Mikihisa Saito

Lightning Electric Field Characteristics Associated With Transmission-Line Faults in Winter

In this paper, lightning electric field waveforms related to notable power transmission-line faults in winter along the coastline of the Sea of Japan were examined. Most of the waveforms of the observed electromagnetic field changes, which were more than five times as intense as an average negative return-stroke fields, are inferred to be associated with upward lightning discharges. Lightning discharges that produce such characteristic waveforms are termed ground-to-cloud flashes, and properties of the associated electric field waveforms are examined. These lightning discharges in winter distribute around the coastline of the Sea of Japan in different ways depending on their polarities.

Reproduction of electromagnetic field waveforms and tower currents associated with return strokes struck Tokyo Skytree

Observation of lightning current by using Rogowski coils started in February 2012 at Tokyo Skytree, which is a 634-m high freestanding broadcasting tower. Electromagnetic field waveforms are also observed by capacitive antennas at several tens of kilometers from the tower. Lightning current and electric field waveforms simultaneously observed in May 2012 are reproduced by using an electromagnetic model of a return stroke with the help of NEC-4. Observed lightning current waveforms are well reproduced by modeling the tower with small number of thin wires. Electromagnetic field waveforms also are reproduced by an inclined lightning channel model reconstructed from still photographs.

Influence of geometrical shape of return stroke channel on associated electromagnetic fields

Geometry of a lightning return-stroke channel influences the associated electric and magnetic field waveforms. Influence of the channel geometry in the larger scale on field waveforms is investigated through numerical electromagnetic analysis by using an electromagnetic model of lightning channel with the help of NEC-4. The geometry of the channel model is determined based on the observation of natural lightning by a lightning mapping array (LMA).

Lightning incidence on wind turbines in winter

In Japan, transmission lines or wind turbines in the coastal area of the Sea of Japan have suffered from higher frequencies of serious troubles due to lightning in winter than those in summer. Lightning which hits tall structures in winter in Japan has turned out to be mostly upward lightning, and the number of lightning hits to such structures is far more than that in summer although lightning flash density in winter is much lower. To evaluate lightning risk to tall structures in such an area, an experimental formula to estimate the frequency of lightning hits on wind turbines in winter in Japan is proposed. From 2008 to 2013, lightning hits to wind turbines were observed at 27 sites in Japan under a research project of NEDO (New Energy and Industrial Technology Development Organization). The aspects of attachment of upward lightning observed in this project are also reported on.

Influence of geometry of — GC strokes on associated electromagnetic waveforms

GC (Ground to Cloud) strokes are specifically observed in winter along the coast of the Sea of Japan, characterized by high range-normalized amplitudes and bipolar waveforms in their electromagnetic radiation. The strokes cause severe transmission line faults, but their modeling, which is necessary to estimate current parameters from electromagnetic radiation, has not been reached to agreement yet. In this paper, relationship between the geometry of lightning channels and electric field waveforms of negative GC strokes are investigated, and estimation of the peak current and location error are discussed.

Probability of lightning hits to tall structures taking account of upward lightning

It was revealed that serious lightning faults of high voltage power transmission lines in winter along the coastal area of the Sea of Japan were due to upward lightning initiated from transmission towers. Wind turbines in the same region, reportedly suffer from high lightning fault rate, must also generate frequent upward lightning. Taking account of the aerological data which indicate the heights of the charged region in thunderclouds, a method to evaluate frequency of upward lightning from tall structures in cold regions of Japan in winter is proposed. With this new method, it is possible to evaluate the probability of lightning hits to tall structures in Japan through the year.

Lightning strokes forming hot spots observed by LLS in winter in Japan

Hot spots, where lightning strokes located by LLS (Lightning location system) concentrate, are formed almost in winter only in Japan in the coastal area of the Sea of Japan. The lightning strokes forming a hot spot are inferred to be related to upward lightning strokes which frequently damage wind turbines and electrical facilities in winter. Characteristics of lightning strokes forming a hot spot are investigated by using directly ob-served current, electric field waveforms and LLS data.

Upward and downward lightning observed at Tokyo Skytree

This paper describes observations of lightning current waveform at Tokyo Skytree, which stands in the eastern area of central Tokyo, located in the Kanto Plain, and is a 634-m high freestanding broadcasting tower. Direct observation of lightning current by using a pair of Rogowski coils started in the end of February 2012. From 2012 to the end of 2013, currents associated with 24 lightning flashes which hit the tower were recorded. In 2012, most of the flashes were upward as was expected; however, in 2013, most of the flashes observed in summer were downward. We have also evaluated statistical characteristics of obtained lightning current waveforms.