A facility is described for studying the properties of lightning and of the effects of lightning on power systems using lightning discharges stimulated to occur between a natural thundercloud and a designated point on ground. Examples of experimental data obtained in 1993 through 1995 are given.

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Since 1986, the authors have been taking part in the triggered lightning experiments in winter, at the site of Okushishiku, Ishikawa, for the purpose of observing the entire lightning current waveforms with coaxial shunt resistors and an electrooptical measuring system. As the results, extra-fast current changes of the order of 1, 000 kA/μ s were recognized by the sharp pulse records superposed on the positive current waveform obtained by a shunt resistor of relatively high residual inductance. This was not the case with the resistor of lower inductance. Furthermore, a positive lightning current waveform which looks like the composite result of the bi-polar discharges has been observed by an advanced measuring system of high performance. This paper deals with those current-related topics of triggered lightning discharges to be analyzed hereafter in detail.

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In the measurement of the rocket-triggered lightning currentt and voltage, executed from 1986 to 1995 at mountain top of Okushishiku in Kanazawa areas, the authors succeeded in artificially inducing the winter lightning to the arresters, Using the data obtained in those measurments, we analyzed the energy absorption characteristics of surge arresters, which are installed on every tower of transmission line for three phases, by EMTP.
The energy withstand capability of individual arrester was verified as approximately same as expected value. Analysis result for energy share of each arrester connected in parallel showed that usual light duty arresters installed on every tower have possibility to be able to absorb the big winter lightning energy even if the lightning bit directly the power line.

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It has been important to maintain high reliability for supplying electric power in recent high- intelligent society. Overhead ground wires (GW) are installed at upper positions of overhead transmission lines in order to prevent lightning attack on conductors. As the sacrifice, GW sometimes gets certain damages including strand-breakage by a severe lightning attack. Recently, optical fiber composite ground wire (OPGW), which has arisen for development of strong GW against lightning attack, in particular for transmission lines passing through areas where “winter lightning” with enormous electric charge occurs, or areas where frequent lightning occurs.
We investigated materials and strand structures to get high lightning-resistant properties and verified by rockettriggered lightning experiments and DC arc tests. As the results, we have developed excellent lightning-resistant OPGW that has fundamental properties equivalent to conventional GW.

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This paper presents the altitude triggered lightning to a test EHV transmission line in winter thund erstorm. The inducing wire was isolated with a nylon line from the ground up to the altitude of 70m to 160m. Thirty one striking events have taken place on the several locations of the transmission line. Current wave forms were measured and the corresponding lightning discharge channels were well reconstructed by an acoustic analyzing method. The characteristics of striking distance vs. peak current, and the histogram of vertical striking angle were investigated and discussed from the view point of lightning striking parameter to a transmission line.

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Measurement of lightning surges originating from natural and rocket-triggered lightning strokes in winter has been carried out on a real-size test transmission line since 1987. During November 1993 to December 1995, fifteen flashes which struck the test transmission line were observed. In one of the fifteen cases, the arresters equipped on the test transmission tower operated and well controlled the voltages across the insulators. In another case, a four-phase backflashover was observed on the upper and middle phases. The calculated results of the multi-phase analysis by EMTP agreed with the measured waveforms when the timing of the flashovers was given. But the four-phase flashover phenomenon was explained neither by the integration method nor by the leader development model.

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Characteristics of upward lightning by interferometric measurement are discussed. The data set is obtained in winter at Hokuriku-district of Japan. We analyzed both positive and negative upward leaders which ware initiated from the top of the 50m-tall tower that is located at 4km away from the observation site. The interferometric system is operating at center frequency of 327MHz with a bandwidth of 350kHz and the time resolution of 1μs.
The comparative study of the positive and negative upward leaders shows that the intensity of UHF radiation caused by a negative leader is 10dBm higher than the upward positive leader. It also shows that a upward negative leader is preceded by a cloud discharge which may enhance the electric field at the tower top. In case of upward positive leader, no cloud discharge prior to the initiation of the upward leader, are observed.

