Basic structure of power system

A power system is the network that generates electrical energy, transmits it over long distances and distributes it to consumers, with protection and control at every stage.

 [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

Parts of the structure

1. Generating stations

• Hydro, thermal, nuclear, solar or wind plants generate 3-phase AC at 6.6–25 kV (Nepal: mostly hydro at 11 kV).
• A step-up transformer raises the voltage for transmission.

2. Transmission system

• Extra-high voltage lines (132, 220, 400 kV in Nepal; up to 765 kV and above elsewhere) carry bulk power to load centres.
• Lines are interconnected to form a grid, allowing power exchange and shared reserve.

3. Sub-transmission system

• At grid substations the voltage is reduced to 66 or 33 kV and carried to distribution substations in towns and districts.
• Large industries may be supplied directly at this level.

4. Primary distribution

• Distribution substations (33/11 kV) feed 11 kV feeders through the load area.
• Medium industries take supply at 11 kV.

5. Secondary distribution

• Distribution transformers (11/0.4 kV) supply LT distributors at 400 V 3-phase and 230 V single-phase for homes, shops and small industries.

6. Protection and control

• Circuit breakers, relays, lightning arresters at all substations; load dispatch centre (SCADA) monitors and controls the whole system, keeping frequency and voltage within limits.