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Lightning-Induced Transients
Overvoltages in a power system can be caused by transient currents and by transient voltages after switching actions during normal operation or after clearing fault situations. The overvoltages originate from the state of the system. There are also overvoltages that come from outside the system as a result of atmospheric discharges. Large parts of the power system are formed by overhead transmission lines interconnected by outdoor substations. Only in densely populated areas, the high-voltage transmission and distribution is done with high-voltage cables interconnected by gas-insulated substations (GIS) placed in buildings. When we realise that on an average every commercial aeroplane and every square kilometre of theearth’s surface in a countrylike the Netherlandsis hit by lightningonce a year, it is obvious that our power systems should be protected against lightning strokes. For the analysis of the lightning-induced overvoltages, a difference is made between the following:
• Lightning strokes in the vicinity of high-voltage transmission lines, which do not hit the conductors themselves,
• Direct lightning strokes on the line conductors injecting a current wave on the line, and
• Lightningstrokesonthetransmissiontowersoronthegroundwires. In addition, the solar system can be the cause of a blackout of the power
system. Heavy eruptions on the solar surface in a so-called solar spot on March 10th, 1989, caused a large blackout of the Hydro Quebec grid in Canada on March 13th. Geomagnetic-induced currents were the cause of a DC component in the system current, and the power transformers were brought into saturation. The system current got strongly distorted and contained, apart from the power frequency ground wave, higher
harmonics. The higher harmonics increased the apparent reactance of the power transformers, and therefore the transformer also consumed more reactive power. The temperature of the transformer went up, and after a certain period of time, the transformer protection tripped the power transformer out of service.
7.1 THE MECHANISM OF LIGHTNING