ABSTRACT
A lightning stroke is dešned as a direct stroke if it hits either the tower or the shield wire or the phase conductor. is is illustrated in Figure 7.1. When the insulator string at a tower Âashes over by direct hit either to the tower or to the shield wire along the span, it is called a backÂash; if the insulator string Âashes over by a strike to the phase conductor, it is called a shielding failure for a line shielded by shield wires. Of course, for an unshielded line, insulator Âashover is caused by backÂash when the stroke hits the tower or by direct contact with the phase conductor. In the analysis of performance and protection of power systems, the most important parameter that must be known is the insulation strength of the system. It is not a unique number. It varies according to the type of the applied voltage, for example, DC, AC, lightning, or switching surges. For the purpose of lightning performance, the insulation strength has been dešned in two ways: basic impulse insulation level (BIL) and critical Âashover voltage (CFO or V50). BIL has been dešned in two ways. e statistical BIL is the crest value of a standard (1.2/50 μs) lightning impulse voltage, which the insulation will withstand with a probability of 90% under specišed conditions. e conventional BIL is the crest value of a standard lightning impulse voltage, which the insulation will withstand for a specišc number of applications under specišed conditions. CFO or V50 is the crest value of a standard lightning impulse voltage, which the insulation will withstand during 50% of the applications. In this chapter, we will use the conventional BIL as the insulation strength under lightning impulse voltages. Analysis of direct strokes to overhead lines can be divided into two classes: unshielded lines and shielded lines. e šrst discussion involves the unshielded lines.
