Browsing by Author "Omidiora, Michael Adebayo"
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- Lightning overvoltage studies in medium voltage distribution lines
Helsinki University of Technology | Master's thesis(2008) Omidiora, Michael AdebayoThis research work involves simulations, laboratory experiments and statistical analyses of lightning overvoltages in MV distribution lines. The present understanding of lightning models and the nature of lightning overvoltages are also critically investigated and evaluated. All simulations are carried out with reference to fast transient simulation guidelines as specified in the lightning literature. In the experimental work, the electrical breakdown characteristics of lightning impulse voltages are observed on both the tree and the grounded conductor with variations in the striking distances from the lightning electrode to the two objects. This is used to determine the attractiveness of the tree on direct strokes to the grounded conductor. A calibration constant is extracted, which corresponds to the probability of occurrence of direct strokes to both the tree and the grounded conductor being equal. This constant is then incorporated in the electro geometric model to determine the influence of tree height on the exposure arc of a grounded conductor. The experimental result is compared with similar work available in the literature. Finally, the statistical analyses are performed in order to estimate the lightning performance level of the two existing distribution line designs in Finland, which are overhead lines and covered conductors, and also investigate improvements. - Modeling and experimental investigation of lightning arcs and overvoltages for medium voltage distribution lines
Sähkötekniikan korkeakoulu | Doctoral dissertation (article-based)(2011) Omidiora, Michael AdebayoIn this dissertation, lightning overvoltages in Medium Voltage (MV) lines are thoroughly investigated. The other goal is to propose new protection schemes for the designs. The lines consist of overhead lines, underground cables and covered conductors. These overvoltage problems range from direct and indirect strokes to lightning arcs. All the models and simulations are developed using the Electromagnetic Transient Program (EMTP) and Finite Element Method (FEM), while MATLAB is used for post-processing the results and identification of the model parameters. Improvement in the surge protection of MV overhead lines is demonstrated with a combination of surge arresters and a shield wire. Using the IEEE 34-node feeder injected with multiple lightning strokes, the feeder is simulated using EMTP. The response of the line is modeled both with and without the surge protection devices. The simulation study extends to the performance of a MV underground cable due to a nearby lightning discharge using FEM. The use of shield wire for limiting the overvoltage stress in the cables is proposed. A numerical analysis and simulations are performed to determine the outage rate of MV covered conductors due to lightning strokes of different characteristics. The optimum distance for surge protective devices on the conductors is also assessed. An enhancement in the surge analysis of distribution lines with the shielding effect of trees is proposed. An experimental study shows that a tree can intercept a lightning stroke in the vicinity of a distribution line. This study also analyzes experimental results of the shielding effectiveness of a tree and the induced voltages existing between the tree and the distribution line. The study is extended to evaluate the induced voltage on a distribution line for larger clearances using a Rusck model. This work investigates the lightning arc between an overhead line and a nearby tree under artificial rainfall. A full-scale laboratory experiment confirms that a direct stroke to a tree can cause severe damage to nearby power lines by initiating an arc channel through air to the conductors. A complete model of this phenomenon is developed by combining the existing static and dynamic arc equations. The model is accomplished by the bilateral interaction between the EMTP and Transient Analysis Control System (TACS) field. The experimental results have been reproduced by the computer simulations. The performance of the arc phenomenon is examined using a typical Finnish distribution network design. Using the modified arc model, the lightning arc performance of the MV/ LV network under the influence of nearby trees and the network characteristics is evaluated.