Why generation and distribution voltages are lower

Transmission voltage is high to reduce losses and conductor cost over long distances, but generation and distribution are kept at lower voltages for practical, safety and cost reasons.

Why generation voltage is low (6.6–25 kV)

• The stator winding insulation must fit in narrow slots; very high voltage would need thick insulation, increasing machine size and reducing slot space for copper.
• Rotating machines are subject to vibration and heating, so high-voltage insulation would be unreliable and costly.
• Hence generators are built for about 11–25 kV and a step-up transformer is used.

Why transmission voltage is high

• For power $P$: $I = P/(\sqrt3V\cos\phi)$, loss $= P^2R/(V^2\cos^2\phi)$.
• Raising $V$ reduces the current ($\propto 1/V$), the loss and the percentage voltage drop (both $\propto 1/V^2$), and the conductor volume ($\propto 1/V^2$), which matters on long lines.

Why distribution voltage is low (11 kV, 400/230 V)

• Safety: Distribution lines pass through towns and enter buildings; high voltage would be dangerous.
• Short distances: Over a few km, losses at 11 kV or 400 V are acceptable.
• Cheap equipment: Low-voltage insulation, clearances, poles and transformers are much cheaper and smaller.
• Utilisation: Appliances and motors are designed for 230/400 V.