Significance of transmission voltage level, 132 vs 400 kV

Significance of voltage level

For power $P$ sent at voltage $V$ and p.f. $\cos\phi$:

$$I = \frac{P}{\sqrt3V\cos\phi},\qquad P_{loss} = 3I^2R = \frac{P^2R}{V^2\cos^2\phi}$$

• Loss and % voltage drop $\propto 1/V^2$; conductor volume for the same loss $\propto 1/V^2$.
• Power-carrying capacity $\propto V^2$ (SIL $= V^2/Z_c$; $P_{max} = V_sV_r/X$).
• But insulation, clearances, tower size and substation cost rise with $V$.

So the voltage level is chosen to balance line losses against insulation cost, depending on power and distance (e.g. Still's formula $V = 5.5\sqrt{l/1.6 + P/100}$ kV, $l$ in km, $P$ in kW).

132 kV vs 400 kV for the same power and distance

Technically feasible: yes, and performance improves:

• Current falls by $400/132 = 3.03$ times, so loss falls by about $3.03^2 \approx 9.2$ times.
• Voltage regulation improves and stability margin rises (SIL of 400 kV ≈ 9 times that of 132 kV).
• Fewer circuits or smaller conductor area may be needed.

Economically: feasible only if the power is large. A 400 kV line needs bigger towers, longer insulator strings, wider right-of-way and costly 400 kV substations; corona and charging current also increase. If the power is small (suitable for 132 kV, e.g. 50–100 MW), the line would be lightly loaded far below its SIL (≈ 500–600 MW), the saving in loss would not pay for the extra cost, and at light load the Ferranti effect would need reactors.

Conclusion: It is technically possible and more efficient, but it is economical only when the transmitted power (now or in future) is large enough to justify the 400 kV investment.