Akira Asakawa

Voltage induced on a test distribution line by negative winter lightning strokes to a tall structure

Simultaneous measurements of lightning-induced voltages and lightning currents are presented and subject to analysis. The lightning current was measured at the top of a 200-m high stack by a current shunt. In the analysis, both the transmission-line model (TLM) and traveling current-source model (TCSM) of a return stroke are tested, taking account of reflection and refraction of the lightning current waves at the top and bottom of the stack. Analysis shows that the measured induced voltages on a test distribution line were influenced by finite ground conductivity. Agreement of measured induced-voltage waveforms with calculations demonstrates the validity of the adopted models of the stack and the lightning stroke. Both TLM and TCSM turn out to be useful in predicting induced voltage associated with a negative lightning stroke to a tall structure in winter.

Flashover Characteristics of Long Air Gaps with Negative Switching Impulses

The electrogeometric model (EGM) proposed by Armstrong and Whitehead has been used to calculate the lightning shielding features of transmission lines. While the striking distance equation in the EGM uses the 50% flashover voltage characteristics of negative switching impulses for rod-rod gaps, those characteristics for long air gaps have not been sufficiently examined. The present study has therefore focused on clarifying the 50% flashover characteristics of negative switching impulses for long air gaps. Consequently, saturating characteristics have been obtained for rod-plane gaps compared with almost linear characteristics for rod-rod and rod-conductor gaps. Furthermore, new knowledge was obtained indicating that, for air gaps of around 4 to 5 m, the 50% flashover voltage may possibly be lower for rod-plane gaps than for rod- rod and rod-conductor gaps.

Observation of lightning flashes to wind turbines

An observation of winter lightning flashes striking wind turbines has been conducted at Nikaho Kougen Wind Farm in the coastal area of the Sea of Japan. We measured the current waveforms of the lightning flashes using wide-frequency-band Rogowski coils and observed the features of lightning flash striking the wind blades with still cameras and a CCTV camera. 128 flashes were captured from 2005 to 2006 and result of measurement was reported in previous papers [1]. We obtained 150 flashes in following season from 2007 to 2008 and 278 flashes were recorded totally. This paper reports overall result of measurement from 2005 to 2008. The median values of the duration, current peak, and charge transfer of bipolar flashes were larger than those of negative and positive flashes. Most of the flashes were initiated by an upward lightning leader. Furthermore, 30% of the flashes are multiple termination strokes to the wind turbines in winter.

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.

Measurement of lightning currents at TOKYO SKYTREE ® and observation of electromagnetic radiation caused by strikes to the tower

TOKYO SKYTREE® has recently been constructed in the eastern area of central Tokyo, located in the Kanto Plain, and is the tallest free-standing broadcasting tower in the world (634m). When a tall structure is located on a plain, it is expected that lightning will often strike the structure when thunderclouds approach. The authors plan to observe the currents of lightning striking TOKYO SKYTREE and have installed Rogowski coils on the tower at a height of 497m. CRIEPI have also started to observe the electromagnetic radiation generated by lightning strikes to TOKYO SKYTREE. Hemispherical antenna to detect electric fields and loop coils to detect magnetic fields are located at various sites in the Kanto Plain. In this paper, the authors report their plan to conduct measurements using these coils and the system used for electromagnetic observation.

Distribution arrester outages caused by lightning backflow current flowing from customer's facility into power distribution lines

Direct lightning strokes are considered to be a main cause of damage to surge arresters on power distribution lines. Recently, lightning performance of distribution lines has been observed using still cameras, and lightning-caused distribution outages on hilltop areas on the coast of the Sea of Japan have been investigated. This research has shown a possibility that lightning backflow current flowing from customer facilities into distribution lines causes damage to surge arresters on the distribution lines. We have investigated the lightning backflow current flowing from customer facilities into distribution lines as a cause of damage to surge arresters. The main results are as follows: (1) The electric charge of the backflow current flowing into distribution lines is more than 60% of that of the lightning stroke current. (2) If the grounding resistance of the customers facility is not low, the failure rates of a surge arrester caused by backflow current due to winter lightning is more than 90% of that caused by direct lightning strokes.

Observational and experimental study of the lightning stroke attraction effect with ground wire system constructions

Although power distribution lines are protected by ground wire systems, there are fears that these ground wires may break at their corrosion points. For practical purposes, they have therefore been removed to avoid maintenance in certain areas. The Tokyo Electric Power Company (TEPCO) has continuously observed lightning strokes and obtained 321 datasets to date. The present study has determined the potential for an anticipated lightning stroke attraction effect based on the observed data, despite the fact ground wires are eliminated while ground wire caps are left in place. Furthermore, a verification experiment using facilities simulating the power distribution system has confirmed the problem of insulated wires being struck by lightning and the extent of the impact of the ground wire cap on the lightning stroke attraction effect. These results are expected to provide a new facility-construction menu, which will help rationalize the lightning protection design.

Reducing lightning current using surge protector for telecommunications cables

We describe the effectiveness of surge protective devices (SPDs) for telecommunication cables and evaluated them in a full-scale experiment and a field test against lightning surges. Because the extent of the impact of communication failure is wide due to lightning damage to telecommunication cables, an effective countermeasure is required. An SPD is installed in connected splicing closure installed in the branch point and the cable end of a telecommunications cable. We also clarified that the lightning damage rate of telecommunications cable decreased to about 1/3, and the lightning damage rate of telecommunications equipment decreased to about 1/5.

A study of rational lightning protection measures for power distribution lines in the Chugoku region

Recently, current-limiting arcing horn with zinc oxide elements have been installed on distribution lines as lightning protection equipment. In this paper, we show an analytical study of rational lightning protection measures with the above equipment on power distribution lines using the assessment method of lightning risk.

Estimation of damage rate of electronic watt-hour meters caused by transient magnetic field due to direct lightning strokes to distribution line

In Japan, the introduction of smart meters into distribution systems has recently been accelerating. Since smart meters are low-voltage electronic equipment, they are considered to be vulnerable to disturbances such as lightning surge. In this study, we first experimentally examined the lighting failure of smart meters caused by a transient magnetic field. Then, we calculated the lightning damage rate of a smart meter due to a magnetic field by lightning surge analysis. To improve the lightning performance of smart meters, it is effective to ensure a suitable relationship between the bus conductor and the processing unit inside the smart meter.