Air clearances, 220 kV line for 200 MW

The air clearances fix the tower head dimensions. They depend on the voltage, not on the power or the length; those decide the conductor. The 200 MW, 120 km data are used later for conductor selection.

Data and assumptions: 220 kV nominal, maximum system voltage 245 kV (phase rms 141.45 kV); 254 × 146 mm discs; maximum string swing 45°; minimum air clearance 6.5 inch per 10 kV (phase rms) plus 8 inch safety; tower body width 1.5 m; single circuit in a triangular (offset vertical) layout, with two phases on one side and one on the other.

Minimum air clearance

$$a = 6.5\times\frac{141.45}{10} + 8 = 99.94\ \text{in} = 2.54\ \text{m}$$

ii) Insulator string length

• 14 discs, the standard 220 kV suspension string. The table check (1-min wet withstand 395 kV and impulse 900 kV) needs 10 discs; extra discs are added for switching surges, pollution and one defective disc.
• Length:

$$l = 14\times0.146 + 0.3 = 2.34\ \text{m}$$

i) Cross arm length

With the string swung 45°, the conductor must still be at least $a$ from the tower body:

$$X = a + l\sin45^\circ = 2.54 + 2.344\times0.707 = 4.20\ \text{m}$$

iii) Separation of conductors

• Vertical: the conductor below one cross arm must clear the next cross arm by $a$:

$$Y = l + a = 2.344 + 2.539 = 4.88\ \text{m}$$

• Horizontal (between the phases on opposite sides of the tower):

$$S_h = 2X + w = 2(4.196) + 1.5 = 9.89\ \text{m}$$

• The resulting phase distances are $\sqrt{9.89^2 + 4.88^2} = 11.03$ m between adjacent phases and $2Y = 9.77$ m between the top and bottom phases. The GMD is 10.59 m.

iv) Height of earth wire above the top conductor

Use one earth wire at the peak with a shielding angle of at most 30°:

$$h_e = \frac{X + w/2}{\tan30^\circ} = \frac{4.946}{0.5774} = 8.57\ \text{m}$$

This is about 6.2 m above the top cross arm.

            E   (1 earth wire)
            |\  30 deg
            | \
       o----+--*  top phase (A)
  (B)  *----+     Y = 4.88 m
            +--*  bottom phase (C)
            |  X = 4.2 m
   <------ 9.89 m ------>