SLD with voltage levels and why three-phase

Single line diagram with typical voltage levels

 [G] 11 kV       Step-up       Transmission
  |---->[11/220 kV]====== 220/400 kV ======+
                                            |
 [G] 11 kV                          [220/132 kV]
  |---->[11/132 kV]====== 132 kV ====== Grid S/S
                                            |
                              Sub-transmission 66/33 kV
                                            |
                                    [33/11 kV] S/S
                                            |
                              Primary distribution 11 kV
                                            |
                                    [11/0.4 kV] DT
                                            |
                         Secondary 400/230 V consumers

Why different voltages are used

Generation (6.6–25 kV): Generator voltage is limited by the insulation that fits in the stator slots and by machine cost. Higher generation voltage is not practical.

Transmission (132–400 kV, up to 765 kV): Bulk power must travel long distances. For a given power:

• current $\propto 1/V$, so loss $\propto 1/V^2$ and % drop $\propto 1/V^2$,
• conductor volume for the same loss $\propto 1/V^2$,
• line capacity $\propto V^2$.

So the voltage is raised as high as insulation economics allow.

Distribution (11 kV and 400/230 V): Near consumers, very high voltage would be unsafe and need large clearances in towns. Distances are short, so 11 kV primary and 400/230 V secondary are used for safety and cheap equipment, matching appliance ratings.

Why 3-phase AC instead of single-phase

1. Less conductor: A 3-phase 3-wire line needs only 75 % of the conductor of a 1-phase line for the same power, voltage and loss.
2. Constant power: Balanced 3-phase power is constant, giving smooth torque; 1-phase power pulsates at $2f$.
3. Bigger, more efficient machines: A 3-phase generator gives about 1.5 times the output of a 1-phase one of the same size.
4. Self-starting induction motors on 3-phase supply.
5. Both 400 V and 230 V available from a 3-phase 4-wire system.
6. Cheaper transformers and switchgear per kVA and easier parallel operation.