This paper discusses characteristics of a transient and difficulties of its simulation, and presents modeling and simulation examples on a low-voltage system including a distribution system. A transmission system is expressed as a line between a generator station and a substation. On the contrary, a distribution is characterized as a surface on which there are a number of circuit elements and short lines. Thus, the time period of the transient is rather small and the frequency range is high. The characteristic requires an accurate modeling of a circuit element in a high frequency region in general. The modeling itself is principally the same as that in a transmission system, but its parameter is not easily determined because its rating and characteristic are not generally available.
Overvoltage generated by lightning is a threat to indoor electrical equipments in information-oriented and computerized society. Countermeasures for preventing the overvoltage on an indoor wiring are demanded. Therefore, we have investigated the effectiveness of some countermeasures, such as low-voltage arresters, communication line arresters, SPDs and a common grounding wire, for reducing the overvoltage on an indoor wiring with a communication line. Especially, we have studied the effect of the combination of these countermeasures and their layout on controlling the overvoltage, using a full-scale test distribution line with a customer's facility. Main results are as follows: (1) A common grounding wire installed along the low-voltage indoor wiring is more effective for reducing the overvoltage on the indoor wiring with a communication line more than that installed under the floor, when the arresters are installed at the panel board. (2) Combination of a SPD and another countermeasures, such as low-voltage arresters and a common grounding wire, is effective for controlling the overvoltage on the indoor wiring, and the effectiveness of these countermeasures for preventing damage of electrical equipment has been verified by the experiment using actual electrical equipment.
In order to clarify the instability and chopping phenomena of a low current in the vacuum, we observed the phenomena by changing parameters such as the arcing time, the breaking current, the gap distance or the parallel circuit parameters. We measured instability initiation current, chopping current, arc voltage, arc voltage oscillation peak and the transient recovery voltage values by using copper, copper chromium and silver tungsten carbide electrodes. It was found that the instability initiation current and the chopping current increases with arcing time and decreases with the breaking current amplitude and the gap distance. It was also found the chopping current, the arc voltage oscillation peak and the transient recovery voltage are dependent on the parallel circuit parameters, but the instability initiation current and arc voltage are not dependent on them. We also found that the instability phenomena depend first of all on the cathode material.
Investigations were conducted on the VFT (Very Fast Transient) surges that propagate into a 500 kV transformer. The disconnector re-striking surge and ground fault surge were discussed. It was supposed that large part of the surge voltage was applied at the just entrance of the transformer winding for the VFT surges. So, an equivalent circuit that models the windings was used for the analysis. The overvoltages that appear between first winding sections at the entrance of the transformer were computed. The followings were clarified. (1) The overvoltage between the first winding sections becomes bigger when the magnitude of the voltage change at the transformer terminal is larger. The overvoltage between the first winding sections is not affected by the magnitude of the transformer terminal voltage. (2) For the disconnectors that are not connected directly to the transformer, the voltage change at the transformer terminal is not so large. (3) In the case of ground fault at the GIS near the transformer, the voltage change at the transformer terminal is the same as that for a disconnector directly connected to the transformer. (4) In actual GIS, the disconnector that is connected directly to the transformer is not used usually. In this situation, the overvoltages that threaten the transformer insulation will not be generated by.the re-striking of the disconnectors.
Generally speaking, lightning overvoltages on overhead power distribution lines can be suppressed more greatly for small earth resistance of an overhead ground wire, which is popularly used for lightning protection of overhead power distribution lines, than for large earth resistance. This paper shows the possibility that overvoltages, which arise between a phase conductor and the earth, are larger on distribution lines with high earth resistance of an overhead ground wire than on those with low earth resistance of an overhead ground wire. Typical examples are demonstrated for induced overvoltages due to nearby lightning strokes as well as for direct lightning hits to distribution lines. These phenomena can be explained by the conversion of common mode overvoltages into normal mode (between a phase conductor and an overhead ground wire) ones. Although above phenomena are not ordinary, we should be careful when considering effective protection method for an overhead power distribution lines supported by wooden poles or for ones constructed in the field of low soil conductivity.
