--- title: "DC supply system in a generating station" subject: "Power Plant Design" chapter: 4 key: "semester_8/power_plant_design:dc-supply-system" source: https://www.bctengineer.com/q/semester_8/power_plant_design/dc-supply-system site: BCT Engineering --- # DC supply system in a generating station Power Plant Design · Chapter 4: Electric System Design of a Power Plant - Asked in: 2080 Chaitra, 4 marks - Video: [Power Plant Basics: DC Bus](https://www.youtube.com/watch?v=4dOcBx-t__8) — Shad Edwards - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/power_plant_design/dc-supply-system - Whole chapter: https://www.bctengineer.com/md/semester_8/power_plant_design/chapter-4 ## Question Discuss briefly about requirements of DC supply system in a power generating station. ## Short answer DC supply system provides reliable power for protection, control, breaker trip/close, emergency lighting, and field flashing. Typically 220/110 V DC from battery banks (lead-acid or Ni-Cd) with redundant float-cum-boost chargers, distribution with earth-fault monitoring, sized per duty cycle with ageing margin. ## Answer The **DC supply system** gives a reliable, uninterrupted power source for the vital control, protection and emergency loads of a power station, even when all AC supply is lost. #### Requirements - **Loads served:** relay and protection circuits, breaker trip and close coils, control and SCADA, alarms, emergency lighting, field flashing, DC emergency oil pumps (governor, bearing lube), communication (often 48 V). - **Voltage levels:** typically **220 V or 110 V DC** for control and switchgear, **48 V DC** for telecom/PLCC, **24 V** for some electronics. - **Battery bank:** lead-acid (VRLA or flooded) or Ni-Cd, sized for the duty cycle (e.g. 1–3 hours of emergency load plus trip and close surges at the end), with ageing and temperature factors (IEEE 485). - **Battery chargers:** float-cum-boost chargers fed from the station AC. Usually **two chargers and two battery banks** for redundancy in large stations. - **Distribution:** DC distribution board with separate feeders, fuses/MCBs, and **earth-fault monitoring** because the DC system is unearthed. - **Reliability:** separate battery room with ventilation (hydrogen), voltage monitoring and alarms, and regular capacity tests. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why is a DC supply system required in a generating station? A. To supply the main generator output B. To give an uninterrupted source for control, protection and emergency loads even when AC is lost C. To charge the grid D. To drive the main turbine Answer: B. Protection, trip/close coils and emergency pumps must work even during a total AC loss. (Key point: Purpose of DC supply) 2. What redundancy is usual in large stations? A. One battery and one charger B. Two chargers and two battery banks C. No chargers, battery only D. Two generators on DC Answer: B. Duplicate chargers and battery banks keep the DC system reliable. (Key point: Redundancy) 3. Why is earth-fault monitoring provided on the DC distribution board? A. The DC system is unearthed, so a first earth fault must be detected before a second causes malfunction B. Because DC is solidly earthed C. Because batteries do not leak D. To measure battery temperature Answer: A. With an unearthed DC system a single earth fault does not trip, so it is monitored and cleared before a second one appears. 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