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The UHF Interferometric observation were conducted during the Rocket Triggered Lightning Experiment in Japan. The experiment site is located in the Hokuriku Area where we have frequent winter thunderstorms. The experiment was held during a winter thunderstorm season. During the project four data sets of the UHF radiation emitted by upward negative leaders which initiated positive ground strokes were recorded. The images of upward leader progressions were derived and the velocity of upward negative leaders as functions of time were estimated. The estimated results seem to be consistent with the former results obtained through optical observations.

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According to fault records, most transmission line outages arc caused by lightning. Recently transmission line arresters using Metal-Oxide elements have been developed up to 500kV in Japan and have been applied to service lines with good service experiences. The most of line faults due to lightning occurs in summer season, while in some areas facing to the Sea of Japan, the line faults have been often caused by the winter lightning which has usually a huge energy owing to longer duration as compared with the lightning in summer.
Until now, the performance of the arrester has not been proved for such a winter lightning. Therefore, field observation has been carried out for rocket-triggered or natural lightning in winter at the Okushishiku test line from 1993 to 1996. This paper reports the results of the measurement and EMTP analysis about performance of the arresters against winter lightning.

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The experiment of triggered lightning by rocket has been carried out for the real power transmission line tower and the real lightning rod. The predischarge phenomena including leader propagation to the test objects were studied with the streak photograph by moving film camera and with the observation of the discharge current wave and the electric field change at ground. The stepped leader of negative polarity was observed to propagate downward to the grounded object, then the upward leader of positive polarity propagated from the rocket to the cloud contineously. The protection effect of lightning rod was also studied by the streak photograph for the triggered lightning test to the real lightning rod.

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If a lightning back-flashover occurs on a transmission line tower, the lightning surge will be transmitted by the line conductor and penetrate into a substation. Overvoltage protection methods for substations are already published in several papers^(1_??_3). In the present analytical method, however, it is assumed that the back-flashover takes place at only one of six line conductors installd on one transmission tower. It is known that when a lightning surge voltage across six insulator strings in one line tower exceeds 1.2_??_1.3 times the lightning impulse 50% flashover voltage of the insulators, flashovers occur on plural strings simultaneously ^(4_??_7). Authors analized the lightning overvoltage in the substation using EMTP. Considering occurrence of multi-phase simultaneous flashover on a lightning stricken tower, the permissible lightning current, which does not cause the insulation damage in the substation, is more than 25% large than those given by the presently adopted analysis method. This analysis leads to the conclusion that the permissible lightning current by the presently adopted analysis practice has an allowance of more than 25%.

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This paper investigates lightning surge overvoltages on a coil system of a magnetic-levitation train, when lightning hits an overhead ground wire(GW) to protect the coils. The paper considers an elevated railroad with height of about 20 m from the earth surface and iron structures which were neglected in the previous investigations. The overvoltage induced to the coil due to the lightning current on the GW increases by about 30% when considering the grounding lead of the GW. The iron structures reduce the lightning overvoltages by about 10% when the structure is assumed to be directly grounded to the earth. If the grounding lead of the structure is taken into account, the reduction effect is not much noticeable. The induced overvoltage to the coil enters into a power feeding cable, and produces an overvoltage which may exceed the cable sheath insulation level. The overvoltage can be controlled by an arrester.

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Gas insulated equipment inevitably employs various kinds of solid dielectrics. Surface electrification (charging) on the solid surface may significantly reduce the insulating ability of the equipment.
This paper reports the results of the experimental work carried out to clarify the characteristics and mechanism of discharge propagation on the surface of an insulating plate which have been electrified by a preceding surface discharge. Oscillating lightning impulse corresponding to disconnector-induced surge was employed in the test together with fundamental square voltage. A streak camera and a still camera were used to examine the mechanism of the surface discharge as well as to measure the length of discharge channel. While the length of discharge channels is scarcely affected by the preceding discharge for the square voltage, there occurs a distinct change of the length with each voltage application in the case of the oscillating impulse. When positive and negative voltages of the same height are applied by turns, the maximum length appears at the application of the first positive voltage. These characteristics are discussed by measuring the charge quantity during the propagation of surface discharge.

