Need for different voltage levels

Different voltage levels are used because no single voltage is best for every part of the system: generators are limited to a moderate voltage, bulk power must be sent far at very high voltage to keep losses low, and consumers need a low, safe voltage.

Reasons

1. Generation limit: Generator insulation in the stator slots limits the voltage to about 11–25 kV (Nepal: 6.6/11 kV).
2. Low transmission loss: For a given power, $I = P/(\sqrt3 V\cos\phi)$, so loss $3I^2R \propto 1/V^2$. Raising voltage from 11 kV to 132 kV cuts the loss by $(132/11)^2 = 144$ times.
3. Less conductor material: Conductor volume for the same loss is $\propto 1/V^2$, so high voltage saves copper/aluminium.
4. Better regulation and capacity: % voltage drop $\propto 1/V^2$; power capacity of a line $\propto V^2$ (e.g. $P_{max} = V_sV_r/X$).
5. Distance and power: Longer lines and larger blocks of power need higher voltage (400 kV for hundreds of km, 33 kV for tens of km).
6. Safety and cost at the user end: Very high voltage needs large clearances and costly insulation; so in towns it is stepped down to 11 kV and finally 400/230 V for safe use.

Interconnected system with 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

Transformers make these levels possible economically; this is one of the main reasons AC is used for power systems.