The high electromagnetic fields surrounding any stricken conductor will induce currents and
couple voltages in nearby, unstricken conductors through their mutual surge impedances. In the
case where lightning strikes a grounded overhead shield wire, this coupling increases common-mode
voltage and reduces differential voltage across insulators. Additional shield wires and corona [11,12]
can improve this desirable surge–impedance coupling to mitigate half of the total tower potential rise
(VRþVL).
The strong electromagnetic fields from vertical lightning strokes can induce large overvoltages in
nearby overhead lines without striking them directly. This is a particular concern only for MV and LV
systems.
17.5 Insulation Strength
Power system insulation is designed to withstand all anticipated power system overvoltages. Unfortu-
nately, even the weakest direct stroke from a shielding failure to a phase conductor will cause a lightning
flashover. Once an arc appears across an insulator, the power system fault current keeps this arc alive
until voltage is removed by protective relay action. Effective overhead shielding is essential on trans-
mission lines in areas with moderate- to high-GFD.
When the overhead shield wire is struck, the potential difference on insulators is the sum of the
resistive and inductive voltage rises on the tower, minus the coupled voltage on the phase conductors.
The potential difference can lead to a ‘‘backflashover’’ from the tower to the phase conductor. Back-
flashover is more frequent when the stroke current is large (5%>100 kA), when insulation strength is
low (<1 m or 600 kV basic impulse level), and=or when footing resistance is high (> 30 V). Simplified
models [11,12] are available to carry out the overvoltage calculations and coordinate the results with
insulator strength, giving lightning outage rates, in units of interruptions per 100 km=year.
17.6 Mitigation Methods
Lightning mitigation methods need to be appropriate for the expected long-term GFD and power
system reliability requirements. Table 17.1 summarizes typical practices at five different levels of
lightning activity to achieve a reliability of one outage per 100 km of line per year on an HV line.
17.7 Conclusion
Direct lightning strokes to any overhead transmission line are likely to cause impulse flashover of
supporting insulation, leading to a circuit interruption. The use of overhead shield wires, located
above the phase conductors and grounded adequately at each tower, can reduce the risk of flashover
by 95–99.5%, depending on system voltage.
TABLE 17.1 Lightning Mitigation Methods for Transmission Lines
Ground Flash Density Range Typical Design Approaches
0.1–0.3 Ground flashes=km^2 per year Unshielded, one- or three-pole reclosing
0.3–1 Ground flashes=km^2 per year Single overhead shield wire
1–3 Ground flashes=km^2 per year Two overhead shield wires
3–10 Ground flashes=km^2 per year Two overhead shield wires with good grounding or line surge arresters
10–30 Ground flashes=km^2 per year Three or more overhead and underbuilt shield wires with good grounding, line
surge arresters; underground transmission cables