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Coupling factor of a ground wire can be increased by
Reducing the footing impedance
Increasing the ground wire size
Using cantilever roads on the crossing along with the arm of ground wire
All of above
All of above
The coupling factor (k) in power transmission lines represents the effectiveness of the ground wire (shield wire) in reducing the voltage induced on phase conductors during a lightning strike. It is defined as the ratio of the potential induced on the conductor to the potential of the ground wire; increasing this factor improves lightning performance by reducing the stress on the line insulation.
The coupling factor (k) in power transmission lines represents the effectiveness of the ground wire (shield wire) in reducing the voltage induced on phase conductors during a lightning strike. It is defined as the ratio of the potential induced on the conductor to the potential of the ground wire; increasing this factor improves lightning performance by reducing the stress on the line insulation.
k=VgтАЛVcтАЛтАЛ тАФ Basic definition of coupling factor where VcтАЛ is induced voltage on conductor and VgтАЛ is ground wire voltage
ZfтАЛтЙИ1тИТkZgтАЛтАЛ тАФ Relationship showing impact of footing impedance ZfтАЛ and ground wire impedance ZgтАЛ on lightning protection
The coupling factor depends on the geometric arrangement and the impedance of the grounding system. Reducing footing impedance allows surge currents to dissipate more effectively into the earth, preventing rise in tower potential. Increasing ground wire size decreases the self-impedance and improves the mutual coupling between the ground wire and phase conductors, while cantilever roads (or auxiliary grounding arrangements) optimize the electromagnetic coupling geometry.
A higher coupling factor implies better shielding against lightning strikes.
Lowering tower footing impedance is the most critical factor in mitigating back-flashovers.
Geometric configuration (height/spacing) directly influences the mutual inductance and the resulting coupling coefficient.
Increasing the size of the ground wire reduces its inductive reactance, facilitating faster dissipation of surge energy.
Reduces probability of insulation failure due to back-flashovers.
Enhances overall power system transient stability during lightning surges.
Increased ground wire size leads to heavier mechanical load on transmission towers.
Complex grounding geometries like cantilever arrangements increase installation and maintenance costs.
EHV and UHV transmission line design
Lightning protection systems for HV substations
Coupling factor typically ranges between 0.1 and 0.3 for standard transmission line designs.
Option B (Increasing size) reduces self-impedance of the ground wire, which indirectly enhances the coupling effect by lowering the effective potential rise at the strike point.
D is correct тАФ All listed methods contribute to either reducing the impedance path or improving the electromagnetic coupling between the lightning protection system and phase conductors.
Always remember that in lightning transient analysis, the 'Back-Flashover' occurs when tower potential exceeds the insulation strength; hence, minimizing footing impedance is the highest priority.