Purposes of substation and indoor substation

Purposes of a substation in an industrial electrical system

1. Step down the utility voltage (11 or 33 kV) to utilization voltage (400/230 V), or to 3.3/6.6 kV for large HT motors.
2. Switching: connect and disconnect feeders and transformers for operation and maintenance.
3. Protection: detect and isolate faults (overload, short circuit, earth fault, surges) to protect equipment and people.
4. Metering and monitoring: energy, maximum demand and power factor measurement for billing and energy management.
5. Power quality: voltage regulation (tap changers) and power factor correction (capacitor banks).
6. Reliability: duplicate transformers, bus coupler and standby generator changeover keep critical loads supplied.

Indoor substation

In an indoor substation all equipment (HT switchgear, transformer, LT switchboard, capacitor bank) is installed inside a building or room. It is used where the air is dusty, corrosive or saline, where space is limited, or where the substation must be placed in the load centre inside the plant.

 11 kV cable from utility / RMU
          |
  +---- HT panel room --------------+
  |  [Isolator]--[VCB/SF6]--[CT]    |
  |       |   O/C, E/F relay        |
  |  [HT metering CT/PT, LA]        |
  +-------|-------------------------+
          | HT cable
  +---- Transformer room -----------+
  |   (TR) 11/0.4 kV dry/oil type   |
  |   oil pit, louvres, fire wall   |
  +-------|-------------------------+
          | LT cable / bus duct
  +---- LT panel room --------------+
  |  [ACB]--MDB busbar--[MCCBs]     |
  |            |--[APFC panel]      |
  +------------|--------------------+
               v  feeders to SDBs

Functions of each part

• HT panel: incomer isolator and VCB/SF₆ breaker with overcurrent and earth fault relays; trips on transformer or cable faults. Surge arresters at cable termination.
• Transformer room: oil-filled transformer with oil soak pit and fire wall, or dry-type (cast resin) transformer for fire safety; natural or forced ventilation through louvres.
• LT panel: ACB as main incomer, busbars and outgoing MCCBs to SDBs, metering, APFC panel.
• Earthing: earth grid with separate electrodes for neutral and body; earth bus around rooms.
• Auxiliaries: battery and charger for breaker tripping, lighting, fire extinguishers, rubber mats, cable trench.

Operating characteristics

• Equipment is protected from rain, dust and pollution, so insulation stays clean and maintenance is lower.
• Safe operation: live parts are enclosed, access is limited to authorised persons.
• Can be placed at the load centre, reducing LT cable lengths and losses.
• Needs good ventilation since the transformer heat must be removed; temperature rise limits loading.
• Higher initial cost (building, fire protection), and extension is harder than for outdoor types.
• Usually limited to 11–33 kV due to clearance and building size.