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A conceptual design of transonic disk MHD channel is carried out, for a power generation system with liquefaction-recovery of CO₂. A previous study has shown that the subsonic disk MHD channel has rather poor performance, and that the supersonic disk channel yields sufficiently high power output although its stability should be improved. The present paper proposes the transonic disk channel which can be stably operated with high power output. It is assumed that the transition between supersonic flow and subsonic flow is accompanied with a cylindrical shock wave in the channel. The transonic channel yields the enthalpy extraction ratio of 20.2% and 22.9%, respectively, for the thermal input of 1100MW and 2000MW, which is nearly equal to the performance of supersonic channel. The stability of transonic disk channel is examined by γ-θ two-dimensional time-dependent calculations. The two-dimensional analysis shows that the transonic disk channel works stably with fewer load sections than the supersonic channel even when inlet perturbations are added.

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Important characteristics of metal oxide surge arresters depend on the duty of specified temporary overvoltage requirement due to Volt-Ampere characteristics. However, there is hardly any literature where the duty of surge arresters in non-effectively grounded system is investigated. Temporary overvotage of 1.43pu proposed in effectively grounded systems is too low compared with nearly √3pu in a line-to-ground fault.
The overvoltage due to the combination of a line-to-ground and load rejection, where not only main power circuit but also control system like AVR participate, is rare but probable as the severest. Here the duty of surge arresters to the overvoltage is clarified to various residual voltage levels and system conditions. The severest condition is the one in the system including a cable line because of the operation of UEL of AVR.

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In the paper, a possibility of the unit step response simulation in a time domain from the resistive divider's dimensions is discussed. This technique consists of two stages: to compute the floating capacitances between the segmented two electrodes as well as stray capacitances to the ground; and to calculate the unit step response for the constructed equivalent circuit using EMTP. To verify the validity of the proposed simulation, a 200kV equi-potential resistive reference divider is to be selected. This is because the equi-potential divider's unit step response can theoretically be estimate. The numerical simulation has been in completely agreement with the expected response. The results showed a possibility that even without constructing prototype dividers, one can determine divider's dimensions which yield the best step response.

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There is a possibility that increasing capacity and load flow of power system may weaken damping of low frequency interarea power swing. The present paper proposes a new control design method based on stepwise application of multi-variable control (MVC) scheme for damping such inter-area oscillations.
For the same purpose, many papers propose two input type power system stabilizer (PSS) which use information on electrical output power (ΔP) and rotor speed (Δω) of generators. In controller design procedures, however, most of them use single machine and infinite bus (SMIB) model which is not suitable for modeling low frequency oscillations in large scale power systems.
The present paper discusses the characteristics of low-frequency inter-area modes which can not be explained by SMIB model. In addition, the paper makes it clear that two input type PSS can be easily designed by the proposed method named stepwise application of MVC, in which modal reduction models are used for controller design instead of conventional SMIB model. The effectiveness of the proposed scheme is demonstrated through numerical examples using a 29-machine system model.

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This paper presents a new approach for solving multistage decision problems in power system operation by using a new fuzzy dynamic programming technique. To investigate its validity, the hydrothermal scheduling problem is used as an example problem. Within this problem, uncertainty elements exist such as load demand and natural hydraulic inflows to reservoirs exist which can be expressed by fuzzy numbers. Two new fuzzy decision making techniques for selecting the optimal operating states at each stage are introduced.
Further, the hydrothermal scheduling solution obtained had an operating band defined by upper and lower bounds. To further enhance the information that this operating band provides, a midpoint operation technique that applies both forward and backward fuzzy DP is also introduced. This allows the system operator to consider current operating conditions to determine a suitable stage by stage operation policy within the bounds of the initial solution. By the optimal range definition of the initial operating band, a cost effective route will always be achieved by operation within this region as verified in the test results.

