Shinichi Sumi

Artificially triggered lightning experiments to an EHV transmission line

In a triggered-lightning experiment carried out on a mountain in Japan, thirteen strikes were obtained for the winter seasons of 1986 and 1987. The lightning occurred on the conductor and the tower of a 275 kV test transmission line, on the ground, etc. The authors outline the triggering method and the experimental results obtained from electrical and optical observations. Comparatively intense lightning with a current greater than 50 kA or a charge greater than 200 C was obtained. >

A study of lightning protection for wind turbine blade by using creeping discharge characteristics

A great number of onshore wind power plants are constructed and planned. And many large-scale offshore wind power plants are constructed and planned for the past some years. Almost large wind turbine blades have the receptor on the blade for the lightning protection, and down conductors are connected to receptor inside the blade. However, the wind turbine blades have been damaged by lightning. So, it is need to develop new lightning protection method for the blade with receptor. The authors thought that it is possible to lead the lightning discharge to the receptor smoothly on the blade by using the backside electrode. In addition, the authors also thought that it is easy to discharge on the blade when the surrounding seawater was sprayed on the blades of offshore wind turbine. The paper shows two experimental results. The lightning impulse discharge on the test model blade was found to lead to the receptor smoothly by using the backside electrode. However, other problem was emerged. The flashover voltage was decreased on a blade peace and glass fiber reinforced plastics (GFRP) board piece when the surfaces of the test pieces were sprayed with salty moisture.

Transient grounding characteristic of wind turbines affecting back-flow lightning current into distribution system

Lightning damages to facilities inside or in the vicinity of wind turbines become remarkably common. Most of the breakdowns and malfunctions of the electrical and control systems inside or in the vicinity of wind turbines are caused by ground potential rise due to lightning current flowing into the grounding system of the turbine and back-flow lightning current due to the ground potential rise. To clarify the transient characteristics of the wind turbine grounding systems and to understand the mechanism of the breakdowns and malfunctions, we have carried out a lot of field tests and simulations. In this paper, transient grounding characteristics of a wind turbines foundation are discussed.

Validations of lightning protections for accidents at a wind farm

In Taikoyama wind farm in Kyoto, Japan, wind turbines have often damaged by lightning in winter. In the winter from 2011 to 2012, two typical accidents have been occurred. In one of the accidents, a fire occurred inside the wind turbine close to the lightning protection tower on January 28th, 2012. The accident is caused by lightning to the wind turbine itself or the lightning protection tower; the BTB (Back-To-Back) system had remarkably burned out. Another accident is the breakdowns of the anemometers placed on the lightning protection tower on March 24th, 2012. In this paper, the causes and the protection methodologies for those accidents have been introduced. To confirm reliability of the protection methods, experiments of transient grounding characteristics and a simulation model of the FDTD (Finite Difference Time Domain) method have been made.

Derivations of Effective Length Formula of Vertical Grounding Rods and Horizontal Grounding Electrodes Based on Physical Phenomena of Lightning Surge Propagations

Grounding is essential to build effective lightning protection design for electric power equipment. The main role of grounding is to disperse abnormal current such as that from lightning into the soil via a grounding electrode. When the lightning current flows into the grounding electrode, the electrical potential of the electrode rises, and overvoltage arises between the equipment connected to the grounding electrode and nearby equipment. The overvoltage causes equipment to breakdown or malfunction. An effective grounding electrode controls the overvoltage at the equipment in a lightning strike and can ensure the normal operation of electrical equipment. The most common grounding electrodes are the vertical grounding rod and the horizontal buried electrode. A property of these grounding electrodes, known as effective length, has been investigated by many researchers. Many formulas to express effective length have already been proposed, all of which are determined heuristically on the basis of formulas that approximate the property from experimental or analytical test results; they are not formulas derived based on the physical phenomenon of grounding. In this paper, we first report on the results of the derivation of a formula for effective length based on the propagation velocity within the soil and the depth of penetration of the lightning surge into the soil. The aforementioned formula still has unknown constants. To decide those parameters, the finite-difference time-domain method is utilized.

Transient characteristic of an actual wind turbine grounding system

Accidents caused by natural phenomena such as lightning are serious problems for the diffusions of wind power generations. Those problems affect the safety and reliability of wind turbine generator systems. Most of the breakdowns and malfunctions of the electrical and control systems inside or in the vicinity of wind turbines are caused by ground potential rise due to lightning. To solve those problems, the field tests have been carried out at 11 sites so far. In this paper, new measurements of transient grounding characteristics carried out at a wind turbine in the vicinity of the Sea of Japan have been presented.

Transient grounding characteristics of a wind turbine foundation with grounding wires and plates

Lightning damages to facilities inside or in the vicinity of wind turbines become remarkably common. Most of the breakdowns and malfunctions of the electrical and control systems inside or in the vicinity of wind turbines are caused by ground potential rise due to lightning current flowing into the foundation with grounding electrodes and wires consisting of the wind turbine grounding system. To clarify the transient characteristics of the wind turbine grounding systems and to understand the mechanism of the breakdowns and malfunctions, we have carried out field tests. In this paper, the measurements and analyses of the transient grounding characteristics carried out at a wind turbine site are described.

EMTP models of a wind turbine grounding system

Installations of wind turbine generator systems are noteworthy examples of eco-friendly energy generation. Because larger-scale facilities and increased electrical capacity can be realized by increasing the number of turbines, wind energy can be utilized more efficiently than other clean energy sources. However, the number of lightning strikes has increased with the popularization of wind turbine generator systems. The grounding potential, which may cause malfunctions of equipment inside and in the vicinity of the wind turbine and accidents, increases owing to the flow of lightning current into the ground. The grounding potential rise depends on the transient characteristics of the grounding systems. Therefore, it is important to clarify these grounding characteristics. In this paper, an EMTP (Electro Magnetic Transients Program) models using lumped parameters are proposed for a wind turbine grounding system. The EMTP models are verified by comparing with the FDTD models. Incidentally, the FDTD (Finite Difference Time Domain) simulation results have already verified by comparing with measurements on an actual wind turbine site.

About 100 years survey of the surface temperatures of Japan sea and lightning days along the coast

On the website of the Japan Meteorological Agency, various weather data are shown. This paper has surveyed the hundred years data of the surface temperatures of Japan Sea and the number of lightning days in cities along the Japan Sea coast. The sea surface temperatures have increased by 1.2 °C to 2.2 °C in the last hundred years, and correspondingly the number of lightning days has increased by 20 to 45 days in the last hundred years. The correlation coefficient for winter lightning shows rather high dependence of the lightning days on the sea temperature in the cities located in a high latitude.

Observation of lightning current in the soil by rocket-triggered lightning

In recent years, there has been a significant increase in the damage to electrical and electronic devices owing to lightning surge-induced overvoltage/overcurrent, in Japan. In particular, the lightning surge overcurrent that flows through grounding lines can cause serious damage to electronic equipment. To understand the phenomenon of the lightning surge current propagation in the ground poles, it is important to understand the lightning current propagation in the soil. We have developed a detection circuit for determining the lightning current distribution in the soil and have measured the lightning current caused by rocket-triggered lightning in the soil. We have observed the lightning current in the soil owing to a negative precursor discharge at a depth of 0.1 m, 30 m from the triggered flash point.