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Examoogle • User • info@examoogle.com • EE-2024-8821
Chapter 1 of 12 • Page 1 of 248🔒 Protected PDF • Watermarked
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Overhead ground wires are used to protect a transmission line against
Line to ground faults
Arcing earths
Voltage surges due to direct lightning stroke
High Voltage oscillations due to switching
Voltage surges due to direct lightning stroke
Overhead ground wires, also known as earth wires or shield wires, are installed above the phase conductors of a transmission line to intercept direct lightning strikes. They provide a low-impedance path to the ground, thereby preventing the lightning stroke from hitting the live conductors and protecting the power system from severe overvoltage transients.
Overhead ground wires, also known as earth wires or shield wires, are installed above the phase conductors of a transmission line to intercept direct lightning strikes. They provide a low-impedance path to the ground, thereby preventing the lightning stroke from hitting the live conductors and protecting the power system from severe overvoltage transients.
V=Is×Zs — where V is the induced surge voltage, Is is the lightning surge current, and Zs is the surge impedance of the shield wire.
θ=21×Protection Angle — characterizes the zone of protection provided by the ground wire.
The ground wire is grounded at every tower, creating a high-frequency path for the lightning current surge. By shielding the phase conductors within a specific protection angle, the wire forces the lightning to strike the shield wire rather than the phase lines. This limits the potential rise of the line insulation by safely discharging the surge current into the tower footing resistance.
Ground wires are typically made of galvanized steel or ACSR (Aluminum Conductor Steel Reinforced).
They are connected to the tower tops and provide continuous grounding along the line route.
The protection angle is usually kept between 20° and 30° for high-voltage lines.
Effectiveness depends significantly on the tower footing resistance.
Prevents direct lightning strikes on phase conductors
Reduces the magnitude of traveling waves entering substations
Provides a path for ground fault currents
Increases the total weight and mechanical load on the transmission towers
Requires proper tower footing resistance maintenance to be effective
Additional capital cost of installation
High Voltage (HV) transmission lines
Extra High Voltage (EHV) and UHV transmission lines
Substation lightning protection
Arcing earths (Option B) are typically related to insulation failure or resonance in ungrounded systems.
Switching surges are caused by circuit breaker operations and are managed via damping resistors or surge arresters.
If the tower footing resistance is too high, 'back flashover' can occur even with ground wires.
C is correct — Overhead ground wires act as a shield to intercept direct lightning strokes and divert the current safely to the ground.
Always remember that ground wires protect against direct strokes, while surge arresters (Lightning Arresters) provide equipment-level protection against both direct and induced surges.