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T/F: V-type string for 400 kV lines

Short answer

FALSE. V‑strings are used at 400 kV and above primarily to eliminate insulator swing, which reduces required air clearance, tower window, and right‑of‑way. The improved mechanical strength is a secondary benefit.

For a given amount of power to be transmitted over a given distance, state whether the following statement is TRUE or FALSE. Justify your answer with a brief explanation: The major reason that V-type insulator string configurations are used for a transmission line of 400 kV and above is to improve the mechanical strength of the string.

FALSE.

The main reason for using V-strings at 400 kV and above is electrical clearance and economy, not mechanical strength.

  • An I-string at 400 kV is long, about 3.5–4 m. Under wind it can swing up to about 45°, and the swung conductor would come too close to the tower. A very long cross-arm and a large tower window would be needed.
  • In a V-string the two legs hold the conductor fixed in the transverse direction, so there is practically no swing. The required clearance is therefore met with:
    • shorter cross-arms and a smaller tower window,
    • closer phase spacing,
    • a narrower right-of-way, and lighter and cheaper towers.

The two legs of a V-string do share the load, so its mechanical strength is higher. That is only a side benefit. When higher strength alone is needed, a double I-string is used instead.

✓ Answer checked by Claude Opus 5.5 (high effort)

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