Layout of indoor substation with single transformer

An indoor substation houses all substation equipment inside a building. With a single transformer the layout is simple: an HT room, a transformer room and an LT room arranged in the order of power flow, with cable trenches between them.

General layout (plan)

          11 kV cable in (from RMU / pole)
                    |
 +------------------|----------------------------+
 |  HT ROOM         v                            |
 |  [Isolator]--[VCB]--[CT]--[relay panel]       |
 |  [HT meter]  [battery + charger]             |
 +------------------|----------------------------+
 |  TRANSFORMER     v           |  LT ROOM       |
 |  ROOM                        |                |
 |   +----------+               |  [ACB incomer] |
 |   |  (TR)    |-- LT cable -->|  == MDB ==     |
 |   | 11/0.4kV |   in trench   |  [MCCBs] out   |
 |   +----------+               |  [APFC panel]  |
 |   oil soak pit, fire wall    |                |
 |   louvres for air            |  feeders ->SDB |
 +------------------------------+----------------+
  door (wide, for TR)    earth pits outside

Description of the layout

1. HT room
   • Incoming 11 kV XLPE cable terminated with surge arresters.
   • HT panel: isolator/load-break switch, VCB or SF₆ breaker, CTs and PTs, overcurrent and earth-fault relays, HT metering.
   • DC battery and charger for breaker tripping and closing.
2. Transformer room
   • Single 11/0.4 kV transformer (oil-filled ONAN or dry-type cast resin).
   • Oil-filled units need an oil soak pit filled with pebbles, a fire-resistant wall and a fire door.
   • Low-level inlet and high-level outlet louvres (or exhaust fans) for cooling; clearance of about 0.75–1 m around the transformer.
   • Wide door and rails for moving the transformer in and out.
3. LT room
   • LT panel with ACB incomer, busbars, outgoing MCCBs to SDBs, meters and indication lamps.
   • APFC capacitor panel.
   • Changeover from DG set if standby supply is provided.
4. Cable trenches covered with chequered plates connect HT panel to transformer and transformer to LT panel; outgoing cables leave through trenches or ducts.
5. Earthing: earth bus inside the rooms connected to separate earth pits for neutral and body; LA earthing separate.
6. Safety items: rubber mats in front of panels, fire extinguishers (CO₂ / DCP), sand buckets, emergency lighting, danger boards, single-line diagram on the wall, adequate lighting and first-aid chart.

Features

• Safe and protected from dust, rain and pollution; good for chemical, textile and cement plants.
• Can be built at the load centre within the factory building.
• Higher cost and need for ventilation and fire protection; extension is limited since there is only one transformer (no redundancy). A second transformer bay may be kept for future.