Michael A. Omidiora

Investigation of lightning arc between conductor and nearby tree under artificial rainfall

In the Nordic countries, most distribution networks are installed in forest areas. Direct strokes to a tree can cause severe damage to nearby power lines by initiating an arc channel through air to bare conductors. In this paper, this phenomenon is examined experimentally using a full-scale laboratory setup considering dry-air and wet-air conditions, and the experimental results were reproduced by computer simulations. In a number of the experimental runs performed, it was found that the conducting arc path allows the establishment of a large long-duration lightning current from the lightning-struck tree to the bare conductor under the wet-air condition. Consequently, a higher induced voltage is measured on the conductor. The lightning-triggered arc phenomenon was monitored closely. It was confirm that lightning strokes to a nearby tree can be more intense in the wet-air than in dry-air conditions, and cause a flashover on power lines. The ATP/EMTP computer simulation using the arc model shows that the experimental results, one of which has been photographed and reported in the paper, are reproducible by means of system modeling. Both the experimental and simulation results provide strong evidence confirming the phenomenon of the arc conduction and the induced voltages due to lightning strokes to nearby trees within the vicinity of power conductors. Using a Rusck model, the study is extended to evaluate the induced voltage from the tree to the power conductor for larger clearances.

A comparative study on the shielding effect of tree, concrete building on direct lightning strokes to medium voltage distribution line

Lightning overvoltages are responsible for the majority of faults and outages that occur in Medium Voltage (MV) distribution networks. The overhead lines can be protected from the above phenomena with adequate installation of surge arresters and shield wires. Insulation enhancement can also play a significant role in minimizing outage rates of the lines, though it will not be so effective. However, high and grounded structures such as trees or buildings will naturally protect the lines from direct lightning strokes, if adequate clearance between the lines and the structures is considered. This paper presents the experimental study and electro-geometric modeling on the shielding effect of trees on direct strokes to medium voltage distribution lines. The shielding effect of a nearby tree on an MV distribution conductor is compared with that of a concrete building which has been previously investigated in the literature. It is expected that the results of the comparison will serve as good recommendations for the enhancement of lightning protection in power distribution circuits.

An Approach to the Lightning Overvoltage Protection of Medium Voltage Lines in Severe Lightning Areas

This paper deals with the effect of shield wires on lightning overvoltage reduction and the energy relief of MOV (Metal Oxide Varistor) arresters from direct strokes to distribution lines. The subject of discussion is the enhancement of lightning protection in Finnish distribution networks where lightning is most severe. The true index of lightning severity in these areas is based on the ground flash densities and return stroke data collected from the Finnish meteorological institute. The presented test case is the IEEE 34‐node test feeder injected with multiple lightning strokes and simulated with the Alternative Transients Program/Electromagnetic Transients program (ATP/EMTP). The response of the distribution line to lightning strokes was modeled with three different cases: no protection, protection with surge arresters and protection with a combination of shield wire and arresters. Simulations were made to compare the resulting overvoltages on the line for all the analyzed cases.

Short interruptions caused by lightning in overhead line medium voltage networks

Lightning strokes represent a considerable cause of short interruptions in overhead line medium voltage networks. The overvoltages caused by lightning can not be avoided but their influence can be limited by appropriate overvoltage protection. This paper presents the results of theoretical studies about the number of short interruptions caused by lightning in various environmental circumstances. The results will be compared to the fault statistics collected from Kainuun Energia. In the calculations, the environmental conditions, ground flash densities and various overvoltage protection types of power distribution transformers are taken into account. At the end, further research is discussed.

Performance analysis of lightning overvoltage on medium voltage distribution lines equipped with covered conductors

This paper gives the performance of lightning overvoltage due real lightning discharges that have occurred in distribution power lines equipped with covered conductors. Evaluation and simulations are based on real lightning data collected from the Finnish Meteorological Institute (FMI) from 1998 to 2008. The presented feeder is an overhead distribution line with a covered conductor subjected to indirect lightning strokes to ground and direct lightning strokes to a line support. Evaluation of induced overvoltage from indirect stroke to the line is analyzed with MATLAB. With the Electromagnetic Transient Program (EMTP), simulations of lightning strokes are performed with different lightning current characteristics. All cases are considered with the modelling guidelines for fast transients as specified in some lightning literature. Simulations are made to compare the resulting overvoltages and energy absorptions of surge arresters for all the analyzed cases

Mitigation of Lightning Flashover from Tree to Medium Voltage Aerial Cable Using Shield Wire

This paper presents the performance and protection of medium-voltage (MV) aerial cables against lightning faults by means of laboratory experiment and digital modeling. With the series of laboratory tests, practical examination of lightning hazard to shielded and unshielded aerial cables is conducted, and the need for shield wire protection is established. For the purpose of simulation studies, digital models are developed by using Alternative Transient Program-Electromagnetic Transient Program (ATP-EMTP) and its transient analysis of control system features. The performance of lightning flashover overvoltage on shielded and unshielded aerial cable is evaluated with the developed digital models. The evaluation is made by considering a direct lightning strike on wooden poles, messenger wires, and nearby trees. Based on the experimental and simulation results, practical recommendations are proposed for improvement in lightning performance of aerial cables. This study offers possible solutions for the protection of aerial cables against lightning flashover, induced overvoltage, and direct lightning strike, which may improve electricity reliability and availability to customers.

Influence of rainfall on lightning flashovers from trees to medium voltage bare and covered conductors

In this paper, a laboratory experiment was conducted to compare the performance of lightning discharge characteristics of bare conductor (BC) and covered conductor (CC), resulting from very-close lightning strokes to trees. Special attention is paid to the influence of rainfall on the lightning arc characteristics. All the experiments are performed using full-scale configurations. Other factor investigated is the effects of tree-to-line clearance. The studies reveal a substantial amount of new information which is required to fully understand the true behavior of very-close lightning interaction with power conductors in rainfall, which is often the true lightning situation.