Department of Electrical Engineering

Performance of nonceramic insulators is strongly related to hydrophobicity of the surface. Therefore, it is useful to evaluate the characteristics of the surface of organic insulating materials. To characterize the methods for the evaluation, some measurements were conducted on silicon rubber (SR), EPDM, PMMA and polyethylene (PE). The influence of irradiation with a mercury lamp was detected through measurements of contact angle, formation time of water film, and mass analysis of released gases under irradiation of electron beam.

Impedance and associated electromagnetic (EM) fields of an EM return-stroke model are investigated with the help of NEC-2. The impedance of a cylindrical vertical conductor increases with time. The impedance of a vertical conductor having additional distributed inductance is initially 700 to 1000 for the current propagation velocity ranging from 170 to 130 m s. This is in good agreement with the apparent impedance of a lightning channel, estimated from lightning-current waveforms measured at the top and bottom of tall towers. A model considering a horizontally bent channel in a thundercloud succeeds in reproducing all of the established features of measured field waveforms except the hump of close magnetic field, while a vertically straight conductor fails to reproduce two of the features. Index Terms-LEMP, numerical electromagnetic (EM) field analysis, return stroke.

In this short paper, a new "engineering" model of subsequent return stroke has been proposed. In the model, the propagation characteristic of a channel current follows the Diendorfer and Uman (DU) model in the bottom region but a current pulse attenuates further in the upper region. This model has succeeded in reproducing all the features of reported typical electric-and magnetic-field waveforms associated with natural subsequent lightning and triggered lightning.

A lightning return-stroke model of transmission-line (TL) type, which incorporates both attenuation and distortion of the waveform of propagating current, is proposed. With the proposed model, influence of distortion of the propagating current on associated electromagnetic field waveforms is investigated and found to be small.

The damages caused by lightning are particularly severe in the case of wind turbine generator systems. The nacelle is composed of steel grids covered with GFRP (glass-fiber reinforced plastics) in order to reduce its weight. When lightning strikes a wind turbine generator system, the current thus generated flows into the ground through down conductors in the blades, the steel grids of the nacelle, and the tower. Further, the current flowing near the nacelle produces large magnetic fields inside the nacelle, and the communication and control systems either break down or malfunction. We have analytically and experimentally investigated the transitions of the magnetic fields in the nacelle by using an FDTD (Finite Difference Time Domain) method and a reduced-size model of a wind turbine generator system.

Cloud-to-ground lightning flashes on and around the island of Java, Indonesia, in the tropical maritime continent region, were observed continuously by a magnetic direction-finder network for lightning location from December 1994 to January 1996. The annual ground flash density, estimated by correcting the raw data by using the detection efficiency of the network, ranged from less th•n 2 to about 16 flashes/y/km 2 on the island. The average annual ground flash density over the entire island was 3.2 flashes/y/km 2, as opposed to 0.24 flashes/y/km 2 on the Indian Ocean about 100 km south of the island. The majority of the flashes on the land occurred during November to April in the rainy season. In the dry season, lightning was active only in the western part of the island. The amount of monthly precipitation over the island was related to the monthly number of ground flashes, but the ratio of the precipitation to the ground flash density differed from season to season, from 4 x 108 kg/flash in the rainy season to 1.3 x 109 kg/flash for other months. The ratio observed in the rainy season is equal to that for the break period in the rainy season observed at Darwin, Australia, in the same maritime continent region. The annual ratio for the entire island was 6 x 108 kg/flash. The diurnal variation of the lightning activity averaged over the island in the set season showed a single peak in the afternoon with the peak time of about 1530 LT, similar to those reported for land stations in the tropics. The diurnal variation over the ocean had a peak in the early morning, showing the same characteristic observed in the western Pacific. The diurnal variation of the lightning activity on the sea of offshore regions around Java showed an almost out-of-phase pattern to the variation on the land, indicating the influence of the land-sea effect. 14,002

The diurnal variation of lightning activity on and around Java Island, Indonesia, is examined by using the observation results from a magnetic direction finder network. The magnetic direction finder network has four direction finders on Java Island and has been operating since 1994. Lighming is active during the rainy season, usually from October to May. The highest ground flash density on Java Island was 16 flashes per km 2 per year. The diurnal variation of the lightning activity over land shows its peak at around 1500 LT. There is a small peak in the early morning around 0300-0500 LT over the sea, several hours prior to the reported peak times of the cold cloudiness, an indicator of deep convection observed from meteorological satellites. Existence of close sea-land interaction related to the activity of deep convection in this area is suggested from the diurnal variations in the lighming activity. The diurnal variation is also found in the peak currents of negative first return strokes over land, medians from 25 kA around noon to 30 kA around midnight. The lightning activity over the ocean does not show such diurnal variation and delivers currents of higher amplitudes with a median of 36 kA.

