400 kV lines: configuration, V-string, earth wires

At 400 kV and above, phase-to-tower clearances (about 3–3.5 m), insulator strings (about 4 m) and conductor bundles are large. This changes the best layout of the tower.

i) Horizontal conductor configuration

• With a vertical or triangular layout, each phase needs its own cross arm and about 8–9 m of vertical spacing. The tower would become very tall, roughly 50–60 m, giving a large overturning moment and a heavy, costly tower.
• A horizontal layout puts all three phases at one level. The tower is the lowest possible, which reduces bending moment, tower weight, foundation cost and wind load.
• A lower tower is also struck by lightning less often.
• A wide right-of-way is needed, but at 400 kV it is wide anyway because of the clearances. Horizontal-configuration towers (waist type or portal/H-frame) are therefore the common choice.

ii) V-type insulator string

• An I-string can swing under wind, up to 45° or more. The cross arm must then be long enough for the swung conductor to keep its clearance from the tower: $X = a + l\sin\theta$. At 400 kV, with $l \approx 4$ m, this is very long.
• A V-string has two legs at about 90° to each other, so the conductor cannot swing. The phase spacing and window size can be reduced. The tower top becomes narrower and lighter, and the right-of-way is reduced.
• The two legs share the load, so a V-string can carry heavy quad-bundle conductors and gives better mechanical security.
• It also has better self-cleaning and pollution performance, because water runs off the inclined discs.

iii) Double earth wire

• In a horizontal configuration the outer phases are 10–12 m from the centre line. A single central earth wire would give a shielding angle far above the required 20–30°, and the outer phases would suffer shielding failures.
• Two earth wires, placed above or near the outer phases, give a small (even negative) shielding angle to all three phases.
• Two parallel earth wires also lower the surge impedance of the earth path and the tower-top potential. This reduces back-flashover, which matters because 400 kV outages are costly. They also share fault current and can carry OPGW for communication.

     E                       E      earth wires
      \                     /
   ----\--------+----------/----   cross arm
       V        V          V       V-strings
      (A)      (B)        (C)      horizontal phases
                |
               tower