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Squirrel-cage induction generators are widely used as generators for windmill power systems, because they are inexpensive, of high durability, and capable of operating asynchronously to power systems. However, induction generators have the drawback that it cause the transient rush current several times as large as the rated value when it is connected to power systems. Also, since wind energy is influenced by geographic and weather conditions, start/stop operations occur frequently for windmill power systems. Therefore, connecting induction generators to power systems cause undesirable voltage vibration in power systems, which is the major obstacle against the practical use of the windmill generator systems. This motivates the study of transient rush current/voltage attenuation in windmill generator systems.
In this paper, a nonlinear state-feedback controller for windmill power systems is proposed. The proposed controller not only globally asymptotically stabilizes the plant, but also effectively attenuate the transient current/voltage of those systems. C omputer simulations using parameters of the actual windmill generator system also give good results.

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A method for SVC damping controller design based on robust pole assignment method is proposed. In the design procedure, the proposed method uses a nominal model which reprepsents weakly damped interarea modes. The ignored higher order modes and perturbation of the low frequency modes are considered as multiplicative perturbations. The controller with high performance and high robustness can be derived easily utilizing H_∞ optimal control theory.
The present paper makes it clear through eigenvalue analyses and digital simulations that the designed controller is effective for damping interarea modes and achieves higher robustness against change of load flow condition than the well tuned conventional controller.

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SUb-transmission Network Probabilistic Reliability Analysis System (SUNPRAS) has been developed to support more rational planning of the sub-transmission network. In the developed system, many fault events are assumed at first and the effects of these faults on the loss of load are analyzed. Then based on both the result of the fault analysis and the occurrence probability of these faults, the several reliability indices are calculated by using the probabilistic method. The fault restoration procedure should be considered in analyzing the fault, and is formulated in the radial network as the combinatorial optimization problem which is very difficult to solve in large system. In this research, a quite efficient algorithm composed of gradual maximum flow method and branch exchange method was newly developed as the fault restoration logic to minimize the loss of load. The gradual maximum flow method efficiently finds the initial system configuration close to the global optimum solution by attaining the flow condition as balanced as possible in the margin of the line capacity. The reliability indices given by the developed system are expected power not supplied, expected energy not supplied, expected duration not supplied and so on. These are given as load point indices at each distribution substation.
The developed system was applied to the model system to confirm the effectiveness of the method. Furthermore, the quantitative evaluation of alternative reinforcement plans from the improvement of reliability point of view was demonstrated as a typical example of its useful applications. The possibility of the new planning criterion taking probabilistic factors into consideration was also discussed.

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The present paper proposes the use of phase-domain (abc frame) generator model in the electro-magnetic transient analysis instead of conventional dqO-domain generator model in order to improve numerical stability.
The drawback of the conventional dqo-domain machine model is that the prediction of a number of eletrical variables as well as machine speed is needed in order to interface the circuit of a machine model with the rest of the power system network and the prediction of these variables leads to numerical instability. In contrast, the prediction of electrical variables is not needed in the proposed phase-domain model.
In the paper, the phase-domain machine model is developed. The proposed model is validated by comparisons with an existing EMTP type-59 machine model. The paper makes it clear that the numerically stable solutions are obtained by using the proposed model even in the cases where numerical instabilities occur when the existing type-59 model is used.

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Longitudinal power systems can transmit a certain amount of power independently of their lengths. This nature is widely different from one-machine-infinite-bus systems. This paper clarifies from where the nature comes. First, we linearize swing equations of a longitudinal system, and show that its coefficient matrix is essentially a band matrix. Furthermore, the matrix is cyclic. Namely, if you shift all elements of a row by one to the right, then you get the next row. Utilizing this feature, we solve a difference equation consisting of the elements to get analytical expressions for the eigenvalues and eigenvectors of the system. The eigenvectors do not change with increase in the power transmission if there is no load at all intermediate buses. Contrarily, several eigenvectors rapidly change their shapes if there are some loads at the buses. Two eigenvectors of low frequency modes lastly change to a couple of conjugate complex vectors. Simultaneously, one eigenvalue comes to have positive real part, which means that the corresponding low frequency mode oscillately diverges. These changes determine the transmission capacity of the system. However, the changes are brought by the row elements of the matrix. Since the elements do not depend on the system length, the transmission capacity is also independent of the length.

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