The authors have developed a method of analysis of induced voltages where the Agrawal's model is adopted as a field-to-line coupling model. In this letter, the sparkover rate of a power distribution line associated with indirect hits is investigated based on numerical calculations taking account of statistics on return-stroke current. It turns out that the sparkover rate associated with a subsequent stroke is higher than that associated with a first stroke when the line is equipped with surge arresters.

The development of the effective and economical protection measures is very important for the railway signalling systems because the lightning damages cause the disruption of the railway transportation systems. In general, the railway signalling systems are set up at a wayside and directly connected to the rails through cables. The lightning surges can invade the railway signalling systems via the cables and cause damages to the systems. Accordingly, clarifying the propagation characteristics of lightning surges along the rails is essential to develop the lightning protection measures for the railway signalling systems. We have carried out the field tests to examine both surge impedance and surge propagation velocity of the rails. This paper proposes a calculation model of the surge propagation characteristics along the rails. The results of surge impedance and surge propagation velocity calculated by the proposed model almost agree with the experimental results. Moreover, this paper proposes a calculation model of the lightning overvoltages on the railway signalling equipment. The calculation model consists of the rail model and the equivalent circuit model of signalling equipment. We indicated the validity of the calculation model of the railway signalling equipment by comparing with the results of the field test. This model is applicable to the development of lightning protection measures for the railway signalling systems.

In high-voltage measurement, the digital measuring system is used widely because of its attractive features, for example, automatic data acquisition. However, the accuracy in the measurement of both the amplitude and time parameters by the digital measuring system is not known in detail, especially in the measurement of single-shot waveforms. In this paper, the accuracy of the peak value and the virtual front time of the standard lightning impulse without any signal processing of raw data is shown. From these results, the required characteristics for the digital measuring system to satisfy the IEC Draft on digital recorders for measurement in high-voltage impulse test techniques are shown. In addition, the accuracy of the peak voltage and the rise time of the Gaussian pulse is shown. A s a part of the identification of the measurement accuracy of the digital instrumentation system, research on the errors accompanying the AID conversion has been done in the high-voltage field. the analog output of an oscilloscope is A digitizer in which.

Cloud-to-ground lightning flashes in the Niigata area in Japan have been observed for a few years by a magnetic direction-finder network. Data obtained so far are analyzed carefully taking into account the operating characteristics of the system. As a result, frequency distributions of return-stroke current for each polarity are produced, and their seasonal variations and the occurrence probabilities of high current exceeding 200 kA are clarified. Based on the estimated current distributions, backflashover rates of 500-kV 2 cct transmission lines are calculated. It is known that the back-flashover caused by downward flashes can account for most of the line outages in the summer for both single-and multiple-line faults. However, this mechanism cannot explain the high outage rate in the winter experienced in the winter thunderstorm area in the late 1970s. In spite of the low lightning flash density in the winter, the multiple-line fault rate in the winter caused by back-flashovers is predicted to be comparable to that in the summer, because of the seasonal variation of the frequency distributions of return-stroke current.

Experiments on the lightning-induced voltage waveforms on an overhead wire influenced by an inclined return-stroke channel are carried out outdoors with a reduced-scale model. The measured voltages are compared with those calculated by solving the Telegrapher's equation in combination with numerical calculation of electric fields associated with return-stroke currents, and the validity of the calculations is verified. The effect of the ground conductivity on the lightning-induced voltage waveform is studied based on numerical calculations and is found to be dependent on the position of the return-stroke channel relative to the overhead wire. When a return stroke hits the ground close to the end of the overhead wire, the influence of the ground conductivity on the induced voltage waveform is significant irrespective of the inclination or the direction of the lightning channel. The degree of the influence is dependent on the inclination and the direction of the channel when a return stroke hits the ground on the side of the overhead wire.

The influence of elevated objects, when they are hit by lightning, on the measured lightning currents and associated fields is studied. For the present study, an electromagnetic model of return strokes with the help of NEC-4 is employed. Cases of strokes to ground, to independent towers, and to a transmission line are compared and the validity of comparison is discussed. The influence of the independent tower of 70 m in height, which simulates Berger's tower, on the peaks of currents is negligible even for the fast-rising current, which corresponds to a subsequent return-stroke current. The shape of the lightning current at the top of a transmission line is less influenced than that at the top of an independent tower of the same height. Elevated objects significantly decrease the amplitude of vertical electric fields in a close range. If the ground conductivity is infinite, the peak amplitude of electromagnetic field at a distant range will be significantly increased by the presence of an elevated object. In actual cases of finitely conducting ground, however, the increase of the peak field is limited due to the propagation effect.

This paper addresses the exploitation of offshore wind energy. A good example is in the shallow coastal waters around the Baltic Sea where there is high potential for wind turbines located at 10 to 30 km from the shore. The underwater transmission of power to shore has to be by cables. Therefore, dc transmission is required to avoid the large capacitive reactance currents of ac cables. This paper describes the optimal acquisition and aggregation of wind power by multiterminal HVdc based on sinusoidal pulse-width-modulated, three-phase voltage-source converters (SPWM-VSCs) connected at their ac terminals to the wind turbine generators.