventies. It is, however, quite uncertain whether or not political muscles will be used specifically for tackling the CO2 problem in the near future. In such a context, this paper presents a new method for optimal power system planning under uncertain CO2 emissions control policies. The derived optimal expansion plan gives us the minimum expected value of the sum of the total system cost and the total amount of carbon taxes levied on net CO2 emissions from fossil fuel-fired power plants. The stochastic dynamic optimization problem discussed here is formulated as a linear programming problem decomposable into several small sub-problems. The proposed method can take into account a specific future scenario of the magnitude of control policy uncertainties, which can be presumably resolved in the first half of the next century due to the increase in scientific wisdom. In order to evaluate the usefulness of the method proposed here, the authors also present a simple case study with the Japanese national power system up to the year of 2050.
This paper investigates the suppression effect of surge arresters against lightning overvoltages on distribution lines caused by indirect lightning strokes. Scale model technique is useful to simulate a large power system network. Then, we use a 1/100th scale model. Geometric dimensions and current waveforms in the return stroke model are determined by the scale factor. Surge arresters are modelled by the base-emitter of a transistor which V-I characteristics are equivalent for a real device. The paper describes as follows on the induced voltages on the distribution line model. (1) Calculated values agree well with experimental results. (2) Surge arresters suppresse the voltages if the locations of lightning paths are nearby them. (3) The voltages are reduced by the simultaneous application of a ground wire and surge arresters more than the case that lightning protections are the arresters or the ground wire alone. (4) The voltages at branced points are 1.2 1.4 times as large as ones on an infinite line.
This paper describes the possibility of high quality performance of the electronic air conditioning systems using semiconductor thermoelectric conversion which are cooperated with water-circulating systems as the interface with the environment. The performance of the systems is expected to be particulary excellent in the districts with relatively mild climate, because they can usually be operated under the low temperature difference between the room and the environment. However, the thermoelectric elements, which are made from the material of Bi-Te family and are assembled into modules, must be much flatter in the heat-flow direction than those for commercially available ones. This paper is to urge the effort of the device manufactures to develope the material saving type thermoelectric modules for the new air conditioning systems free from fluorocarbon-problem.
This paper describes the evaluation of contamination withstand characteristics of porcelain type surge arresters for 275-345kV power systems on artificial contami-nation test. Contamination evaluation methods have been applied to surge arresters combining of the JEC-217-1984 artificial contamination test method and proposed IEC TC37 WG4 TF4 1993 pollution test techniques using measurements of coulombs of surge arrester leakage current. The comparison of contamination classification of surge arresters between JEC-217-1984 standard and IEC TC37 WG4 TF4 1993 draft is shown.