--- title: "Distribution substation with single line diagram" subject: "Industrial Power Distribution and Illumination" chapter: 4 key: "semester_6/industrial_power_distribution_and_illumination:sld-distribution-substation" source: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/sld-distribution-substation site: BCT Engineering --- # Distribution substation with single line diagram Industrial Power Distribution and Illumination · Chapter 4: Distribution Substation of Industrial Plant - Asked in: 2070 Magh, 8 marks - Video: [How to make Low Voltage distribution single line drawing (SLD )and calculation (11KV Substation)](https://www.youtube.com/watch?v=yEG6V4eXxHA) — electrical mastar - 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/sld-distribution-substation - Whole chapter: https://www.bctengineer.com/md/semester_6/industrial_power_distribution_and_illumination/chapter-4 ## Question With the help of single line diagram, explain the distribution substation in an industry. ## Short answer 11 kV supply comes via LA, isolator, VCB/DO fuse with O/C & E/F relays, HT metering, to 11/0.4 kV Dyn11 transformer. LT side: ACB incomer, MDB busbar with MCCBs feeding SDBs for motors/lighting, and APFC panel. Provides step-down, switching, protection, metering, and power factor correction. ## Answer 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 ```text 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the role of the industrial distribution substation? A. Receives 11 kV, steps down to 400/230 V and distributes via protective switchgear B. Generates power C. Converts to DC D. Stores energy Answer: A. It is the heart of the plant's electrical system. (Key point: Role) 2. Which HT protection suits small versus large plants? A. VCB for small; DO fuses for large B. Only AB switches C. DO fuses for small; VCB/SF6 with O/C and E/F relays for large D. Only lightning arresters Answer: C. Larger plants need relay-controlled breakers. (Key point: HT protection) 3. What do HT CTs and PTs feed? A. A trivector/kWh meter for energy and maximum demand billing B. The lightning arresters C. The earthing system D. The APFC capacitors Answer: A. Instrument transformers scale down HT quantities. (Key point: HT metering) 4. How are essential loads supplied during an outage? A. The lightning arresters B. A second AB switch C. A diesel generator with changeover (ATS) at the MDB D. Reverse feeding from SDBs Answer: C. ATS transfers essential loads. 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