--- title: "Layout of indoor substation with single transformer" subject: "Industrial Power Distribution and Illumination" chapter: 4 key: "semester_6/industrial_power_distribution_and_illumination:indoor-substation-single-transformer" source: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/indoor-substation-single-transformer site: BCT Engineering --- # Layout of indoor substation with single transformer Industrial Power Distribution and Illumination · Chapter 4: Distribution Substation of Industrial Plant - Asked in: 2069 Bhadra, 8 marks - Video: [LECTURE-15 layout of indoor substation. floor mounted type](https://www.youtube.com/watch?v=PgEAXBg7SUA) — VISHNU CHANDRAN - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/indoor-substation-single-transformer - Whole chapter: https://www.bctengineer.com/md/semester_6/industrial_power_distribution_and_illumination/chapter-4 ## Question Explain the general layout of an indoor-substation with single transformer in an industry. ## Short answer Layout: HT room has isolator, VCB, relays, battery; transformer room houses single 11/0.4 kV unit with oil pit and ventilation louvres; LT room contains ACB, MDB, MCCBs, APFC panel. Cable trenches connect rooms. Protected indoors, placed at load centre, but no redundancy. ## Answer 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) ```text 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. In the indoor substation with one transformer, how are the rooms arranged? A. LT room, transformer room, HT room B. HT room, LT room, transformer room C. Transformer room, HT room, LT room D. HT room, transformer room, LT room in the order of power flow Answer: D. The rooms follow the power flow: 11 kV HT room, then the 11/0.4 kV transformer room, then the LT room. (Key point: Order of rooms) 2. Why is an oil soak pit filled with pebbles provided in the transformer room of an oil-filled unit? A. To earth the transformer body B. To collect and quench leaked or burning oil and reduce fire spread C. To cool the transformer by conduction D. To support the weight of the transformer Answer: B. Pebbles in the pit take up leaked oil and cool it, limiting fire; it goes with a fire wall and fire door. (Key point: Oil soak pit) 3. How is the transformer room cooled? A. Low-level inlet and high-level outlet louvres or exhaust fans B. High-level inlet and low-level outlet louvres only C. A sealed room with no openings, to keep dust out D. Water cooling pipes in the cable trench Answer: A. Cool air enters low and hot air leaves high through louvres or fans, so the transformer losses are carried away. (Key point: Ventilation) 4. Which is a drawback of this layout? A. It cannot be placed inside a factory B. It is exposed to dust and rain C. Higher cost, ventilation and fire protection needs, and no redundancy with one transformer D. It cannot supply LT loads Answer: C. Indoor substations cost more and need ventilation and fire protection; with a single transformer there is no standby unit. 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