To protect the transmission line from lightning stroke and avo... | Electrical MCQ | Examoogle

ElectricalPower System

To protect the transmission line from lightning stroke and avoiding Back Flashover, Tower footing resistance (Rt) should not be more than _________ ohm in HV line and ________ ohm in EHV line.

10, 10

10, 20

5, 15

Correct Answer

⚙️ TE • Technical Concept & PrincipleElectricalPower System

Option B

10, 20

Quick Summary:

To effectively protect high-voltage (HV) and extra-high-voltage (EHV) overhead transmission lines from lightning strokes and to avoid back flashover, the tower footing resistance ( $R_t$ ) must be kept as low as possible. For HV lines (up to $66\text{ kV}$ / $132\text{ kV}$ ), $R_t$ should not exceed $10\ \Omega$ , whereas for EHV lines ( $220\text{ kV}$ and above), $R_t$ should ideally not exceed $10\ \Omega$ to $20\ \Omega$ depending on line insulation levels, with standard guidelines allowing a upper operational ceiling of up to $20\ \Omega$ in difficult terrains.

⚙️TETechnical SolutionConcept & Principle

📜 Standard Reference & Code Clause

IEEEStd 1243 / IS 3043:2018 Cl. 12

💡 Explanation

🔢 Key Formulas

$V_{top} = I \times R_t$ — Tower top potential during lightning discharge

$V_{flashover} > V_{insulation}$ — Condition causing Back Flashover across insulator string

⚙️ Working Principle

When lightning strikes a ground wire or top of a transmission tower, a steep surge current ( $I$ ) flows to the ground through the tower footing resistance ( $R_t$ ). This creates a massive voltage surge at the tower top given by $V_{top} = I \times R_t + L \frac{di}{dt}$ . If $V_{top}$ exceeds the impulse flashover voltage level of the insulator string, an arc discharges from the tower structure across the insulator to the phase conductor; this phenomenon is known as Back Flashover.

📌 Key Points

▸
Tower footing resistance ( $R_t$ ) is the resistance offered by the tower foundation to the earth discharge path.
▸
High $R_t$ increases potential at tower top during lightning strikes, promoting back flashover from tower to conductor.
▸
Methods to reduce $R_t$ include counterpoise wires, treated earth electrodes, and chemical grounding compounds.

✅ Advantages

▸
Low $R_t$ prevents back flashovers and limits insulator damage.
▸
Increases the operational reliability and critical impulse withstand voltage ( $CIL$ ) of the transmission system.

❌ Disadvantages / Limitations

▸
Achieving low $R_t$ (below $10\ \Omega$ ) in high resistivity rocky or sandy soils requires expensive counterpoise installations.

🛠️ Applications / Uses

▸
High Voltage (HV) transmission lines ( $66\text{ kV} - 132\text{ kV}$ ).
▸
Extra High Voltage (EHV) transmission lines ( $220\text{ kV} - 765\text{ kV}$ ).

📄 Additional Information

▸
In HV lines, $R_t$ should ideally not exceed $10\ \Omega$ .
▸
In EHV lines, values up to $20\ \Omega$ are considered acceptable upper design limits in difficult terrains, though $< 10\ \Omega$ is preferred wherever economically feasible.
▸
Option B ( $10,\ 20$ ) correctly captures these standard design upper limits for HV and EHV lines.

📊 Diagram / Illustration

✅

B is correct — Tower footing resistance should not exceed $10\ \Omega$ in HV lines and $20\ \Omega$ in EHV lines to effectively safeguard against back flashovers.

Core Concepts Used

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Tower Footing Resistance ($R_t$) Back Flashover Lightning Impulse Protection

💡 EXAM TIP

Always remember that back flashover occurs from the tower structure to the phase conductor (reverse direction of normal flashover), and its severity is directly proportional to tower footing resistance $R_t$ .

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