Substation classification and 11 kV schematic

Classification of industrial distribution substations

According to service

• Step-down (transformer) substation: 11/0.4 kV or 33/11 kV.
• Switching substation: only switchgear, no transformer.
• Converting substation: AC to DC or frequency conversion.
• Power factor correction substation: capacitor banks.

According to design

• Outdoor: pole-mounted (single pole), H-pole (double pole, up to about 250 kVA), plinth-mounted (above 250 kVA).
• Indoor: equipment inside a building.
• Underground and packaged/compact (kiosk) substations.

Schematic diagram from 11 kV to end use

 11 kV overhead feeder (utility, NEA)
            |
        [LA]  lightning arresters (3 nos)
            |
        [AB]  air-break / gang-operated isolator
            |
        [DO]  drop-out fuse  (or HT VCB + relay)
            |
        [HT metering CT/PT]  (for HT consumers)
            |
        (TR)  11/0.4 kV, Dyn11 transformer
            |      neutral + body earthed
        [ACB / MCCB]  LT main breaker + LT meter
            |
 ======== MDB  400 V, 3-ph 4-wire busbar ========
     |            |            |            |
   MCCB         MCCB         MCCB         MCCB
     |            |            |            |
   SDB-1        SDB-2        SDB-3       APFC panel
  (motors)    (lighting)    (sockets)   (capacitors)
     |            |            |
   MCB/MPCB      MCB          MCB/RCCB
     |            |            |
   400 V M     230 V lamps   230 V outlets

Explanation

1. 11 kV incoming line from the utility feeder.
2. Lightning arresters protect the transformer from lightning and switching surges.
3. AB switch isolates the substation (off-load) for maintenance.
4. DO fuse (or VCB with overcurrent and earth fault relays in bigger plants) protects the transformer from internal faults and overloads.
5. HT metering for energy billing of HT consumers.
6. 11/0.4 kV Dyn11 transformer gives 400 V line and 230 V phase voltage with a solidly earthed neutral.
7. LT ACB/MCCB is the main LT switch and protection; LT meters show current, voltage and energy.
8. MDB busbars divide power into feeders with MCCBs; an APFC panel improves power factor.
9. SDBs near each shop feed motors (through starters and overload relays) at 400 V and lighting and sockets at 230 V through MCBs and RCCBs.
10. Earthing of neutral, equipment bodies and arresters completes the system.