Tower dimensions for 220 kV single circuit line

The tower dimensions follow from the minimum air clearance, the insulator string length and the 45° swing. They are taken from the given appendix rules and common IS practice.

Data assumed: maximum system voltage $V_m = 245$ kV. Tower width at cross-arm level is 1.5 m. Discs are 254 × 154 mm.

Minimum air clearance $a$

Using 6.5 inch per 10 kV of maximum phase rms voltage plus 8 inch:

$$\begin{aligned} V_{ph} &= \frac{245}{\sqrt{3}} = 141.45\ \text{kV} \\ a &= 6.5 \times 14.145 + 8 = 99.94\ \text{in} = 2.54\ \text{m} \end{aligned}$$

As a check, 1 cm per kV of phase peak voltage plus 30 cm gives $200 + 30 = 230$ cm. The larger value, $a = 2.54$ m, is used.

(ii) Insulator string length

For 220 kV, 10 discs meet the wet (395 kV) and impulse (900 kV) withstand levels. One spare disc is added, giving 11 discs:

$$l = 11 \times 0.154 + 0.30\ (\text{hardware}) \approx 2.0\ \text{m}$$

(i) Cross-arm length

When the string swings by $\theta = 45°$, the conductor must stay at least $a$ from the tower:

$$\begin{aligned} L_{ca} &= a + l\sin\theta \\ &= 2.54 + 2.0 \times 0.707 = 3.95\ \text{m} \end{aligned}$$

Take 4.0 m from the tower face.

(iii) Conductor separation

• Vertical (between cross-arms): the conductor hanging from the upper arm must keep clearance $a$ from the arm below.

$$V = l + a = 2.0 + 2.54 = 4.54\ \text{m} \approx 4.6\ \text{m}$$

• Horizontal (conductors on opposite sides of the tower):

$$H = 2L_{ca} + w = 2 \times 4.0 + 1.5 = 9.5\ \text{m}$$

This is well above the phase-to-phase clearance (about $1.15a \approx 2.9$ m).

(iv) Number of earth wires

A 220 kV single-circuit line in a vertical or delta configuration is narrow enough for one earth wire at the peak to give a 30° shielding angle.

(v) Height of earth wire above the top conductor

For a shielding angle $\alpha = 30°$, the horizontal distance from the tower centre to the outermost conductor is $4.0 + 0.75 = 4.75$ m. So:

$$y = \frac{4.75}{\tan 30°} = 8.23\ \text{m} \approx 8.3\ \text{m}$$