In Japan, local power systems (77kV) are served from the 275kV or 154kV substations. For enhancement of power supply reliability, the transmission lines are connected to several substations, and the operational configuration is radial. The local power system's configuration is determined by connecting and disconnecting transmission lines so as to keep the radial structure and satisfy the operational constraints. When a local power system has a number of transmission lines, many configuration candidates occurs. Recently, an IEEJ committee made a practical scale local system model (IEEJ Local System Model). Since IEEJ Local System Model has 76 transmission lines, the total number of configuration candidates is 276(=7.5×1022 approximately). In this paper, the authors try to strictly obtain the loss-minimum configuration under constraints such as substation capacity, line capacity and radial structure in IEEJ Local System Model. In order to obtain the optimal configuration, a new computation algorithm is proposed. In the proposed algorithm, the configuration determination problem is replaced as two combinatorial optimization problems based on the operational constraints (1)substation capacity, (2)line capacity and (3)radial structure). One combinatorial optimization problem (subprobleml) is to pick up all partial configurations satisfied with line capacity and radial structure constraints. The, other one (subproblem2) is to select the partial configurations so as to minimize total line loss under the substation capacity constraint. By using the enumeration method, subprobleml is solved. Subproblem2 is solved by using the reduced ordered binary decision diagram (ROBDD). Since the proposed method is based on enumeration and Boolean function, the optimality of obtained solution is guaranteed.
Recently, the electric utility industry in the whole world has been facing pressure to be deregulated or restructured in order to increase its efficiency, to reduce operational cost or to give consumers more alternatives. For this aspect, a great deal more research is needed to achieve better intelligent knowledge process. The present centralized system for the power system control, operation and planning must be remodeled to cope with these situations. With the promotion for the deregulation of the electric power system, the definition of the objective function for the optimization problem such as outage work operation for electric power system is becoming critical. Currently, agents are focus of intense on many sub-fields of computer science and artificial intelligence. Agents are being used in an increasingly wide variety of applications. In this paper, we developed a power system normal operation application by multi-agent architecture. Our multi-agent system consists of several Facilitator-agents, Equipment-agents and Switch-box-agents. Facilitator-agent acts as a manager for negotiation process between agents. Equipment-agent corresponds to the element of the electric power system, such as bus, transformer and transmission line, while Switch-box-agent is the pseudo object which consists of neighboring current breakers and disconnecting switches. The proposed system realizes the appropriate switching operations by interacting with corresponding agents. The proposed approach is applied to a simple network, and the results show that the proposed multi-agent system is an efficient decentralized approach for solving power system normal operations.
Applying ATP-EMTP for the purpose of thorough understanding of rotating machine transients, several kinds of model synchronous machines with typical features were analyzed in time domain as for sudden short circuiting, the calculated currents and flux linkages of which were plotted on the d-q domain planes. Many interesting current/flux vector traces, representing rotating machine transients during short circuiting, were obtained. These could, hopefully, be good help for understanding rotating machine transients.
To achieve a rational insulation design for transformers, it is important to evaluate dielectric strength against surges actually impinging on equipments on-site. This paper deals with the breakdown voltage characteristics of an oil gap under non-standard lightning surge waveforms combined with oscillatory voltages. As the results, the breakdown voltages of the oil gap under non-standard impulse waveforms are higher than standard lightning impulse voltages. This results can be ascribed to V-t characteristics of oil gap in short time impulse voltage ranges.
This paper describes outline and result of the lightning impulse test for 275kV full GIS substation. The behavior of lightning surges is very important for rational design of substations and low voltage and control circuits inside substations. For above reason, we carried out a lightning impulse test for a new 275kV full GIS substation. In this test, we measured induced voltage of low voltage and control circuits, transient characteristics of grounding mesh, injected voltage and current waveforms, and so on. By investigating these data, we confimed the behavior of lightning surges inside the substation. We also compared simulated waveform by EMTP analysis with measured waveforms. The simulated results agreed well with the measured results.
To study galloping phenomena of two spanned transmission line with four-bundled conductors, the field observation has done for three years in the full scale Tsuruga Test Line, and the influences of the shape of ice accretion and the input wind velocity on the mode, the frequency and the amplitude of galloping vibration were examined by using the Matsubayashi's formula in the present design code of transmission line, a complex eigenvalue analysis and a nonlinear response analysis. This paper reports characteristics of galloping phenomenon in the transmission line which has been clarified analytically.
Recently the wind power generation has attracted special interest and many wind power stations are being in service in the world. In the wind power stations, induction machines are mostly used as a generator. Since induction machines can become unstable during a fault in power system, like a step-out of synchronous machines, it is important to analyze the stability of power system including wind generators. This paper presents a consideration on the stability of induction generators from a point of view of voltage stability.