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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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How we can compensate for the higher value of Tower footing resistance (Rt) in transmission line?
increase insulation disc
increase tower hight
Increase Earth wite count
All of these
increase insulation disc
High tower footing resistance (Rt) leads to a significant rise in tower potential during a lightning strike, which can cause back-flashover to the transmission line conductors. Increasing the number of insulation discs effectively raises the insulation level (Basic Insulation Level - BIL) of the string, allowing it to withstand higher potential differences without flashing over.
High tower footing resistance (Rt) leads to a significant rise in tower potential during a lightning strike, which can cause back-flashover to the transmission line conductors. Increasing the number of insulation discs effectively raises the insulation level (Basic Insulation Level - BIL) of the string, allowing it to withstand higher potential differences without flashing over.
Vtower=I×Rt — Potential rise of the tower due to lightning stroke
BIL∝Ndiscs — Basic Insulation Level is directly proportional to the number of insulator discs
When a lightning stroke hits the tower, the tower potential rises by V=I×Rt. If this potential exceeds the insulator string's withstand voltage, an arc forms across the insulator string (back-flashover). By adding insulation discs, the total withstand voltage capacity of the string increases, reducing the probability of flashover even if Rt is high.
Tower footing resistance is the primary determinant of lightning-induced back-flashover severity.
Increasing Rt increases the tower surge potential.
Counterpoise wires are an alternative method to reduce Rt physically.
Increasing insulation is a compensatory measure for higher voltage withstand requirements.
Improved protection against back-flashover
Increases the reliability of transmission line insulation
Increases the cost of the transmission tower string
May require larger tower heights to maintain ground clearance
High Voltage (HV) and Extra High Voltage (EHV) transmission lines
Areas prone to high soil resistivity and frequent lightning
Option B (increase tower height) actually increases surge impedance and potential risk, not safety.
Option C (increase earth wire count) improves shielding but does not directly compensate for existing footing resistance in the same way as increasing BIL.
A is correct — Increasing the number of insulation discs raises the insulator string's withstand voltage capacity, effectively preventing back-flashover caused by high tower footing potential.
Always remember that back-flashover occurs when the potential difference across the insulator string exceeds its flashover voltage; focus on increasing string strength or decreasing footing resistance.