--- title: "Battery DOD and station service schemes" subject: "Power Plant Design" chapter: 4 key: "semester_8/power_plant_design:battery-dod-service-station" source: https://www.bctengineer.com/q/semester_8/power_plant_design/battery-dod-service-station site: BCT Engineering --- # Battery DOD and station service schemes Power Plant Design · Chapter 4: Electric System Design of a Power Plant - Asked in: 2073 Bhadra, 6 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/power_plant_design/battery-dod-service-station - Whole chapter: https://www.bctengineer.com/md/semester_8/power_plant_design/chapter-4 ## Question What is DOD of a battery? Draw the typical schemes of service station. ## Short answer DOD (Depth of Discharge) is the percentage of rated capacity discharged; deeper DOD shortens battery life. Typical station service scheme: unit auxiliary transformer from generator bus feeds unit board; station transformer from HV bus feeds common board; diesel generator provides emergency and black-start supply; essential loads on emergency board. ## Answer #### Depth of discharge (DOD) **Depth of discharge (DOD)** is the fraction of a battery's rated capacity that has been taken out, expressed in percent: $$ \text{DOD} = \frac{\text{Ah discharged}}{\text{Rated Ah capacity}} \times 100\,\% $$ It is the complement of the **state of charge**: $\text{SOC} = 100\,\% - \text{DOD}$. For example, taking 60 Ah from a 200 Ah battery gives a DOD of 30 %. Importance: - A deeper DOD **shortens battery life** (fewer cycles). Lead-acid batteries are usually designed for 50–80 % maximum DOD, Ni-Cd and Li-ion can go deeper. - Battery sizing for a station uses the allowed DOD and the end-of-discharge voltage (e.g. 1.75–1.8 V/cell for lead-acid). #### Station service (auxiliary supply) schemes Station service supplies the auxiliaries: pumps, cooling, governors, cranes, lighting, battery chargers. Common schemes: **1. Unit auxiliary transformer (UAT) tapped from the generator bus, with station transformer backup** ```text Gen G1 --+-- GSU T1 --+-- HV bus (132 kV) | | UAT1 Station service tr (from HV bus) | | Unit aux board 1 ----+---- Common station board | DG set (standby / black start) ``` **2. Common station service board fed from two sources (main bus and tertiary/grid)** ```text 11 kV gen bus --> SST-1 --+ |-- 415 V board --> loads 33/11 kV feeder -> SST-2 -+ (bus coupler) | DG set --> Emergency board | Battery charger --> DC board ``` Key features: - At least **two independent sources** with automatic changeover (bus coupler). - An **emergency diesel generator** for black start and essential loads. - **Unit boards** for each generating unit's own auxiliaries, plus a **common board** for shared loads. - Essential loads moved to the emergency board; DC system with its own battery for vital loads. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the depth of discharge (DOD) of a battery? A. Ah remaining divided by rated Ah B. Ah discharged divided by rated Ah capacity, in percent C. Voltage drop in percent D. Charging current ratio Answer: B. DOD = (Ah discharged / rated Ah) x 100%. (Key point: DOD definition) 2. Taking 60 Ah from a 200 Ah battery gives what state of charge? A. 30% B. 60% C. 70% D. 140% Answer: C. DOD = 60/200 = 30%, so SOC = 100 - 30 = 70%. (Key point: DOD calculation) 3. How does a deeper DOD affect battery life? A. It lengthens life B. It shortens life (fewer cycles) C. No effect D. It doubles the capacity Answer: B. Deeper discharge reduces the number of cycles; lead-acid is usually limited to 50-80% DOD. (Key point: DOD and life) 4. Which feature is characteristic of good station service schemes? A. A single source with no changeover B. At least two independent sources with automatic changeover and an emergency DG C. Only a DG set D. Only a battery Answer: B. Two sources with a bus coupler and an emergency diesel generator keep essential auxiliaries supplied. (Key point: Station service scheme) ## Related in Power Plant Design - [Layout of a medium head Francis station](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Afrancis-medium-head-layout) — Power Plant Design, chapter 3, asked 2× ## Related in other subjects - [Battery charging schemes and most effective](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Abattery-charging-schemes-effective) — Industrial Power Distribution and Illumination, chapter 9 - [Battery backup design with inverter at 48 V](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Abattery-backup-design-48v) — Industrial Power Distribution and Illumination, chapter 9 - [Reactor schemes in stations and substations](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Areactor-schemes) — Power Plant Equipment, chapter 4 - [SLI and station batteries and charging methods](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Asli-station-battery-charging) — Industrial Power Distribution and Illumination, chapter 9