| As an important part of the power system, distribution network is responsible for distrib-ute power to users, and its safe, reliable and stable running is crucial. However, due to the lowinsulation level and weak lightning protection of overhead distribution lines, lightning mayaccount for many power interruptions. Lightning may cause flashovers on distribution linesfrom both direct strikes and induced voltages from nearby strikes. Over-voltages due to directstrike can be calculated with the classic circuit analysis method. Induced voltage, produced bycoupling between Lightning Electro Magnetic Pulse(LEMP) and lines, however, should beanalyzed in more complex way, due to the different mechanism compared with direct strikes.Lightning performance assessment and protection has been investigated for a long time in ourcountry, whereas, relative studies of overhead distribution lines are still weaknesses. For thisreason, this paper focused mostly on the calculation of lightning induced over-voltages andlightning performance assessment of distribution networks, and on the effectiveness analysisof different lightning protection measures. The main content of this paper is described as whatfollows.(1)With similarity transformation, the Agrewal model-expression describing couplingrelationship between LEMP and lines, is transformed into mode domain. After that, using themethod of characteristic to analyze the expression, an equivalent circuit for the calculation oflightning induced voltage composed with lumped parameter component is obtained. On thebase of an simplification of calculation method for LEMP, this paper propose a calculationmodel for LIV, applied in Electro-magnetic transient Program, as an example, inPSCAD/EMTDC in this paper. Lightning induced voltages of different lightning and line parameters is calculated with proposed model. The results shows that lines located in highresistivity region should be protected with particular emphasis, due to the enhancement ofresistivity on lightning induced voltage. Lightning induced voltage can be decreased byinstalling surge arresters and grounded shield or neutral conductor. The smaller the altitudedifference between line conductors and neutral conductor, the lightning induced voltagesdecreasing larger.(2)A procedure for assessment of overhead distribution lines lightning performance,considering the influence of both direct strokes and nearby strokes, is proposed in this paperbased on the Monte Carlo method and on using of the PSCAD/EMTDC program. Thelightning performance after adopting different protection measures is calculated by applyingthe proposed procedure, and effectiveness of these measures is analyzed. The result showsthat flashover rate of line insulators due to indirect strokes is low when the insulation level isgreater than300kV, and there is no need to take measures. In order to improve the indirectlightning performance to a satisfying degree, for low insulation level lines located in low orhigh resistivity region, spacing between surge arresters better less than400m or200mrespectively, for high insulation level lines, the spacing better less than600m when resistivityis high, while there is no need for installing arresters for the case of low resistivity. Theshielding wire is effective only for distribution lines with high insulation level and low towerfooting resistance,.(3)Provided that the indirect lightning performance of distribution network with differentarresters placement is not the same, this paper propose a method to optimize its placementusing the Improved Genetic Algorithm. The method firstly simplifies model of the network,based on spatial arrangement of lines, tower geometries and number of surge arresters. Thecoding, selection, cross, mutation method of Simple Genetic Algorithm is improved. With thegoal of minimum the indirect lightning flashover rate, placement of arresters is optimized. |
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