A lightning stroke is deﬁned as a direct stroke if it hits either the tower or the shield wire or the phase conductor. This is illustrated in Figure 1. When the insulator string at a tower ﬂashes over by direct hit either to the tower or to the shield wire along the span, it is called a backﬂash; if the insulator string ﬂashes 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 ﬂashover is caused by backﬂash 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 know n is the insulation strength of the system. It is not a unique number. It varies according to the ty pe of the applied voltage, e.g. DC, AC, lightning , or switching surges.
For the purpose of lightning performance, the insulation strength has been deﬁned in two ways:
- Basic impulse insulation level (BIL),
- Critical ﬂashover voltage (CFO or V50 )
BIL has been deﬁned in two ways.
The statistical BIL is the crest value of a standard (1.2 =50- ms) lightning impulse voltage, which the insulation will withstand with a probability of 90% under speciﬁed conditions. The conventional BIL is the crest value of a standard lightning impulse voltage, which the insulation w ill withstand for a speciﬁc number of applications under speciﬁed conditions. CFO or V 50 is the crest value of a standard lightning impulse voltage, which the insulation will withstand during 50% of the applications.
Analysis of direct strokes to overhead lines can be divided into two classes: unshielded lines and shielded lines.
SOURCE: Power Systems, Pritindra Chowdhuri – Tennessee Technological University