The insulation level of 400 KV overhead transmission line is decided on the basis of

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Option B: Switching over voltage

Explanation & Concepts:
For Extra High Voltage (EHV) transmission lines of 400 kV and above, the insulation level is primarily governed by switching surges rather than lightning overvoltages. While lightning causes higher magnitude transients, switching operations (like circuit breaker closing/reclosing) produce surges with longer durations, making them the critical design factor for the dielectric strength of the insulation.

Working Principle: Switching surges arise from sudden changes in the network state during switching operations (e.g., energizing a long line). These surges have a longer wave front compared to lightning impulses, resulting in a lower impulse ratio for insulators, which forces engineers to design insulation systems that can withstand these specific transient voltages.

Key Formulas: $V_{sw} = k \cdot V_{ph}$ — where $V_{sw}$ is the switching surge voltage, $k$ is the switching surge factor, and $V_{ph}$ is the phase voltage.; $B.I.L. \ge V_{impulse} \cdot SF$ — where $B.I.L.$ is Basic Insulation Level and $SF$ is the Safety Factor.

Key Points: Lightning overvoltage is dominant for lines below 220 kV.; Switching overvoltage becomes dominant for lines 400 kV and above.; Switching surge magnitude typically ranges from 2.0 to 3.5 p.u. of the nominal phase voltage.; Insulation coordination is the process of selecting the dielectric strength of equipment in relation to the voltages that will appear on the system.

Advantages: Reduces the risk of insulation failure during routine switching.; Ensures system reliability and longevity of terminal equipment like transformers.

Disadvantages: Increases the cost of transmission towers and line insulators due to higher clearance requirements.; Requires complex insulation coordination studies.

Applications: Design of 400 kV, 765 kV, and 1200 kV transmission systems.; Specification of Surge Arresters and transformer insulation levels.

Additional Info: For lines. Option A is critical for low-voltage distribution systems but is not the limiting factor for EHV lines.. Options C and D relate to the design of conductor diameter and bundle spacing, not insulation dielectric withstand.

Answer: B is correct — For overhead transmission lines rated at 400 kV and above, the insulation level is primarily determined by switching overvoltages rather than lightning surges.

Exam Tip: Always remember the threshold: 220 kV is the dividing line where switching surges start to dictate insulation requirements over lightning surges.

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Option A: Lightning over voltage

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Option C: Corona inception voltage

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Option D: Radio and TV interference

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