Modelling and Analysis of Lightning Overvoltage Protection of MV Cable Laterals Connected with Overhead Lines

摘要

The lightning strokes and switching actions taking place in electricity distribution networks result in overvoltages, which most of the times cannot be avoided. The most common type of fault is the single-phase earth fault, however, short circuits may represent a considerably higher share of experienced faults during lightning, which are stressful and causing in addition to interruptions, sags [1, 2]. Three types of overvoltages are essentially distinguished as temporary, switching, and lightning overvoltages [3]. Temporary overvoltages occur during load rejection or because of earth connection faults and the duration of such operating frequency oscillating overvoltage lies between 0.1 seconds and several hours with the surge magnitude not exceeding above 3 p.u. Switching overvoltages occur during switching actions and consist mostly of heavily damped oscillations with frequencies up to several kHz and a magnitude up to 3 p.u. Lightning overvoltage originate in atmospheric discharges. They reach their peak value within a few us and subsequently decay very rapidly. The magnitude of these uni-polar overvoltages in a MV network can reach well over 10 p.u. and they are considered the greatest threat to the MV networks. The lightning can touch a power line by striking either a conductor or a tower or an earth wire, causing the significant overvoltages classifies like the most dangerous constraints for the electric systems [3,4]. Overvoltage protection must be arranged in such a way as to confine the overvoltage to non-damaging values. The earth wire protection in MV networks is generally not very effective. Due to the small distance between the earth wire and the line wires, a direct lightning stroke hits usually the line wires as well. In addition, induced overvoltages in the line wires (indirect effects of the lightning strokes) cannot be avoided by the earth wires. The most effective protection against overvoltages in a MV network is therefore the use of surge arresters in the vicinity of the electrical equipment [5-9].