Distribution substation with single line diagram

A distribution substation in an industry receives power from the utility at 11 kV, steps it down to 400/230 V and distributes it to the shops through switchgear and protective devices. It is the heart of the plant's electrical system.

Single line diagram

 11 kV overhead line / cable (NEA)
            |
        [LA]   lightning arresters
            |
        [AB]   gang-operated isolator
            |
     [DO fuse] or [VCB + O/C, E/F relay]
            |
     [HT metering: CT/PT, kWh, kVA MD]
            |
        (TR)   11/0.4 kV, Dyn11
            |      neutral & body earthed
        [ACB/MCCB] LT incomer + meters
            |
 ====== MDB 400 V, 3-ph 4-wire busbar ======
    |          |          |          |
  MCCB       MCCB       MCCB      MCCB
    |          |          |          |
  SDB-1      SDB-2      SDB-3    APFC panel
  motors    lighting   sockets   (capacitors)
    |          |          |
 starters    MCB        MCB+RCCB
    |          |          |
  400 V M   230 V      230 V

Explanation of the parts

1. Incoming 11 kV supply: tapped from the utility feeder by overhead line or HT cable.
2. Lightning arresters: one per phase, protect the transformer and switchgear from lightning and switching surges by diverting them to earth.
3. AB switch / isolator: off-load switch to isolate the whole substation for maintenance.
4. HT protection: drop-out fuses for small (pole/H-pole) substations; VCB or SF₆ breaker with overcurrent and earth-fault relays for larger plants.
5. HT metering: CTs and PTs feed the trivector/kWh meter for energy and maximum demand billing.
6. Transformer: 11/0.4 kV, delta-star (Dyn11). The delta HV winding traps triplen harmonics; the star LV gives 400 V between lines and 230 V phase to neutral. Rating is chosen on maximum demand.
7. LT incomer (ACB/MCCB): main switch and protection against overload and short circuit on the LT side, with ammeter, voltmeter and kWh meter.
8. Main distribution board (MDB): copper/aluminium busbars with outgoing MCCBs or switch-fuse units to each SDB.
9. APFC panel: automatic capacitor bank to keep power factor near unity, reducing current, losses and demand charges.
10. Sub-distribution boards: near the load centres; feed motors through starters (DOL, star-delta, soft starter) and lighting/socket circuits through MCBs and RCCBs.
11. Earthing: separate electrodes for transformer neutral, equipment bodies and lightning arresters.
12. Standby supply: a diesel generator with changeover switch (ATS) at the MDB feeds essential loads during outages.

Types used: pole/H-pole mounted (up to about 250 kVA), plinth-mounted outdoor (above 250 kVA), indoor or packaged substations where space or environment demands.