Overvoltages Caused by Direct Lightning Strokes

This chapter describes the conventional basic impulse insulation level (BIL) as the insulation strength under lightning impulse voltages. A lightning stroke is defined as a direct stroke if it hits either the tower or the shield wire or the phase conductor. If lightning hits one of the phase conductors, the return-stroke current splits into two equal halves, each half traveling in either direction of the line. One or more conductors are strung and parallel to the phase conductors of single- and doublecircuit overhead power lines to shield the phase conductors from direct lightning strikes. For a shielded line, lightning may strike either a phase conductor or the shield wire or the tower. The statistical BIL is the crest value of a standard lightning impulse voltage, which the insulation will withstand with a probability of 90% under specified conditions. The minimum return-stroke current that causes an insulator flashover is called the critical current, I_c, of the line for the specified BIL.